System and method for improving resistance welding electrode life

Abstract

Shape memory alloy (SMA) actuators may include elements (e.g., SMA wires) configured to act when an electric current is supplied. SMA elements can be joined to a beam (e.g., stainless steel) by resistance welding, which joins two metals by passing an electric current through them. Resistance welders with smaller step sizes in power, lower total power, smaller step sizes in electrode clamping force, and smaller time-to-time variations may produce additional test samples. Approaches to redistribute the system's heat may be employed. The resistance of the lower electrode (R6) can be increased by changing the electrode material from tungsten copper to a more resistant tungsten alloy. Tungsten alloys may be used. A short-time pulse welding recipe using a high-resistivity lower electrode may be a reference resistance welding process for attaching SMA wires to stainless steel.

Description

[Technical Field]

[0001] The present invention generally relates to performing resistance welding between metals. More specifically, it relates to an improved method for performing resistance welding between a metal actuator and a shape memory alloy material. [Background Art]

[0002] This application claims the benefit and priority of U.S. Provisional Application No. 63 / 534,718, filed on August 25, 2023, and U.S. Patent Application No. 18 / 812,143, filed on August 22, 2024. Each of these is incorporated herein by reference in its entirety.

[0003] Actuators may be used in various contexts. For example, as part of an autofocus system, an actuator can move a lens back and forth to focus the lens. In many cases, it may be desirable to move a moving component in a desired direction (e.g., the Z direction) to increase efficiency in implementing such an autofocus system.

[0004] Shape memory alloy (SMA) actuators may include elements configured to act when supplied with electric current (e.g., shape memory alloy SMA wires). Such actuation may be used to move an object such as a moving carriage or a lens within an autofocus (AF) or optical image stabilization (OIS) system in an imaging apparatus. The shape memory alloy SMA element may be an alloy of nickel and titanium, commonly referred to as nitinol.

[0005] Shape memory alloy (SMA) elements can be joined to a beam (e.g., made of stainless steel) by resistance welding, a process that joins two metals by passing an electric current through them to induce Joule heating and bond the two materials together. The shape memory alloy (SMA) wire and the stainless steel are pressed together between opposing electrodes, and an electric current is supplied through the electrodes to induce Joule heating. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2023 / 029366 [Overview of the project] [Problems that the invention aims to solve]

[0007] Regarding the resistance welding process described above, further improvements are desired, and there is room for improvement. [Means for solving the problem]

[0008] Shape memory alloy (SMA) actuators may include elements (e.g., shape memory alloy SMA wires) configured to actuate when an electric current is supplied. Shape memory alloy SMA elements can be joined to a beam (e.g., stainless steel) using resistance welding, a process that bonds two metals together by passing an electric current through them. Resistance welders with smaller power step sizes, lower total power, smaller electrode clamping force step sizes, and smaller time-to-time variability may produce additional test samples. Furthermore, approaches to redistribute the system's heat may be employed. The resistance (R6) of the bottom electrode can be increased by changing the electrode material from tungsten copper to a more resistant tungsten alloy. A short-time pulse welding recipe using a high-resistivity bottom electrode may serve as a baseline resistance welding process for attaching shape memory alloy SMA wires to stainless steel.

[0009] In one exemplary embodiment, a method is provided for performing a resistance welding process between a metal actuator and a shape memory alloy (SMA) wire. This method may include positioning a resistance welding machine at the first end of the shape memory alloy SMA wire and the first end of the metal actuator such that the top electrode is positioned adjacent to the shape memory alloy (SMA) wire and the second electrode of the resistance welding machine is positioned adjacent to the metal actuator. The electrode material may include a tungsten alloy with increased resistance compared to the more commonly used tungsten copper.

[0010] The method may also include the step of performing a first resistance weld between the first end of the shape memory alloy SMA wire and the metal actuator by passing an electric current through the shape memory alloy SMA wire and the metal actuator using a resistance welding machine. The method may also include the step of positioning the resistance welding machine at the second end of the shape memory alloy SMA wire and the second end of the metal actuator such that the upper electrode is positioned adjacent to the second end of the shape memory alloy SMA wire and the second electrode of the resistance welding machine is positioned adjacent to the second end of the metal actuator. The method may also include performing a second resistance weld between the second end of the shape memory alloy SMA wire and the metal actuator by passing an electric current through the shape memory alloy SMA wire and the metal actuator using a resistance welding machine. Each of the first and second resistance welds may be performed with a welding pulse duration of approximately 9 milliseconds (ms).

[0011] In some cases, the process of performing each of the first and second resistance welds includes a 2-millisecond ramp-up period and a 2-millisecond ramp-down period during the duration of each weld pulse. Here, the pulse current for each of the first and second resistance welds is approximately 110 amperes.

[0012] In some cases, metal actuators are made of stainless steel. In some cases, resistance welding machines feature a power step size of approximately 0.001 watt-seconds (W·s), pulse time step sizes of approximately 0.1 milliseconds (ms) and approximately 1 millisecond, an electrode clamping force step size of approximately 0.1 pounds (lbs) (approximately 45.36 grams), and smaller time-to-time variations.

[0013] In some cases, the method may include the steps of: obtaining a set of coupons made of 302 stainless steel with a thickness of 0.004 inches (0.1016 mm); performing resistance welding on each coupon to a shape memory alloy SMA wire such that the pulse duration is approximately 10 milliseconds, the welding energy is approximately 2.2 to 4 watt-seconds, and the weld strength is in the range of approximately 25 to 35 grams; and testing one or more conditions of the set of coupons to determine the weld peel strength between the wire attached to each coupon and the coupon.

[0014] In some cases, the process of testing one or more conditions further includes a step of performing a cross-section of the weld on each coupon using a focused ion beam (FIB).

[0015] In some cases, this method may include performing nanoindentation on shape memory alloy SMA wires to measure the nanohardness and modulus of samples welded under different settings.

[0016] In some cases, metal actuators are part of an optical image stabilization (OIS) system. Other features and advantages of embodiments of the present invention will become apparent from the accompanying drawings and the following detailed description.

[0017] Embodiments of the present invention are shown in the accompanying drawings, not as examples or limitations, where similar reference numerals indicate similar elements, and are as follows: [Brief explanation of the drawing]

