Method and industrial installation for analysing a process of resistance spot welding

Abstract

A method and industrial installation for monitoring a process of resistance spot welding, wherein the method comprises the steps of: disposing two or more steel sheets (1, 2) one on another, such that a surface of one of the steel sheets (1) becomes in contact with another surface of the other steel sheet (2), wherein at least one of the contact surfaces of the steel sheets (1, 2) has an AlSi coating, applying a current pulse with a certain period to the steel sheets (1, 2), thus removing the AlSi coating on a spot where the steel sheets (1, 2) are going to be welded, measuring on the steel sheets (1, 2) a voltage and a current at the beginning of the current pulse applied, from the measured voltage and the measured current, calculating a dynamic resistance at the beginning of the applied current pulse, and evaluating the thickness of the interdiffusion layer of the contact surfaces of the steel sheets (1, 2) from the calculated dynamic resistance.

Description

[0001] METHOD AND INDUSTRIAL INSTALLATION FOR ANALYSING A PROCESS OF RESISTANCE SPOT WELDING

[0002] TECHNICAL FIELD

[0003] The present invention relates to the technical field of spot-welding in stamping manufacturing. In particular, it relates to a method and to an industrial installation for analysing a process of resistance spot welding, and more particularly to a method of analysing the coating removal of hot stamped steels during resistance spot welding.

[0004] STATE OF THE ART

[0005] Resistance spot welding (RSW) is a welding method that uses electrical resistance to join two metal sheets at a specific point. More than two metal sheets could also be joined. Welding is produced by the heat generated by the electrical resistance that occurs at the contact surface between the metal sheets under pressure and the flow of a high current. Due to the Joule effect, heat is generated that melts the metal sheets in the contact area, as the Joule’s Law of Heating indicates:

[0006] Q = 0,24 * l2*R *t (cal)

[0007] being Q the heat energy generated, measured in calories; I the electric current flowing through the conductor, in this case the metal sheets; R the resistance of the conductor; t the time duration for which the current flows.

[0008] Particularly, RSW works by positioning two or more metal sheets on top of each other and applying pressure with two electrodes. A power supply then generates a high current flow between the electrodes, causing the metal to heat up and melt at the spot where the electrodes are applying pressure, due to electrical resistance. As the electrodes maintain pressure, the workpieces fuse together and form an inextricable joint upon cooling.

[0009] Therefore, during the welding, an electrical circuit is generated, wherein electrical current circulates between the two electrodes and through the metal sheets. Along this path, several resistances appear. Firstly, the internal resistance of the electrodes and the cables to the power supply. Secondly, the contact resistance between the electrodes and the surface of the metal sheets. Thirdly, the internal resistance of the metal sheets. And finally, the interfacial contact resistance between the metal sheets. This is the point with the maximum resistance,and where a nugget is formed with the highest temperature due to Joule’s law of heating, being a weld nugget the fused volume of metal that joins the overlapping metal sheets together during resistance spot welding.

[0010] The main advantages of RSW are: high welding speed; low distortion of the workpieces (metal sheets); no additional filler metal is required, reducing the weight of the workpiece; can weld thin and thick materials and relatively low cost.

[0011] RSW is applied in several industries, such as automotive industry, appliance manufacturing, metal structure construction and metal furniture manufacturing.

[0012] The quality control of RSW is extremely challenging, as it requires a trial-and-error method to find and optimize the welding parameters. Two methods are primarily used to test the spot welds: destructive tests, such as peeling or chisel tests, and non-destructive testing, such as ultrasound tests. Both methods are costly, time-consuming, and require a great deal of expertise, particularly in the case of ultrasound testing. Furthermore, after testing, it is difficult to determine which welding parameter needs to be modified to optimize the weld.

[0013] Moreover, the use of ultra-high strength steels (LIHSS) is becoming more widespread in the automotive industry due to increasingly stringent safety and fuel efficiency regulations. This type of steel is often used in car body components such as front bumpers, reinforced pillars, and roof rails. However, traditional manufacturing techniques such as RSW can be challenging when working with LIHSS.

[0014] To overcome these challenges, a special hot forming technology called press hardened steel (PHS) is being used. This method avoids the limitations of traditional cold stamping, which can result in springback and reduced formability. Despite its benefits, PHS poses new challenges for the traditional RSW technology commonly used in car body construction. The high carbon content and strength of PHS makes it difficult to weld, resulting in a narrow welding zone, hardening of the weld nugget, and softening of the surrounding area. This can lead to difficulties in achieving a strong and reliable weld.

