Methods for resistance spot welding of components II

Abstract

Method for resistance spot welding of components (110, 120), wherein - in a first step at the welding point (S) an electric current (I1) is passed through the components (110, 120) by means of electrodes arranged on both sides of the components (110, 120) and thereby a substantially martensitic spot weld (P) is produced; and - in a second step at the same welding point (S) an electric current (I2) is passed through the components (110, 120) by means of electrodes (210) arranged on both sides of the components (110, 120), whereby the martensitic structure of the previously produced spot weld (P) is austenitized and, after switching off the electric current (I2), is converted into neumartensite (N), wherein the neumartensite (N) forms a neumartensite island (A) which is enclosed by a martensite (M) produced in the first step and tempered in the second step.

Description

[0001] The invention relates to a method for resistance spot welding of components according to claim 1.

[0002] In resistance spot welding, the components to be welded are pressed together to a greater or lesser degree at a welding or joining point using electrodes and heated by an electric current passed over the electrodes until they melt. After the current flow stops, a spot-shaped weld forms at the welding point, which is also called a spot weld or simply a weld point.

[0003] German patent DE 10 2012 018 098 A1 describes a method for resistance welding components in which the components are welded together by means of a welding current applied via the electrodes, forming a weld nugget. The welding current has a variable time profile characterized by alternating high-current phases and low-current or current-free cooling and hardening phases, resulting in a weld nugget composed of several layered and / or adjacent molten areas. This method proves particularly advantageous for the resistance welding of press-hardened sheet metal components (with a martensitic microstructure), as it produces a weld joint with high strength and good ductility (elongation properties).

[0004] The invention is based on the objective of providing a further method for resistance spot welding with which high joint strengths can be achieved.

[0005] The problem is solved by the method according to the invention with the features of claim 1. Advantageous further developments and embodiments of the method according to the invention are shown in the dependent claims, the following description of the invention (this expressly includes features described by way of example and as optional) and the figures.

[0006] The inventive method for resistance spot welding of components, whereby at least two components are meant, provides that - in a first step, an electric current is passed through the components at the welding point by means of electrodes arranged on both sides of the components, thereby creating (between the components) an essentially martensitic spot weld; and - in a second step, an electric current is passed through the components at the same welding point by means of electrodes arranged on both sides of the components, whereby the martensitic structure of the previously created spot weld is at least partially austenitized and, after switching off the electric current or after the end of the current-energizing phase, is converted into neumartensite, wherein the neumartensite forms a neumartensite island which is enclosed, at least partially enclosed and, in particular, completely enclosed by a martensite produced in the first step and tempered in the second step.

[0007] Preferably, the electrodes are positioned opposite the components at the welding point and are typically subjected to an electrode force, optionally a varying electrode force. It is preferred that the same electrodes are used for both the first and second steps, and that these electrodes, in particular, continuously contact or are placed on the components. In principle, different electrodes can also be used in the first and second steps, differing in their shape or geometry (i.e., having different electrode shapes or geometries), which may even be advantageous.

[0008] In the first step, a spot weld is created between the components by resistance heating in a known manner. This spot weld has, in particular, a weld nugget, or, in the case of more than two components, optionally several weld nuggets. Preferably, only one current-energizing phase is provided in the first step. It is preferred that the weld nugget be as large as possible, in particular the largest possible size. During the cooling phase following the resistance heating, a martensitic microstructure with very high hardness and / or strength forms in the spot weld or weld nugget; that is, the spot weld or weld nugget has a substantially martensitic microstructure. A martensitic microstructure can also form, at least partially, in the so-called heat-affected zone (HAZ). (The heat-affected zone is the area around the spot weld or weld nugget that undergoes a structural change in the first step of resistance spot welding.)

[0009] In the second step provided for in the invention, the martensitic microstructure produced at the weld point in the first step is heat-treated, also by resistance heating. Preferably, only one current application phase is provided in the second step. The resistance heating propagates from the center of the spot weld or weld nugget. With a suitable ratio of current intensity (where a varying current intensity can also be provided) and pulse duration (time length of the current application phase), austenitization of the microstructure occurs from the center of the spot weld or weld nugget, which can also be referred to as re-austenitization. The extent of the re-austenitization depends in particular on the current intensity and pulse duration. During the heat treatment or re-austenitization, the temperature at the weld point or weld nugget is...In the relevant area, the temperature is preferably equal to or greater than the AC1 temperature, or preferably equal to or greater than the AC3 temperature, of the components or at least one of the components. Preferably, however, the temperature is lower than the melting point of the components or at least one of the components. During the second heat treatment step, the temperature can locally exceed the melting point, allowing at least one area of ​​molten and resolidified neumartensite to form within the neumartensite or within the neumartensite island. During the heat treatment, the surrounding martensite area, which remains below the AC1 temperature, is tempered. Upon subsequent cooling following the resistance heating, an island-like or core-like area of ​​neumartensite (referred to as the neumartensite island or core) forms from the neumartensite, this area being surrounded or enclosed by tempered martensite.In particular, the neumartensite island is smaller than the spot weld or weld lens produced in the first step.