[0018] [Figure 1] In resistance welding, several embodiments demonstrate that electrodes are used to apply pressure (tightening force) and supply current to the workpiece. [Figure 2] According to some embodiments, there is shown resistance welding of shape memory alloy (SMA) wires to a buckler autofocus prototype. [Figure 3A] According to some embodiments, there is shown an exemplary process flow for assembling an autofocus device prototype by using shape memory alloy (SMA) wire welding, such as for a buckler-type autofocus device. [Figure 3B] According to some embodiments, there is shown an exemplary process flow for assembling an autofocus device prototype by using shape memory alloy (SMA) wire welding, such as for a buckler-type autofocus device. [Figure 4] According to some embodiments, there is shown that an actuator device, such as a bimorph actuator device, may use a shape memory alloy (SMA) wire with both ends resistance-welded. [Figure 5] According to some embodiments, there is shown that a resistance-welded joint connects a shape memory alloy (SMA) wire to a stainless steel substrate both electrically and mechanically. [Figure 6] According to some embodiments, there are shown examples of weld tensile strength in the range of 91 to 99 grams, and the fracture mode in all cases is within the wire material. [Figure 7] According to some embodiments, there are shown examples of weld peel strength in the range of 20 to 26 grams, and the fracture mode in all cases is within the wire material. [Figure 8] According to some embodiments, there is shown that the welded structure is evaluated by sectioning the weld using a focused ion beam (FIB). [Figure 9A] According to some embodiments, there is shown a focused ion beam (FIB)-cut sample of a welding interface between a shape memory alloy (SMA) wire and stainless steel, imaged by a scanning electron microscope (SEM). [Figure 9B] According to some embodiments, there is shown a focused ion beam (FIB)-cut sample of a welding interface between a shape memory alloy (SMA) wire and stainless steel, imaged by a scanning electron microscope (SEM). [Figure 9C] According to some embodiments, there is shown a focused ion beam (FIB) cut sample of a welding interface between a shape memory alloy (SMA) wire and stainless steel, which is captured by a scanning electron microscope (SEM). [Figure 9D] According to some embodiments, there is shown a focused ion beam (FIB) cut sample of a welding interface between a shape memory alloy (SMA) wire and stainless steel, which is captured by a scanning electron microscope (SEM). [Figure 9E] According to some embodiments, there is shown a focused ion beam (FIB) cut sample of a welding interface between a shape memory alloy (SMA) wire and stainless steel, which is captured by a scanning electron microscope (SEM). [Figure 9F] According to some embodiments, there is shown a focused ion beam (FIB) cut sample of a welding interface between a shape memory alloy (SMA) wire and stainless steel, which is captured by a scanning electron microscope (SEM). [Figure 10] According to some embodiments, there is shown a resistance welding machine installed and used for preparing resistance welding test samples. [Figure 11] According to some embodiments, there is shown that none of the welded parts fractured in the cycle durability test. [Figure 12] According to some embodiments, mechanical analysis tests performed on a sample of a shape memory alloy (SMA) wire with both ends welded to steel show that the sample exhibits behavior closer to that of an as-received wire than that of a wire annealed in a red-hot state. [Figure 13] According to some embodiments, there is shown an example of tensile test results indicating that a short welding pulse more closely matches the strength of an as-received shape memory alloy (SMA) wire than a sample produced with a long pulse. [Figure 14] According to some embodiments, scanning electron microscope (SEM) images show that the grain structure of the shape memory alloy (SMA) wire is very small and can be less than 100 nm. [Figure 15]Several embodiments demonstrate that if the resistance (R1) at the interface between the electrode and the workpiece is too high, an unintended amount of heat is generated, causing the electrode to stick. [Figure 16] Several embodiments demonstrate that many wire attachment machines are equipped with automated functions for feeding shape memory alloy (SMA) wire and crimping the shape memory alloy (SMA) wire. [Figure 17] Several embodiments demonstrate that a crimping die set (currently coining by closing the wire crimping section to attach a shape memory alloy SMA wire) can be replaced by an electronically controlled low-pressure resistance welding head. [Figure 18] Several embodiments demonstrate that a crimping die set (currently coining by closing the wire crimping section to attach a shape memory alloy SMA wire) can be replaced by an electronically controlled low-pressure resistance welding head. [Figure 19] Several embodiments demonstrate that a crimping die set (a) that currently coins a wire by closing the wire crimping section to attach a shape memory alloy SMA wire can be replaced with a wire welding electrode and a wire cutting punch (b). [Figure 20] Several embodiments demonstrate that the precision stage is equipped with a single electrode for welding both ends of a wire. [Figure 21] Several embodiments demonstrate that the precision stage is equipped with a single electrode for welding both ends of a wire. [Figure 22] Several embodiments demonstrate that once the wire is welded to the workpiece at both ends, a cutting punch on a precision stage moves downward to cut the wire. [Figure 23A] Several embodiments show scanning probe microscope (SPM) images of nanoindentations in the cross-section of a shape memory alloy (SMA) wire weld. [Figure 23B]In some embodiments, the cross-sectional area and nanoindents show a welded shape memory alloy SMA wire, circled in white. [Figure 24] Several embodiments demonstrate that the nanohardness H of the wire in the as-received state is 4.249 GPa, and that the nanohardness H of the wire increases by cold working, even when the wire is clamped with electrodes but no welding current is applied. [Figure 25] Several embodiments demonstrate that the Young's modulus E of the wire in the acceptance state is 67.711 GPa. [Figure 26] Several embodiments demonstrate that wire welding is tolerant of variations in welding machines at mass production levels, as material properties can be similar at low nominal, high welding energies. [Figure 27A] Several embodiments demonstrate the development components of a shape memory alloy SMA optical image stabilization (OIS) platform used to test the reliability of shape memory alloy SMA wire welding. [Figure 27B] Several embodiments demonstrate shape memory alloy SMA_Optical Image Stabilization (OIS) platform development components featuring shape memory alloy SMA wire welding. [Figure 27C] Several embodiments illustrate shape memory alloy SMA wire welds. [Figure 28] Several embodiments demonstrate that two optical image stabilization (OIS) actuators are placed in a simulated smartphone housing and dropped onto a concrete floor from a height of 1.6 meters. [Figure 29] Figures 29A to 29H show that, in several embodiments, shape memory alloy SMA wire welds on optical image stabilization (OIS) platform development components pass drop tests. [Figure 30A] Several embodiments demonstrate that shape memory alloy SMA wire welding includes a lens-shift optical image stabilization (OIS) system (e.g., a bimorph lens-shift optical image stabilization OIS). [Figure 30B] Several embodiments demonstrate that shape memory alloy SMA wire welding includes an autofocus (AF) system (e.g., a buckler-type autofocus AF). [Figure 30C] Several embodiments demonstrate that shape memory alloy SMA wire welding is equipped with an optical image stabilization (OIS) long wire platform development. [Figure 30D] Several embodiments demonstrate that shape memory alloy SMA wire welding includes a sensor-shift optical image stabilization (OIS) system (e.g., a bimorph sensor-shift optical image stabilization OIS). [Figure 31A] The dimensions of the upper electrode tested are shown in several embodiments. [Figure 31B] The dimensions of the upper electrode tested are shown in several embodiments. [Figure 32] The dimensions of the lower electrode tested are shown in several embodiments. [Figure 33] Several embodiments illustrate examples of electrodes having a burn-in depth over 1500 welding cycles in a stationary position. [Figure 34A] Examples of static welding according to several embodiments are shown. [Figure 34B] Examples of static welding according to several embodiments are shown. [Figure 34C] Examples of static welding according to several embodiments are shown. [Figure 34D] Examples of static welding according to several embodiments are shown. [Figure 35A] Examples of 1000 cycles in several embodiments are shown. [Figure 35B] Examples of 1000 cycles in several embodiments are shown. [Figure 36A] Examples of 1500 cycles in several embodiments are shown. [Figure 36B] Examples of 1500 cycles in several embodiments are shown. [Figure 36C]Examples of 1500 cycles in several embodiments are shown. [Figure 37] Examples of stepped welding with a 4 μm deviation across the entire electrode surface after 1000 cycles are shown according to several embodiments. [Figure 38A] Examples of step welding over 1 to 1000 cycles are shown according to several embodiments. [Figure 38B] Examples of step welding over 1 to 1000 cycles are shown according to several embodiments. [Figure 38C] Examples of step welding over 1 to 1000 cycles are shown according to several embodiments. [Figure 39A] Examples of stepwise welding measurements according to several embodiments are shown. [Figure 39B] Examples of stepwise welding measurements according to several embodiments are shown. [Figure 39C] Examples of stepwise welding measurements according to several embodiments are shown. [Figure 40] An exemplary set of welding locations as part of resistance welding techniques, according to several embodiments, is shown. [Figure 41] An exemplary set of rotatable welding positions is shown according to several embodiments. [Modes for carrying out the invention]

[0019] Shape memory alloys (SMAs) may include elements (e.g., wires) configured to act when an electric current is supplied. Such actuation can be used to move objects such as moving carriages or lenses in autofocus (AF) or optical image stabilization (OIS) systems in imaging devices. Shape memory alloy SMA elements may be alloys of nickel and titanium (commonly called nitinol).