[0015] Also, to prevent surface oxidation, an aluminium-silicon (AlSi) coating is being used on press hardened steels (PHS), but this has had a negative impact on its resistance spot weldability. Interfacial fracture is the dominant failure mode in RSW of these materials. This fracture is dueto the high carbon equivalent and the presence of a sharp notch at the weld boundary, caused by the AlSi coating.

[0016] Particularly, the interdiffusion layer (IDL) in hot stamped steels refers to a layer that forms at the interface between the steel substrate and the AlSi coating during the hot stamping process. This layer is created by the diffusion of elements, primarily aluminium and iron atoms across the interface, resulting in the formation of intermetallic compounds.

[0017] The IDL is characterized by a gradual change in composition from the steel substrate to the AlSi coating, with an increasing concentration of aluminium and a decreasing concentration of iron. This layer can be composed of various intermetallic phases, such as AlsFe2Si, AlsFe2, and a-Fe which form because of the reaction between the aluminium in the AlSi coating and the iron in the steel substrate.

[0018] The formation of the IDL is driven by the diffusion of aluminium and iron atoms during the hot stamping process, which involves heating the steel to high temperatures (typically above 900°C) for a short period of time. The thickness and composition of the IDL can vary depending on factors such as the soaking temperature, heating rate, and AlSi coating thickness.

[0019] The IDL plays a crucial role in determining the properties of the hot stamped steel, including its corrosion resistance, mechanical strength, and weldability. Also, excessive growth of the IDL can lead to a decrease in the AlSi coating's protective properties and affect the overall quality of the hot-stamped part. In general, the IDL is a critical layer in hot stamped AlSi coated steels, as it can affect the performance and durability of the final product.

[0020] The key effects of the interdiffusion layer on RSW are:

[0021] Formation of intermetallic compounds (IMCs): during welding, the IDL can lead to the formation of IMCs, especially in coated hot-stamped steels. For example, aluminiumsilicon (AlSi) coatings can transform into a liquid film that covers the faying interface, affecting the weld nugget formation.

[0022] Mechanical strength: the presence of IMCs within the IDL can influence the mechanical strength of the weld. While some IMCs can enhance strength, others may introduce brittleness, reducing the overall toughness of the weld joint.

[0023] Heat development and nugget growth: the properties of the IDL affect heat development and nugget growth during welding. AlSi coatings tend to have low contactresistance, while the IDL increases the electrical resistance at the contact surfaces lowering the current and affecting the nugget growth.

[0024] Corrosion resistance: the IDL can also impact the corrosion resistance of the weld. Properly formed IDL can act as barriers to corrosion, enhancing the durability of the weld in harsh environments.

[0025] Weld integrity: managing the IDL is crucial for achieving a homogeneous and defect- free weld. This is particularly important in automotive applications where the integrity of the weld is critical for safety and performance.

[0026] Using a stepped current pulse scheduled with an appropriate current increment can help to eliminate the AlSi coating in the spots where the sheets are going to be welded. By removing the AlSi coating in these spots, the IDL is also totally or at least partially removed, improving the weldability of the sheets, as has been reported in the following document.

[0027] Document “Improving weldability of Al-Si coated press hardened steel using stepped current pulse schedule", Ruiming Chen, Ming Lou, Yongbing Li, Blair E. Carlson, Journal of Manufacturing Processes 48 (2019) 31-43, proposes a stepped current pulse schedule to improve the resistant spot weldability of Al-Si coated 1.5 mm HS1300T. This procedure helps to eliminate residue IDL layer decreasing the notch root stress concentration. This schedule enhances the mechanical performance of joints, changing the fracture mode from interfacial fracture to button pullout by growing the weld nugget without expulsion, eliminating the residual Al-Si coating.

[0028] In view of all the above, the increasing use of PHS, for example in car manufacturing, is driving the need for new optimized welding technologies and techniques that can effectively join these high-strength steels, avoiding the problems derived from the presence of an interdiffusion layer.