[0010] Thus, during the previously described heat treatment of the second step, defined microstructure regions form at the weld joint. This heat treatment is facilitated by a suitable electrode shape (or electrode cap shape, su). Suitable settings for current, pulse duration, and / or electrode force (where a varying electrode force can also be provided), as well as suitable electrode shapes, can be determined by calculation or simulation and / or tests (e.g., using micrographs and, in particular, a so-called hardness mapping). For the second step, the residual heat from the first step can also be taken into account or specifically adjusted (su).

[0011] In both the first and second steps, active cooling of the weld area can occur after the electric current is switched off or the current-energizing phase has ended. This can be achieved, in particular, using existing electrode or electrode cap cooling. In the first step, this can be used for rapid cooling and the formation of the martensitic microstructure and / or for the targeted control of residual heat. In the second step, this can be used for the rapid cooling of the new austenite and its conversion to new martensite. Regardless, temperature monitoring or control can also be implemented, especially in the second step.

[0012] Tests conducted by the applicant have shown that the heat treatment carried out in the second step according to the invention significantly increases the joint strength between the components. In particular, a marked increase in the tensile strength at the weld end is observed. This applies especially to components with martensitic microstructure properties and particularly to components with a martensitic microstructure (su). A significant increase in shear strength was also observed. Firstly, the number of weld points required for a given component joint can be reduced. Secondly, cost and / or weight savings are possible through the use of metal materials with which high joint strengths have not previously been achievable in resistance spot welding.The method according to the invention is particularly suitable for vehicle manufacturing, especially for body construction. Furthermore, the invention can be implemented with existing welding equipment without complex modifications. In addition, the invention is also very well suited for series production.

[0013] The hardness and / or strength of the tempered martensite surrounding or enclosing the neumartensite island can be at most 80%, preferably at most 85%, and particularly at most 90% of the strength of the neumartensite of the neumartensite island (this refers to average values). It is also preferred that the neumartensite island be spaced at least 0.4 mm, preferably at least 0.5 mm, away from the joint gap (meaning the gap between the components surrounding the weld). It is further preferred that the area of ​​tempered martensite enclosing the neumartensite island extends beyond the joint gap and / or reaches the surface of at least one of the components. These features have proven to be very advantageous with regard to joint strength, both individually and in combination.

[0014] Residual heat remaining at the weld point after the first step can be taken into account during the heat treatment in the second step. Preferably, residual heat present at the weld point at the beginning of the heat treatment, i.e., at the beginning of the current application phase in the second step, is adjusted, i.e., specifically set to a certain temperature value or range. This can be achieved by setting a pause between the first step (or current application phase) and the second step (or current application phase), during which active cooling of the weld point takes place. This enables cycle time reductions and / or energy savings. Furthermore, this can promote the formation of defined microstructures at the weld point, since the residual heat influences the current flow, particularly at the beginning of the current application phase.

[0015] Prior to the first step, a pre-current phase can be performed, ideally using the same electrodes as those used for the first step. During this phase, an electric current is passed through the components using electrodes positioned on both sides (this can also be referred to as a pre-pulse). This serves primarily to condition the weld area and reduce spatter formation. The heat input should be kept to a minimum. After the pre-current phase, a pause is preferably provided before the current phase of the first step begins. This pause is intended to serve as a cooling pause, and active cooling can also be employed. Any residual heat present before the first step or at the start of the current phase in the first step can be taken into account and, in particular, specifically adjusted (analogous to the preceding explanations).

[0016] The electrodes may have electrode caps, as is generally known from the prior art. The preceding and following explanations apply analogously to electrodes with electrode caps and electrodes without electrode caps.

[0017] The electrodes used in the first and second steps, or at least in the second step, are preferably so-called type F electrodes or electrodes with a type F cap. Electrode caps for resistance spot welding are classified into different types in DIN EN ISO 5821 (formerly DIN 44750), which differ in terms of shape and electrode geometry. Type F electrodes or caps have a spherical shape, possibly with a flattened section; that is, the working surface is spherical and may have a flattened end.

[0018] The components to be welded are primarily steel components. At least one of the components may exhibit martensitic microstructure properties. Preferably, at least one of the components (at least at the weld point) has a martensitic microstructure, preferably a fully martensitic microstructure, and is in particular a press-hardened sheet metal part (e.g., made of 34MnB5) or a sheet metal part made of dual-phase steel. Preferably, all components have martensitic microstructure properties or a martensitic / fully martensitic microstructure and are in particular press-hardened sheet metal parts and / or sheet metal parts made of dual-phase steel.