[0020] Shape memory alloy (SMA) elements can be joined to actuators (e.g., stainless steel) by resistance welding, a process that involves joining two metals electronically and mechanically through the passage of electric current. The shape memory alloy (SMA) wire and the stainless steel are pressed together between opposing electrodes, and an electric current is supplied through the electrodes to produce Joule heating.

[0021] Test samples can be prepared using state-of-the-art resistance welding machines with smaller step sizes in power output, lower total power output, smaller step sizes in electrode clamping force, and smaller time-to-time variations. Furthermore, a baseline welding recipe is established by preparing multiple coupons made of 0.004-inch (0.1016 mm) thick 302 stainless steel at different welding energies and then performing peel tests on them. Weld strength plateaus at 25-35 grams for welding energies in the range of 2.2-4 watt-seconds (technically welding work in watt-seconds) for a pulse duration of 10 milliseconds. A baseline setting of 2.4 watt-seconds can be selected.

[0022] Welded structures can be evaluated by cross-sectionalizing the weld using focused ion beam (FIB) and observing the two materials being welded together with a scanning electron microscope (SEM). Functional testing can be performed by subjecting coupon components to cycle endurance testing (life cycle testing). The purpose of welding shape memory alloy (SMA) wires may be to create a strong mechanical connection while minimizing the impact on the wire material as much as possible.

[0023] Dynamic mechanical analysis can be performed to measure the effect of heat on the mechanical properties of shape memory alloy (SMA) wires. Two samples can be compared: (1) a shape memory alloy (SMA) wire in its as-received state, and (2) a shape memory alloy (SMA) wire that has been annealed by applying an electric current until it glows red in about 1 second.

[0024] The tensile strength of the welded wire can be compared to the shape memory alloy SMA wire in its "acceptance state" (i.e., shape memory alloy SMA wire directly from the wire spool, unwelded, as shipped from the supplier). Two conditions can be compared for a shape memory alloy SMA wire welded to a 30 μm thick 302 / 304 stainless steel substrate: (1) under short-duration pulses and (2) under long-duration pulses. Scanning electron microscope (SEM) images of the shape memory alloy SMA material in the unaffected region and the welded region can be compared.

[0025] Novel approaches can be employed to redistribute the system's heat. The resistance of the lower electrode (R6) can be increased by changing the electrode material from tungsten copper to a more resistive tungsten alloy. This improves the balance of heat generation between the two electrode interfaces, thus resolving the sticking problem at the upper electrode. For example, a tungsten alloy can be used. A short-time pulse welding recipe using a high-resistivity lower electrode (as shown in Table 3) could be a reference resistance welding process for attaching shape memory alloy SMA wire to stainless steel.

[0026] In many cases, shape memory alloys (SMAs) can be difficult to weld with high reliability and good performance. Nitinol welded to stainless steel can be inherently weakened by brittle intermetallic compounds at the interface between the nitinol and the stainless steel substrate. Welding energy also affects the shape memory properties of nitinol, which can lead to performance degradation. Solid-state welding, formed by resistance welding, can have minimal to no intermetallic compounds and can form atomic bonds at lower temperatures than fusion welding, so it may be studied to see if it overcomes the risk of loss of intermetallic compounds and shape memory properties.

[0027] Certain shape memory alloys (SMAs) used in buckle, bimorph, or other shape memory alloy SMA actuators may include nitinol, whose name is derived from an alloy of nickel and titanium. Shape memory alloy SMAs generally have the property of shrinking when heated. In buckle actuators, current can be wired through a shape memory alloy SMA wire. Due to the electrical resistance of the shape memory alloy SMA wire, Joule heating proportional to the amount of current can occur. The wire can reach its phase transformation temperature and transition from the martensite phase to the austenite phase, which can cause strain in the wire.

[0028] Shape memory alloy (SMA) wires can be fixed to mechanisms consisting of partially or entirely stainless steel components by resistance welding the SMA wires to stainless steel. Because resistance welding can yield promising results in initial development, small-scale prototype actuators constructed using resistance welding have demonstrated good performance and reliability. Conventional generations of actuators used crimping to connect the wires to the actuator mechanism.

[0029] Crimping (or crimping) can create both low-resistance electrical connections and high-holding-force mechanical connections by shaping and coining the material around the wire. Crimping can be undesirable in some cases due to its very small processing window. Excessive coining can cause electrical short circuits by collapsing the crimp's insulating layer against the wire. Insufficient coining can result in low holding force of the mechanical connection, potentially causing the wire to loosen under load. While Nitinol can be welded to steel in some medical device applications, such as guide wires, there is generally not extensive industry knowledge on how to perform such welding. Therefore, empirical testing and development were necessary to truly understand the limitations and reliability of shape memory alloy (SMA) wire welding.

[0030] Shape memory alloy (SMA) wires possess "shape memory" properties, meaning they shrink when heated. Therefore, in addition to other potential applications, SMA is a useful "engine" for actuators equipped with optical image stabilization (OIS) and autofocus (AF) actuators used in smartphone cameras. Conventional applications of SMA have involved crimping to attach the wire to the actuator. Welding the SMA wire to the actuator offers an opportunity to miniaturize the actuator design and reduce the number of assembly steps.

[0031] The roadmap for future optical image stabilization (OIS) and autofocus (AF) actuators will utilize shape memory alloy (SMA) wire welding to attach the SMA wires and replace the crimping method used for wire attachment. Resistance welding is being pursued because it allows for a larger processing window than current crimping methods. Resistance welding also offers design flexibility in determining where to attach the wires within the mechanism and how much space to allocate for creating connections to the SMA wires.

[0032] Welding shape memory alloy (SMA) wires to actuators presents an opportunity to miniaturize shape memory alloy (SMA) actuator designs for smartphone cameras and reduce the number of assembly steps for the actuators. Resistance welding may involve joining two metals by passing an electric current through them to induce Joule heating, causing the two materials to bond together. The resistance-welded joint connects the shape memory alloy (SMA) wire to the stainless steel substrate both electrically and mechanically.

[0033] A standard recipe has been developed for welding shape memory alloy (SMA) wire to stainless steel, and welds produced using this recipe have been proven to be strong and reliable. Actuators with welded shape memory alloy (SMA) wire achieve 1 million cycles without failure and pass drop impact tests. Nanoindent testing shows that the material properties are similar at low nominal and high welding energies, indicating that the wire welding is tolerant of variations in welding machines at mass production levels.