[0029] DESCRIPTION OF THE INVENTION

[0030] The present invention aims to overcome the above-mentioned drawbacks and proposes a method and an industrial installation for analysing a process of resistance spot welding (RSW). Particularly, the invention proposes a method of analysing the coating removal of metal sheets being welded with RSW, improving their weldability. The invention also proposes an industrial installation for analysing a process of RSW.A first aspect of the invention is a method for analysing a process of resistance spot welding, wherein the method comprises the steps of:

[0031] -disposing two or more steel sheets one on another, such that a surface of one of the steel sheets becomes in contact with a surface of another steel sheet, wherein at least one of the two contact surfaces of the steel sheets has an AlSi coating,

[0032] - applying a current pulse with a certain period to the steel sheets, thus removing the AlSi coating on a spot where the steel sheets are going to be welded,

[0033] - measuring on the steel sheets a voltage and a current at the beginning of the current pulse applied,

[0034] - from the measured voltage and the measured current, calculating a dynamic resistance at the beginning of the applied current pulse, and

[0035] - evaluating the thickness of an interdiffusion layer of the contact surfaces of the steel sheets from the calculated dynamic resistance.

[0036] In an embodiment of the invention, the method is applied to two metal sheets. In an embodiment of the invention, the method is applied to more than two metal sheets, such as to three metal sheets or to four metal sheets. In an embodiment of the invention all the sheets are made of the same type of steel. In an embodiment of the invention, the sheets are of different types of steel.

[0037] In an embodiment of the invention, only one of the contact surfaces between two metal sheets has an AlSi coating. In an embodiment of the invention, both of the contact surfaces between two metal sheets have an AlSi coating.

[0038] The measured voltage is also referred to as coating removing voltage, meaning that it is the voltage intended to remove the AlSi coating of the surfaces on which the current pulse is applied.

[0039] The measured current is also referred to as coating removing current, meaning that it is the current intended to remove the AlSi coating of the surfaces on which the current pulse is applied.

[0040] In an embodiment of the invention, the steel is a hot stamped steel, also called press hardened steel. In an embodiment of the invention, the steel is an Ultra High Strength steel (UHSS). Examples of UHSS are Usibor® 2000 commercially available from ArcelorMittal, MBW® 1900 which is a manganese-boron steel from ThyssenKrupp™, B1800HS and CR1900T-MB-DS,among others. In an embodiment of the invention, the steel is an AlSi coated boron steel. Examples of AlSi coated boron steel are llsibor® 1500 (22MnB5) commercially available from ArcelorMittal and MBW-W®1500 which is commercially offered by ThyssenKrupp. In an embodiment of the invention the steel is a high strength material such as 37MnB5 steel. Other examples of steels that can be used are Ductibor® 500 and Ductibor® 1000. Further examples of steels that could be used are Nippon steels, such as NSSQAS 500, NSSQAS 1000, NSSQAS 1500 and NSSQAS 2000.

[0041] Different AlSi coating thickness may be used, depending on the desired final anticorrosion properties. Non-limiting examples of AlSi coatings that may be used are AS150 (AS60 / 60m according VDA 239-100), AS80 (AS30 / 30, VDA 239-100), AS120 (AS45 / 45 according to VDA 239-100), among others.

[0042] The thickness of the AlSi coating is in the order of microns (10-6m). For example, the thickness of AS80 typically ranges between 15 and 30 pm, or the the thickness of AS150 typically ranges between 30 and 50 pm.

[0043] In an embodiment of the invention, the thickness of the steel sheets is within 0.5-3mm, which is range suitable for steel sheets used in car manufacturing. Other ranges of thickness may be used alternatively, for example when steel sheets intended for a different application, such as truck manufacturing, are selected.

[0044] The steel sheets are placed one on top of the other, overlapping at least partially, sharing at least partially their contact surfaces. An electric high current is passed through the sheets during the application of the current pulse. The electric current is applied by means of electrodes placed on the sheets or in proximity thereto. The electric current is applied on the surfaces of the steel sheets which are opposite to their contact surfaces. The current circulates from the electrodes, through the sheets, to the contact surfaces. In this point is where the coating is totally or at least partially removed by the applied current.

[0045] In an embodiment of the invention, the applied current is in a range between 1 kA and 50 kA depending on several parameters, such as the IDL thickness, material properties, metal sheet thicknesses and electrode’s diameter and geometry. Preferably, the applied current is in a range between 3kA and 40kA, such as between 4kA and 30kA, or between 4kA and 20kA, or between 4kA and 13kA, or between 5kA and 13kA, or between 8kA and 12kA. The higher thethickness of the metal sheet and the I DL thickness, the higher the current that has to be applied to remove the AlSi coating.