[0019] The invention is explained in more detail below by way of example and in a non-limiting manner with reference to the figures. The features shown in the figures and / or explained below can, even independently of specific combinations of features, be general features of the invention and further develop the invention. Fig. Figure 1 schematically illustrates the first step of a method according to the invention. Fig. 2 illustrates analogously to Fig. 1 the second step of a method according to the invention. Fig. Figure 3 shows a possible current-time diagram for the first and second steps of the process in Fig. 1 and Fig. 2 illustrated procedures.

[0020] Fig. Figure 1 shows the first step of a method according to the invention, in which the components 110, 120 are joined at a weld point S by resistance spot welding. An electric current I1 is passed through the components 110, 120 by means of electrodes 210 arranged on both sides of the components 110, 120, thereby creating a spot weld P in the form of a weld nugget. The electrodes 210 are, for example, type F electrodes. During resistance spot welding, a heat-affected zone W is formed around the spot weld P. During the cooling that follows the resistance heating, a martensitic microstructure forms in the spot weld P. A martensitic microstructure can also form, at least partially, in the heat-affected zone W.

[0021] Fig. Figure 2 shows the second step of the process, in which an electric current I2 is passed through the components 110, 120 at the same weld point S using the same electrodes 210, thereby performing a heat treatment. Here, the martensitic microstructure of the previously created spot weld P is at least partially austenitized (starting from the center, as described above) and, after switching off the electric current I2, is converted into neumartensite N, wherein the neumartensite N forms a neumartensite island A, i.e., a core-like region of neumartensite N, which is enclosed, in particular completely enclosed, by a martensite M produced in the first step and tempered in the second step.

[0022] Fig. Figure 2 shows the final state achieved in the second step. The neumartensite island A is at least 0.4 mm (i.e., ≥ 0.4 mm) away from the joining gap F (as illustrated by arrow d). The area B of the tempered martensite M extends beyond the joining gap F and to the component's (outer) surfaces. The area B of the tempered martensite M can also extend to the edge of the heat-affected zone W. In the example shown, the neumartensite island A is smaller than the spot weld or weld nugget P produced in the first step. The strength of the tempered martensite M is at most 90% (i.e., ≤ 90%) of the strength of the neumartensite N.

[0023] Fig.Figure 3 shows a current-time diagram illustrating possible current intensities and pulse durations for the energization phase (first pulse) of the first step and for the energization phase of the second step (second pulse). Compared to the energization phase of the first step, the current in the energization phase of the second step is lower but longer. The pause between the energization phase of the first step and the energization phase of the second step is comparatively long and is several times the pulse duration of the first energization phase. This longer pause is intended as a cooling pause. An optional pre-energization phase V, as explained above, is also shown in dashed lines.

[0024] Further embodiments and implementation possibilities of the invention are described above. Reference symbol list 110 component 120 components 210 electrode(s) F Joining gap I1 electric current I2 electric current A Neumartensite Island Area B (tempered martensite) M tempered martensite N Neumartensite P spot weld S weld point V Pre-energization phase W Heat-affected zone d distance

Claims

[1] Method for resistance spot welding of components (110, 120) wherein - in a first step at the welding point (S) an electric current (I1) is passed through the components (110, 120) by means of electrodes arranged on both sides of the components (110, 120) and thereby a substantially martensitic spot weld (P) is produced; and - in a second step at the same welding point (S) an electric current (I2) is passed through the components (110, 120) by means of electrodes (210) arranged on both sides of the components (110, 120), whereby the martensitic structure of the previously produced spot weld (P) is austenitized and, after switching off the electric current (I2), is converted into neumartensite (N), wherein the neumartensite (N) forms a neumartensite island (A) which is enclosed by a martensite (M) produced in the first step and tempered in the second step. [2] Method according to claim 1, characterized bythat the same electrodes (210) are used in the first step and in the second step. [3] Method according to any one of the preceding claims, characterized by , that the strength of the tempered martensite (M) is at most 90% of the strength of the neumartensite (N). [4] Method according to any one of the preceding claims, characterized by , that the neumartensite island (A) is at least 0.4 mm away from the joining gap (F). [5] Method according to any one of the preceding claims, characterized by , that the area (B) of the tempered martensite (M) extends beyond the joining gap (F) and / or reaches the component surface of at least one of the components (110, 120). [6] Method according to any one of the preceding claims, characterized by , that the residual heat present at the welding point (S) at the beginning of the second step is adjusted. [7] Method according to any one of the preceding claims, characterized bythat a pre-energization phase (V) takes place before the first step. [8] Method according to any one of the preceding claims, characterized by , that the electrodes used (210) are type F electrodes. [9] Method according to any one of the preceding claims, characterized by , that at least one of the components (110, 120) is a press-hardened sheet metal part.

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