[0034] The heat generation equation 1 (Equation 1) is Q = I 2 The equation may include R·t, where Q is the heat generated, I is the resistance welding current, R is the total resistance, and t is the welding time. This equation may be the governing equation for heat generation in resistance welding.

[0035] Resistance welding is a process that joins two metals by causing Joule heating through the passage of an electric current, thereby bonding the two materials together. The bond formed by resistance welding can be a fusion bond, where both materials reach their melting points and mix (intermix), or it can be a solid-state bond, where there is little to no fusion, but there is interdiffusion of atoms between the two surfaces being joined.

[0036] Electrodes can be used to apply pressure (tightening force, squeezing force) and supply current to the workpiece. Welding begins at a high-resistance interface between two metals. When this interface collapses, heat continues to transfer outward towards the electrodes until the current flow is stopped by the resistance welding machine's controller.

[0037] As shown in Equation 1, some of the main factors in resistance welding are the heat generated due to the resistivity of the two metals being joined, the amplitude of the current flowing between the workpieces, the duration for which the current is maintained, and the magnitude of the force that presses the workpieces together by the electrodes. Resistance welding systems may commonly be equipped with spring-loaded electrodes so that the electrodes can "follow" the workpiece if it becomes thinner during the welding process.

[0038] Figure 1 shows that in resistance welding, the upper electrode 102 and the lower electrode 104 are used to apply pressure (tightening force, squeezing force) and supply current to the workpiece 106. Furthermore, Figure 2 shows that the upper electrode 204 is used to resistance weld a shape memory alloy SMA wire 202 to an autofocus prototype.

[0039] In some cases, static electrode systems have been shown to require a maximum of approximately 1000 welding cycles before electrode readjustment is necessary. Manually stepping the wire across the electrode (stepping wire) has the potential to significantly increase the number of welding cycles before readjustment is required. This embodiment may also relate to an automated electrode stepping method that enables mass production.

[0040] In such examples, this embodiment may include small incremental rotational movement of the electrode and small incremental translational movement of the electrode. Small incremental rotational movement of the electrode may be a viable approach for applications with round electrode tips. Translational movement of the electrode can dramatically improve wear performance and the number of cycles until electrode regrinding (resurfacing). Furthermore, a challenge when joining dissimilar materials such as shape memory alloy SMA wire and stainless steel in resistance welding applications may be achieving the appropriate thermal balance of both materials to achieve solid-state bonding. Optimal solid-state bonding can be achieved by using an appropriate welding recipe and electrode materials having appropriate resistance, hardness, and high-temperature properties, in addition to the upper and lower electrode shapes. Solid-state bonding can be achieved during the welding cycle when the weld interface temperature reaches approximately 80% of the melting points of both materials. At this temperature, there is little or no fusion, but interdiffusion of atoms between the two materials being joined may occur. Annealed tungsten electrode material can be used, which can result in significant improvements in other resistance welding processes.

[0041] Switching to tungsten electrode material for both the upper and lower electrodes reduces the "component-electrode" adhesion (sticking) problem and mitigates premature electrode cracking. Furthermore, reducing electrode wear led to the idea of ​​"micro-movement" in the translational direction. The electrodes may be crack-free, annealed, and eddy current tested, conforming to ANSI / AWS_A5.12, DIN_EN_ISO_6848, and JIS_Z_3233 standards.

[0042] <Design> The resistance weld joints used can connect shape memory alloy (SMA) wire to a stainless steel substrate both electrically and mechanically. Both the buckler autofocus prototype and the bimorph actuator prototype use resistance-welded shape memory alloy (SMA) wire. The shape memory alloy (SMA) wire and stainless steel are pressed together between opposing electrodes, and current is supplied through the electrodes to induce Joule heating. This heating can raise the temperatures of the steel and shape memory alloy (SMA) to near their respective melting points, and the pressure causes them to bond together. So far, the wire typically becomes wider due to flattening caused by the pressure of the electrodes.

[0043] Figures 3A and 3B show exemplary process flows for assembling an autofocus prototype, such as a buckler-type autofocus prototype, using shape memory alloy (SMA) wire welding. As shown in Figures 3A and 3B, a wire welding process 302 may be incorporated after the laser detab (tab removal) process and before the wire resistance welding check. In some embodiments, the wire welding process 302 may be performed by performing a resistance welding process between a metal actuator and a shape memory alloy (SMA) wire. The resistance welding machine is positioned between the first end of the shape memory alloy (SMA) wire and the metal actuator, such that the upper electrode is positioned adjacent to the shape memory alloy (SMA) wire and the second electrode of the resistance welding machine is positioned adjacent to the metal actuator. Here, the electrode material is a tungsten alloy with increased resistance compared to tungsten copper. The resistance welding machine performs a first resistance weld between the first end of the shape memory alloy (SMA) wire and the metal actuator by passing an electric current through the shape memory alloy (SMA) wire and the metal actuator. The resistance welding machine is positioned between the second end of the shape memory alloy SMA wire and the metal actuator, such that the upper electrode is positioned adjacent to the second end of the shape memory alloy SMA wire and the second electrode of the resistance welding machine is positioned adjacent to the second end of the metal actuator. The resistance welding machine then performs a second resistance weld between the second end of the shape memory alloy SMA wire and the metal actuator by passing current through the shape memory alloy SMA wire and the metal actuator. Here, each of the first and second resistance welds is performed with a welding pulse duration of approximately 9 milliseconds (ms).

[0044] Figure 4 shows that the bimorph actuator prototype 402 can use a shape memory alloy SMA wire 404 that is resistance-welded at both ends 406a and 406b. Figure 5 is a photograph showing that the resistance-welded joint connects the shape memory alloy SMA wire to the stainless steel substrate both electrically and mechanically. The wire may typically become wider due to flattening caused by the pressure of the electrodes.

[0045] Another welding machine design (welder design) used in the test may have limited capabilities. It has a coarse adjustment step size and a high minimum welding power for shape memory alloy SMA wires with diameters of 25 μm to 30 μm. A resistance welding machine with smaller power step sizes, lower total power, smaller electrode clamping force step sizes, and smaller time-to-time variability has been purchased and will be used to produce additional test samples.

[0046] <Exam> Weld strength and quality are evaluated in several ways. Tensile testing (pull testing) is used to test the shear strength of a weld. Peel testing involves peeling the wire from the steel and is another indicator of strength. Tensile and peel testing are performed until the sample breaks, and the mode of failure in both tests is recorded. Typical modes of failure in weld strength testing are fracture at the interface between the two members being welded (weld fracture) or fracture of the material being welded itself (the latter indicating that the weld is stronger than the materials being joined).

[0047] Figure 6 is a graph showing examples of weld pull strength in the range of 91–99 grams, indicating that the failure mode in all cases is within the wire material. Figure 7 is a graph showing examples of weld peel strength in the range of 20–26 grams, indicating that the failure mode in all cases is within the wire material.

[0048] To understand whether the condition of the steel affects weld quality, several steel conditions are tested, using standard 302 stainless steel material. Functional testing is performed by subjecting coupon parts to cycle endurance testing. Weld structures are evaluated by using focused ion beam (FIB) to cut out cross-sections of the weld, as shown in Figure 8, and by observing the two welded materials together with a scanning electron microscope. Several conditions for shape memory alloy (SMA) wire and stainless steel are considered to understand whether there are conditions that promote or inhibit material bonding.