[0046] The applied voltage value is determined by the electronic (transformer, inverter, rectifier, etc) characteristics of the RSW machine and is normally below 5 volts. The duration of the required current pulse is also usually related to the thickness of the metal sheets and the material properties. The thicker the metal sheets the longer must be the duration of the current pulse. In an embodiment of the invention, the duration of the current pulse is in a range between 5 ms and 500 ms, such as between 10ms and 400 ms, or between 20 ms and 300 ms, or between as 30ms and 250ms, or between 30ms and 100ms.

[0047] For example, for two steel sheets of NSSQAS 1000 AS150 (1,5mm thickness), a current of 60 ms and 9kA may be needed to remove the AlSi coating. For example, for two steel sheets of llsibor 1500 AS80 (1,2mm thickness) a pulse of 300ms and 4.5kA may be needed. For example, for three sheets, the first one of llsibor 1500 AS150 (2,3mm thickness), the second one of DP980 Gl Z100 (1,8mm) and the third one of E270 Z140 (0,7mm thickness) the required current pulse may be of 50ms and 11 kA.

[0048] The AlSi coating that is removed by applying the current pulse is the AlSi coating of the contact surfaces of the steel sheets, particularly on the spot where the steel sheets are going to be welded. In this way, by removing at least partially the AlSi coating of the contact surfaces the interdiffusion layer is also totally or at least partially removed, thus improving the weldability of the steel sheets.

[0049] The measured coating removing voltage and the coating removing current refer to the voltage and current values measured on the metal sheets when the current pulse is applied. The coating removing voltage and the coating removing current being measured at the beginning of the pulse means that they are measured at an initial moment of the application of the current pulse. This voltage and current are preferably measured as soon as possible at the beginning of the applied pulse, irrespective of the time duration of the current pulse. The precise measuring instant is mainly dependent on the sampling frequency of the equipment with which they are measured, because the time instant at which the current and voltage are measured at the beginning of the pulse depends on the sampling frequency. The higher the sampling frequency, the shorter the time instant at the beginning of the current pulse which triggers the measuring of the current and voltage values. With the proviso that voltage and current are measured as soon as possible at the beginning of the applied pulse, and that this is dependenton the sampling frequency, the initial moment can for example be at any time during the first during the first 20 ms thereof, or during the first 10 ms thereof, or during the first 5 ms, or during the first 1 ms, or during the first 0.1 ms.

[0050] In an embodiment of the invention, the current and voltage on the sheets at the beginning of the pulse are obtained directly from measurements obtained by the RSW machine, from the machine’s analog outputs or by sensors placed on the welding gun. The sensors used to measure the current and the voltage should be as close to the welding electrodes as possible to accurately measure the current and voltage involved in the formation on the weld nugget.

[0051] The dynamic resistance at the beginning of the pulse is associated with the presence of contaminants or intermetallic components in the contact surfaces of the steel sheets that are being welded. It is calculated using equation W , wherein V; is the coating removing voltage measured on the metal sheets and is the coating removing current measured on the metal sheets.

[0052] The value of the dynamic resistance at the beginning of the pulse is directly correlated with the thickness of the interdiffusion layer (I DL). When the IDL layer is thicker the electrical resistance value is higher. Through comparison of the electrical resistance values with reference values taken in a system calibration process, the IDL layer thickness can be estimated. The value of the dynamic resistance is therefore used to estimate the thickness of the interdiffusion layer.

[0053] With the proposed method, different problems associated to RSW in hot stamped steel are reduced, in particular the identified problems caused by the interdiffusion layer (IDL). Proper control and analysis of the IDL is thus achieved. This control and analysing are, as previously explained, very important for optimizing the quality and performance of resistance spot welds in hot stamped steels and, in general, for optimizing the RSW process and for ensuring reliable weld joints. As previously explained, the IDL plays a vital role in determining the quality and performance of resistance spot welds in hot stamped steels. Proper control and monitoring of this layer are essential for optimizing this kind of welding and ensuring reliable weld joints. Furthermore, the method of the invention provides information about the quality of the sheet before it is welded. Therefore, it helps to optimize and correct process parameters.

[0054] In an embodiment of the invention, the value of the dynamic resistance at the end of the applied current pulse for removing the AlSi coating is used to optimize the coating removing pulse. It is possible to know whether to shorten / lengthen this coating removing pulse or toincrease / decrease the current in this current removing pulse. The coating removing current pulse can be lengthened or shortened and / or the current can be increased or decreased until the dynamic resistance at the end of the current pulse is within its respective tolerance bands. If the tolerance bands cannot be reached by adjusting the time, then the applied welding current during the coating removal pulse is adjusted.