[0049] <Result> A baseline welding process can be established by building multiple coupons made of 0.004-inch (0.1016 mm) thick 302 stainless steel with different welding energies and then performing peel tests on them. Weld strength plateaus at 25-35 grams for a pulse duration of 10 milliseconds with welding energies ranging from 2.2 to 4 watt-seconds (technically welding work in watt-seconds). A baseline setting of 2.4 watt-seconds can be selected.

[0050] Several conditions of steel, including standard 302 stainless steel material, can be tested at a baseline setting of 2.4 watt-seconds. The weld tensile strength ranges from 91 to 99 grams, with the failure mode in all cases being within the wire material and breaking at the edge of the weld. The weld peel strength ranges from 20 to 26 grams, with the failure mode in all cases being within the wire material and breaking at the edge of the weld. In both strength tests, the failure mode is within the shape memory alloy SMA wire, meaning that the condition of the steel surface is not a factor in these results. Tensile strength and peel strength can be determined based on the strength and condition of the shape memory alloy SMA wire at the edge of the weld.

[0051] Weld strength tests may include samples with a failure mode in the weld, but in all cases the failure mode was fracture of the shape memory alloy SMA wire. Since weld strength is a function of welding energy, it is clear that it can be reduced. Resistance welding machines with smaller step sizes in power, lower total power, smaller step sizes in electrode clamping force, and smaller time variability can be purchased and used to prepare additional test samples.

[0052] Weld structures can be evaluated by cross-sectionalizing the weld using focused ion beam (FIB) and observing the two materials being welded together with a scanning electron microscope (SEM). Focused ion beam (FIB) cross-section samples of the weld interface between shape memory alloy (SMA) wire and stainless steel are shown in Figures 9A to 9D. Roughening or polishing the surface (Figures 9A and 9B, respectively) does not affect the weld interface. Oxide films (Figures 9C, 9D, and 9F) on the steel or shape memory alloy (SMA) wire did not inhibit welding. Nickel plating on steel (Figure 9E) may be expected to increase fusion with nickel in the shape memory alloy (SMA) wire, but in all cases, regardless of material conditions, no material fusion is observed in the images. Based on the high strength of the weld (the wire breaks, not the weld, in peel tests), strong solid-state welding can result from diffusion and bonding at the atomic level.

[0053] A resistance welding machine may be installed and used to prepare resistance welding test samples. This welding machine may have a smaller step size in power, lower total power, a smaller step size in electrode clamping force, and lower time variability than the welding machine being replaced. Encoder readings and manual operation stages may be added to improve consistency of welding position.

[0054] Functional testing can be performed by subjecting coupon components to cycle endurance testing. The results show that no welds fractured during the cycle endurance testing. Half of the tested components survived 1 million cycles and were stopped there. Half of the components showed early fracture related to stress loading in the wire (fracture (failure) before 20,000 cycles) (stress-related fracture is outside the scope of the shape memory alloy SMA wire welding project and is addressed by a separate project).

[0055] Figure 8 shows the evaluated weld structure obtained by cross-sectionalizing the weld using focused ion beam (FIB). Figures 9A to 9F show focused ion beam (FIB) cut samples of the weld interface between a shape memory alloy (SMA) wire and stainless steel, as imaged by a scanning electron microscope (SEM). In all cases, no material fusion is observed in the images. Based on the high strength of the weld (in peel tests, the wire breaks, not the weld), diffusion and bonding at the atomic level are present, resulting in a strong solid-state weld.

[0056] Figure 9A shows that surface roughening or polishing (Figures 9A and 9B, respectively) does not affect the weld interface. Furthermore, Figure 9C shows that oxide films on steel or shape memory alloy SMA wires (Figures 9C, 9D, and 9F) did not inhibit welding.

[0057] The purpose of welding shape memory alloy (SMA) wires may be to create a strong mechanical connection while minimizing the impact on the wire material. Dynamic mechanical analysis can be performed to measure the effect of heat on the mechanical properties of shape memory alloy (SMA) wires. Two samples can be compared: (1) a shape memory alloy (SMA) wire in its as-received state, and (2) a shape memory alloy (SMA) wire annealed by applying an electric current until it glows red in about 1 second. It is reasonable to assume that a resistance-welded wire will have material properties that fall between these two samples. Mechanical analysis of a sample of shape memory alloy (SMA) wire welded to steel coupons at both ends showed behavior closer to the "as-received" wire than to the red-hot annealed wire.

[0058] Figure 10 shows that a resistance welding machine is installed and used to prepare resistance welding test samples. Figure 11 is an illustrative chart showing that no welds fractured during the cycle endurance test. The tested parts survived 1 million cycles before being stopped.

[0059] Figure 12 is an illustrative chart showing that the mechanical analysis of a sample of shape memory alloy (SMA) wire welded to steel at both ends was tested, and that it exhibited behavior closer to that of a “received” wire than to a red-hot annealed wire. For reference, the average tensile strengths of samples welded with short and long pulses can be superimposed on the stress-strain graph.

[0060] The tensile strength of welded wire can be compared to that of shape memory alloy SMA wire in its "accepted state" (i.e., unwelded shape memory alloy SMA wire directly from the wire spool as shipped from the supplier). Two conditions are compared for shape memory alloy SMA wire welded to a 30 μm thick 302 / 304 stainless steel substrate: (1) short pulse and (2) long pulse. The tensile test results show that short welding pulses (average tensile strength 1,323 MPa) more closely match the strength of the "accepted state" shape memory alloy SMA wire (1,463 MPa) than samples prepared with long pulses (average tensile strength 1,148 MPa). Due to the lower tensile strength, longer pulses may have a greater impact on the metallurgical properties of the shape memory alloy SMA wire than short pulses.

[0061] Figure 13 is an illustrative graph showing an example of tensile test results demonstrating that short-duration welding pulses more closely match the strength of the "accepted" shape memory alloy SMA wire than samples prepared with long-duration pulses. Because the tensile strength of long-duration pulses is lower, longer pulses can have a greater impact on the metallurgical properties of the shape memory alloy SMA wire than shorter pulses.

[0062] Scanning electron microscope (SEM) images of shape memory alloy (SMA) material in unaffected and welded regions can be compared. This analysis aims to compare grain structures to better understand the effect of resistance welding on shape memory alloy (SMA) wires. A focused ion beam (FIB) is used to cut the material to show a cross-section of the shape memory alloy (SMA) wire. The SEM images show that the grain structure of the shape memory alloy (SMA) wire is very small, less than 100 nm. Images of the wire at 30kx magnification (both unaffected and welded regions) do not allow for the identification of grain boundaries in the shape memory alloy (SMA) material. These SEM images do not allow for conclusions regarding similarities or differences between the unaffected wire and the resistance-welded wire.

[0063] Figure 14 shows that scanning electron microscope (SEM) images of the shape memory alloy (SMA) wire demonstrate that the grain structure of the wire is very small, and can be less than 100 nm. Images of the wire in both the unaffected and welded regions show that grain boundaries of the shape memory alloy (SMA) material cannot be identified at 30kx magnification.

[0064] During the development process, electrode adhesion (sticking) can become a significant problem. Newly dressed upper electrodes begin to adhere to the shape memory alloy (SMA) wire within the first or second hit. Since the shape memory alloy (SMA) wire can be welded to thin stainless steel foil with a thickness of only 30 μm, separating the SMA wire from the adhered electrode can bend and damage the component.

[0065] From Equation 1, the heat generated in resistance welding can be proportional to the resistance of the electrical path through the electrode and the workpiece. See Figure 15. If the resistance (R1) at the interface between the electrode and the workpiece is too high, an unintended amount of heat will be generated, causing the electrode to stick.

[0066] Thermal balance can be an important consideration. The upper electrode can decrease in size over time due to repeated resurfacing (regrinding) and shaping of the electrode. This can increase the resistance through the upper electrode (R4 and R1) and the resistance between the upper electrode and the workpiece, leading to increased heat in that region and consequently causing electrode adhesion.

[0067] This approach was adopted to redistribute the system's heat. The resistance of the lower electrode (R6) was increased by changing the electrode material from tungsten copper to a tungsten alloy with higher resistance. This improved the balance of heat generation between the two electrode interfaces, thus resolving the adhesion problem at the upper electrode.

[0068] Figure 15 shows that if the resistance (R1) at the interface between the electrode (R4) and the workpiece is too high, an unintended amount of heat is generated, causing the electrode to adhere. This improves the balance of heat generation between the two electrode interfaces, resolving the adhesion problem at the upper electrode (R4). Depending on the case, various specifications for tungsten electrodes for arc welding and cutting, as well as oxide-dispersed tungsten electrodes, may be used.

[0069] A short-time pulse welding recipe using a high-resistivity lower electrode can be a reference resistance welding process for attaching shape memory alloy (SMA) wire to stainless steel. Many wire attachment machines feature automated functions for part handling, shape memory alloy (SMA) wire feeding, and shape memory alloy (SMA) wire crimping. In one initial concept, many of the part handling and shape memory alloy (SMA) wire feeding functions in the wire attachment machine are maintained. The crimping die set (currently coining by closing the wire crimping section to attach the shape memory alloy (SMA) wire) could be replaced with an electronically controlled low-pressure resistance welding head with upper and lower electrodes. Work is continuing on integrated concepts for resistance welding.

[0070] Figure 16 shows that many wire attachment machines have automated functions for feeding shape memory alloy SMA wire and crimping shape memory alloy SMA wire. The crimping die set can be replaced with a resistance welding head. As shown in Figure 16, the shape memory alloy SMA wire crimping process may include a shape memory alloy SMA wire crimping section 1602. Furthermore, the shape memory alloy SMA wire crimping or shape memory alloy SMA wire welding process may include a shape memory alloy SMA wire feeding capillary 1604 and a shape memory alloy SMA wire welding concept 1606 for replacing the wire crimping section.

[0071] Figure 17 shows that a crimping die set (currently coining by closing the wire crimping section to attach a shape memory alloy SMA wire) can be replaced with an electronically controlled low-pressure resistance welding head. In Figure 17, the wire spool and tension control component 1702 and the electronically controlled low-pressure resistance welding head 1704 can be incorporated into the system described herein.

[0072] Figure 18 shows that a crimping die set (currently coining by closing the wire crimping section to attach a shape memory alloy SMA wire) can be replaced by an electronically controlled low-pressure resistance welding head. As shown in Figure 18, the shape memory alloy SMA wire welding concept may include an electronic low-pressure welding head 1802, a wire handling component 1804, and a part handling component 1806.

[0073] Figure 19 shows that a crimping die set, currently used to coin a shape memory alloy (SMA) wire by closing the wire crimping section, can be replaced with a wire welding electrode and a wire cutting punch. The same wire attachment machine capillary for wire feeding can be retained. As shown in Figure 19, an ultrasonic welding additive manufacturing (UWAM) capillary 1902 for wire feeding and an ultrasonic welding additive manufacturing UWAM wire crimping punch 1904 can be used in the system described herein. Furthermore, an ultrasonic welding additive manufacturing UWAM capillary 1906 for wire feeding, a wire welding electrode, and a wire cutting component 1908 can be used in such a system.

[0074] The precision stage is equipped with a single electrode that welds both ends of the wire. By moving from a first position to a second position, the need for a second welding head can be eliminated. Once the wire is welded to the workpiece at both ends, a cutting punch on the precision stage moves downward to cut the wire. The rotary stage of the original machine design is retained, and the welding and cutting cycle of the shape memory alloy SMA wire is repeated by rotating the workpiece a quarter turn. After the wire has been attached to all four sides of the workpiece by resistance welding, many parts handling automations can swap the finished part with the next part to be processed.

[0075] Figure 20 shows that the precision stage is equipped with a single electrode for welding both ends of the wire. By moving from the first position to the second position, the need for a second welding head is eliminated. In Figure 20, an upper electrode and a lower electrode 2000 for resistance welding and an ultrasonic welding additive manufacturing UWAM capillary 2002 for wire feeding may be used in this system.

[0076] Figure 21 shows that the precision stage is equipped with a single electrode for welding both ends of the wire. By moving from the first position to the second position, the need for a second welding head can be eliminated. In Figure 21, an upper electrode and a lower electrode 2100 for resistance welding and an ultrasonic welding additive manufacturing UWAM capillary 2102 for wire feeding may be used in this system.

[0077] Figure 22 shows that when the wire is welded to the workpiece at both ends, a cutting punch on a precision stage moves downward to cut the wire. In Figure 22, a wire cutting punch 2200, upper and lower electrodes 2202, a wire spool 2204, and a wire cutting punch 2206 may be incorporated into the system.

[0078] <Materials science testing> In the smartphone camera industry, high reliability and long lifespan are expected. The effect of resistance welding on the material properties of shape memory alloy (SMA) wire has been studied, ensuring that the shape memory alloy (SMA) wire maintains its reliability even when resistance welded. Because shape memory alloy (SMA) wire is extremely small, with a diameter of 30 μm, nanoindentation is used to obtain its material properties.

[0079] Samples of welded wire are cross-sectioned and polished. The nanomechanical testing system allows for the application of a 2,000 μN load to the nanoindent sample while a scanning probe microscope (SPM) simultaneously measures the depth of the nanoindents. The load and depth are used to calculate the nanohardness H and the reduced modulus Er. Young's modulus can be calculated from the reduced modulus Er through a relationship based on Poisson's ratio.

[0080] Figures 23A and 23B show (a) scanning probe microscope (SPM) images of nanoindents in the cross-section of a welded shape memory alloy (SMA) wire, and (b) a welded shape memory alloy (SMA) wire with the cross-sectional area and nanoindents indicated by white frames.

[0081] The nanohardness H of the wire in its as-received state is 4.249 GPa. When the wire is clamped with electrodes but no welding current is applied, cold working increases it to 4.875 GPa. The heat generated when welding current is applied has an offsetting effect on cold working, so it softens the wire to 4.286 GPa. Resistance welding of shape memory alloy SMA wire can result in a net result where the nanohardness of the shape memory alloy SMA wire is virtually unchanged.

[0082] Figure 24 is an illustrative graph showing that the nanohardness H of the wire in its acceptance state is 4.249 GPa, and that it increases with cold working when the wire is clamped with electrodes but no welding current is applied. The heat generated when welding current is applied has an anti-inflammatory effect on cold working, softening the wire, so the net result is that the nanohardness hardly changes.

[0083] Figure 25 is an illustrative graph showing that the Young's modulus E of the wire in its acceptance state is 67.711 GPa. When the wire is clamped with electrodes but no welding current is applied, the modulus can increase to 76.986 GPa. When the wire is clamped and welded, the modulus is 99.386 GPa. The modulus does not typically increase with cold working (clamping) or heating (resistance welding). Austenite-phase shape memory alloy SMA has a significantly higher modulus than martensite. Therefore, it is suggested that cold working and heating resulting from resistance welding of the wire cause the wire to transition to a higher proportion of austenite, resulting in a higher modulus. Shape memory alloy SMA wires used in smartphone actuators likely start with a certain proportion of austenite due to the drawing and training processes used in wire manufacturing. This may explain why the modulus of elasticity of the wire in its accepted state is 67.711 GPa, and not in the range of 28-41 GPa for nitinol in its fully martensite state (see reference data in "Table 4").

[0084] [Table 1]

[0085] Nanoindentation demonstrates that resistance welding of shape memory alloy (SMA) affects material properties. The modulus of elasticity of the shape memory alloy (SMA) wire within the weld is 40% higher than that of the wire in its acceptance state. Based on durability test results (see Figure 12), this increase is considered acceptable, and the durability tests indicate that the wire weld is reliable in its application. These nanoindentation results serve as a benchmark for comparing future nanoindentation results and predicting the cycle durability performance when wire welds are applied to designs.

[0086] Nanoindentation can be used as a limit test to demonstrate robustness against processing variations, measuring nanohardness and modulus in multiple samples welded under different settings. Low nominal, high welding current (amperage) settings are used, and nanohardness and Young's modulus are obtained by preparing nanoindented samples. The results in Figure 26 show that the material properties are similar at low nominal, high welding energy, and that wire welding is tolerant of mass-production level variations in welding machines.

[0087] Figure 26 shows an exemplary graph demonstrating that wire welding is tolerant of mass-production level variations in welding machines because material properties can be similar at low nominal, high welding energies.

[0088] Figures 27A to 27C show (a) a shape memory alloy SMA optical image stabilization OIS platform development component used to test the reliability of shape memory alloy SMA wire welding, (b) a shape memory alloy SMA optical image stabilization OIS platform development component equipped with shape memory alloy SMA wire welding, and (c) the shape memory alloy SMA wire weld.

[0089] Impact resistance is evaluated in free-fall drop tests. Two optical image stabilization (OIS) actuators are placed on a simulated smartphone casing, and the device is dropped from a height of 1.6m onto a concrete floor for a total of 24 drops, four times for each edge and both sides.

[0090] Figure 28 shows that two optical image stabilization (OIS) actuators are placed in a simulated smartphone casing and then dropped onto a concrete floor from a height of 1.6 meters.

[0091] Resistance-welded wires remain undamaged and pass the drop test. Passing is indicated by a resistance of 26–28 ohms for the shape memory alloy (SMA) wire. Failing components indicate that one or more wire circuits are open, resulting in breakage of one of the wires.

[0092] Figures 29A to 29H (Figure 29) show that the shape memory alloy SMA wire welds on the optical image stabilization (OIS) platform development components pass drop tests. Figures 30A to 30D show that shape memory alloy SMA wire welding is progressing toward applications in products featuring (a) HTI_Bimorph® lens-shift optical image stabilization (OIS), (b) buckler autofocus (AF), (c) development of an optical image stabilization (OIS) long wire platform, and (d) bimorph sensor-shift optical image stabilization (OIS).

[0093] Resistance welding of shape memory alloy (SMA) wire can be a reliable solution for attaching shape memory alloy (SMA) wire to the actuators of camera modules. A resistance welding machine with improved capabilities can be deployed. A reference resistance welding process can be established. The shape memory alloy (SMA) wire weld has been shown to be robust in tensile tests and reliable for 1 million cycles in life tests. Electrode adhesion issues were resolved by changing the lower electrode material. Dynamic mechanical analysis shows that shape memory alloy (SMA) wire welded to steel coupons behaves more closely to "as-received" wire than red-hot annealed wire. Mechanical properties of the resistance-welded shape memory alloy (SMA) wire are measured using nanoindentation and scanning probe microscopy to provide reference hardness and Young's modulus for comparison with future applications. Passing cycle endurance tests (life cycle tests) and drop impact tests demonstrate the reliability of the welded shape memory alloy (SMA) wire in applications.

[0094] Figures 31A and 31B show the dimensions of the tested upper electrode. Figure 32 shows the dimensions of the tested lower electrode. Figure 33 shows an example of an electrode with burn-in depth after 1500 welding cycles in a stationary position.

[0095] Figures 34A to 34D show examples of static welding (static wells). Figures 35A to 35B show an example of 1000 cycles. Figures 36A to 36C show an example of 1500 cycles.

[0096] Figure 37 shows an example of stepped welding with a deviation of 4 μm across the entire electrode surface after 1000 cycles. Figures 38A to 38C show examples of stepped welding from 1 to 1000 cycles. Figures 39A to 39C show examples of measurements of stepped welding.

[0097] Figure 40 shows an exemplary set of welding locations as part of resistance welding techniques. As shown in Figure 40, the shape memory alloy SMA wires may be positioned at different welding locations relative to the upper electrode 4002, the shape memory alloy SMA wire 4004, the second electrode 4008, and the metal actuator 4006.

[0098] In some cases, electrodes may be positioned adjacent to the previous welding position, approximately 4 micrometers away, to further equalize electrode wear by dispersing wear of the electrode material during the welding cycle, and to increase the number of cycles between stops for resurfacing (regrinding) the electrode surface. The electrode positioning may be repeated to another adjacent position, approximately 4 micrometers away from the previous welding position and approximately 8 micrometers away from the initial welding position.

[0099] Figure 41 shows an exemplary set of rotatable welding positions. For example, in Figure 41, the electrodes can be rotated at each welding position. The upper electrode 4102 may be positioned adjacent to the shape memory alloy SMA wire 4104, and the second electrode of the resistance welder may be positioned adjacent to the metal actuator 4106. To further equalize electrode wear by distributing the wear of the electrode material during the welding cycle, and to increase the number of cycles between stops for resurfacing (regrinding) the electrode surface, the electrodes may be positioned adjacent to the previous welding position, rotated by approximately 10 degrees. The electrode positioning may be repeated to another adjacent position, which is approximately 10 degrees from the previous welding position and approximately 20 degrees from the initial welding position.

[0100] In some cases, a method is provided for performing a resistance welding process between a metal actuator and a shape memory alloy (SMA) wire. This method may include the step of positioning a resistance welding machine toward the first end of the shape memory alloy SMA wire and the first end of the metal actuator such that the upper electrode is positioned adjacent to the shape memory alloy SMA wire and the second electrode of the resistance welding machine is positioned adjacent to the metal actuator. The electrode material may include a tungsten alloy with increased resistance compared to tungsten copper.

[0101] The method may also include the step of performing a first resistance weld between the first end of the shape memory alloy SMA wire and the metal actuator by passing an electric current through the shape memory alloy SMA wire and the metal actuator using a resistance welding machine. The method may also include the step of positioning the resistance welding machine at the second end of the shape memory alloy SMA wire and the second end of the metal actuator such that the upper electrode is positioned adjacent to the second end of the shape memory alloy SMA wire and the second electrode of the resistance welding machine is positioned adjacent to the second end of the metal actuator. The method may also include the step of performing a second resistance weld between the second end of the shape memory alloy SMA wire and the metal actuator by passing an electric current through the shape memory alloy SMA wire and the metal actuator using a resistance welding machine. Here, each of the first and second resistance welds is performed with a welding pulse duration of approximately 9 milliseconds (ms).

[0102] In some cases, the process of performing each of the first and second resistance welds includes a 2-millisecond ramp-up period and a 2-millisecond ramp-down period during the duration of each weld pulse. Here, the pulse current for each of the first and second resistance welds is approximately 110 amperes.

[0103] In some cases, metal actuators are made of stainless steel. In some cases, resistance welding machines feature a power step size of approximately 0.001 watt-seconds (W·s), lower total power, pulse time step sizes of approximately 0.1 milliseconds and 1 millisecond, electrode clamping force step sizes of approximately 0.1 pounds (lbs) (approximately 45.36 grams), and smaller time-to-time variability.

[0104] In some cases, the method may further include the steps of: obtaining a set of coupons having a thickness of 0.004 inches (0.1016 mm) of 302 stainless steel; performing resistance welding on each coupon to a shape memory alloy SMA wire such that the pulse duration is approximately 10 milliseconds, the welding energy is in the range of 2.2 to 4 watt-seconds, and the welding strength is in the range of 25 to 35 grams; and testing one or more conditions of the set of coupons to determine the weld peel strength between the wire attached to each coupon and the coupon.

[0105] In some cases, the process of testing one or more conditions further includes a step of creating a cross-section of the weld on each coupon using a focused ion beam (FIB). In some cases, the method may further include a step of performing nanoindentation on shape memory alloy SMA wires to measure the nanohardness and modulus of multiple samples welded under different settings.

[0106] In some cases, metal actuators are part of an optical image stabilization (OIS) system. In some cases, the upper electrode is positioned adjacent to the shape memory alloy SMA wire, and the second electrode of the resistance welding machine is positioned adjacent to the metal actuator. Here, the electrodes are positioned adjacent to the previous welding position, approximately 4 micrometers away, in order to further equalize electrode wear by distributing the wear of the electrode material during the welding cycle, and to increase the number of cycles between stops for resurfacing (regrinding) the electrode surface. Here, the electrode positioning is repeated to another adjacent position, approximately 4 micrometers away from the previous welding position and approximately 8 micrometers away from the first welding position.

[0107] In some cases, the upper electrode is positioned adjacent to the shape memory alloy SMA wire, and the second electrode of the resistance welding machine is positioned adjacent to the metal actuator. Here, the electrodes are positioned adjacent to the previous welding position, rotated approximately 10 degrees from the previous welding position, in order to further equalize electrode wear by distributing the wear of the electrode material during the welding cycle, and to increase the number of cycles between stops for resurfacing (regrinding) the electrode surface. This electrode positioning is then repeated to another adjacent position, which is approximately 10 degrees from the previous welding position and approximately 20 degrees from the initial welding position.

[0108] Those skilled in the art will understand that other dimensions and materials may also be used to satisfy the desired design characteristics. According to some embodiments, the processes described herein are used to form one or more of any mechanical and electromechanical structures.

[0109] While these embodiments have been described in relation to the present invention, those skilled in the art will recognize that modifications can be made in form and detail without departing from the spirit and scope of the invention.

Claims

1. A method for performing resistance welding between a metal actuator and a shape memory alloy SMA wire, wherein the method is A step of positioning a resistance welding machine to the first end of the shape memory alloy SMA wire and the metal actuator, wherein the upper electrode is positioned adjacent to the shape memory alloy SMA wire, the second electrode of the resistance welding machine is positioned adjacent to the metal actuator, and the electrode material is a tungsten alloy with increased resistance compared to tungsten copper, and the step of positioning the resistance welding machine to the first end, The process involves performing a first resistance welding between the first end of the shape memory alloy SMA wire and the metal actuator by passing an electric current through the shape memory alloy SMA wire and the metal actuator using the resistance welding machine, A step of positioning the resistance welding machine to the second end of the shape memory alloy SMA wire and the metal actuator, wherein the upper electrode is positioned adjacent to the second end of the shape memory alloy SMA wire, and the second electrode of the resistance welding machine is positioned adjacent to the second end of the metal actuator, and The process involves performing a second resistance welding between the second end of the shape memory alloy SMA wire and the metal actuator by passing an electric current through the shape memory alloy SMA wire and the metal actuator using the resistance welding machine, It is equipped with, Each of the first resistance weld and the second resistance weld is performed with a welding pulse duration of approximately 9 milliseconds (ms). method.

2. The process of performing the first resistance welding and the second resistance welding includes a ramp-up period of 2 milliseconds and a ramp-down period of 2 milliseconds during the duration of each welding pulse. The pulse current for each of the first and second resistance welds is approximately 110 amperes. The method according to claim 1.

3. The metal actuator includes stainless steel. The method according to claim 1.

4. The aforementioned resistance welding machine is With an output step size of approximately 0.001 watt-seconds (W·s), Lower total output and Pulse time step sizes of approximately 0.1 milliseconds and approximately 1 millisecond, The step size of the electrode clamping force is approximately 0.1 pounds (lbs) (approximately 45.36 grams), and Smaller time-interval variability and The method according to claim 1, comprising:

5. The above method further, The process of obtaining a set of coupons made of 302 stainless steel with a thickness of 0.004 inches (0.1016 mm), A step of performing resistance welding on the shape memory alloy SMA wire for each of the coupons such that the welding pulse duration is approximately 10 milliseconds, the welding energy is in the range of 2.2 to 4 watt-seconds, and the welding strength is in the range of 25 to 35 grams, and A step of testing one or more conditions of the set of coupons in order to determine the weld peel strength between the shape memory alloy SMA wire attached to each of the coupons and the coupon, It is equipped with The method according to claim 1.

6. The step of testing one or more of the above conditions further includes: The process includes a step of creating a cross-section of the weld on each of the coupons using a focused ion beam (FIB). The method according to claim 5.

7. The method further comprises a step of performing nanoindentation on the shape memory alloy SMA wire in order to measure the nanohardness and elastic modulus of a plurality of samples welded with different settings. The method according to claim 1.

8. The aforementioned metal actuator is part of an optical image stabilization system. The method according to claim 1.

9. The upper electrode is positioned adjacent to the shape memory alloy SMA wire, The second electrode of the resistance welding machine is positioned adjacent to the metal actuator. To further uniformize electrode wear by dispersing the wear of the electrode material during the welding cycle, and to increase the number of cycles between stops for resurfacing the electrode surface, the electrodes are positioned adjacent to the previous welding position, approximately 4 micrometers away. The electrode placement is repeated at another adjacent location, approximately 4 micrometers from the previous welding position and approximately 8 micrometers from the initial welding position. The method according to claim 1.

10. The upper electrode is positioned adjacent to the shape memory alloy SMA wire, The second electrode of the resistance welding machine is positioned adjacent to the metal actuator. To further uniformize electrode wear by dispersing the wear of the electrode material during the welding cycle, and to increase the number of cycles between stops for resurfacing the electrode surface, the electrodes are positioned adjacent to the previous welding position, rotated approximately 10 degrees from the previous welding position. The electrode placement is repeated to another adjacent position, approximately 10 degrees from the previous welding position and approximately 20 degrees from the initial welding position. The method according to claim 1.

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