[0055] In an embodiment of the invention the duration of the current pulses used for AlSi coating removal are corrected based on the calculated dynamic resistance at the end of the applied current pulses.

[0056] In this way, welding is only carried out once it has been checked that the AlSi coating removal has been carried out correctly after the first current pulse has been applied. This ensures the quality of the weld and avoids welds with defects caused, among other things, by the IDL.

[0057] In an embodiment of the invention, the calculations described, as well as others described below, can be carried out in a computing means. In an embodiment of the invention, the computing means comprise at least a processor and a memory. The computing means may be disposed adjacent to a welding installation, or at a remote location. The computing means is configured to receive the voltage and current measurements and calculate the dynamic resistance and estimate the IDL thickness.

[0058] In an embodiment of the invention, to evaluate the thickness of the interdiffusion layer, the calculated dynamic resistance at the beginning of the applied current pulse is compared to a predefined tolerance band. If the dynamic resistance is within the tolerance band, the IDL initial thickness is acceptable, and it is therefore determined that the metal sheets are correct. This means that the metal sheets have an IDL layer of acceptable thickness, i.e. it can be sufficiently removed to ensure that the sheets will be welded correctly. If the calculated dynamic resistance is not within said tolerance band, it is determined that the metal sheets are defective. This can be, for example, because they have been treated with an incorrect soaking temperature or a heating rate out of range.

[0059] Accurate determination of the thickness of the IDL is highly dependent on the sampling frequency of the coat removing current and the coat removing voltage and requires high sampling rates to achieve reliable results. By comparing the dynamic resistance with a tolerance band, the method is simplified.In an embodiment of the invention the method further comprises, when the calculated dynamic resistance is not within the tolerance band, generating an alert signal indicating that metal the sheets are defective.

[0060] In an embodiment of the invention the method further comprises:

[0061] - measuring on the metal sheets a voltage and a current applied by the current pulse at the end of the current pulse,

[0062] - from the voltage and the current at the end of the current pulse, calculating a dynamic resistance,

[0063] - estimating the effectiveness of the coating removal from the calculated dynamic resistance at the end of the applied current pulse.

[0064] The coating removing voltage and the coating removing current measured at the end of the current pulse refer to the voltage and current values measured on the metal sheets just before the current pulse applied ends (i.e. at a final instant of the pulse). Therefore, the coating removing voltage and the coating removing current being measured at the end of the current pulse means that they are measured at a final moment of the application of the current pulse. This voltage and current are preferably measured as late as possible at the end of the applied pulse, irrespective of the time duration of the current pulse. The precise measuring instant is mainly dependent on the sampling frequency of the equipment with which they are measured, because the time instant at which the current and voltage are measured at the end of the pulse depends on the sampling frequency. The higher the sampling frequency, the shorter the time instant at the end of the current pulse which triggers the measuring of the current and voltage values. With the proviso that voltage and current are measured as late as possible at the end of the applied pulse, and that this is dependent on the sampling frequency, the final moment can for example be at any time during during the last 20 ms of the applied pulse, or during the last 10 ms thereof, or during the last 5 ms, or during the last 1 ms, or during the last 0.1 ms.

[0065] The dynamic resistance at the end of the applied current pulse is calculated using the coating removing voltage and the coating removing current at the end of the current pulse, applying the following equation Vjt / lft. The dynamic resistance at the end of the applied current pulse used to remove the coating of the steel sheets is associated to the effectiveness of the coating removal. A decrease in the IDL layer thickness causes a decrease in the electrical resistance value. Through comparison of the electrical resistance values with reference values taken in a system calibration process, the IDL layer thickness after removal can be evaluated.In an embodiment of the invention, to estimate the effectiveness of the coating removal the calculated dynamic resistance is compared to a predefined tolerance band. If the dynamic resistance is within the tolerance band, the IDL thickness after the coating removal is acceptable and the coating removal has been effective.

[0066] When the dynamic resistance at the end of the applied current pulse is above the tolerance band it means that the IDL layer has been insufficiently removed and / or the duration of the current pulse is insufficient and / or the current level is insufficient. When the dynamic resistance at the end of the applied current pulse is below the tolerance band it means that the duration of the current pulse is excessive and / or that the current level is excessive.

[0067] In an embodiment of the invention the method further comprises calculating the total energy applied during the current pulse with the voltage and the current and comparing it to a tolerance band.

[0068] The total energy during the current pulse for removing the coating from the metal sheets is calculated as f Vt* Ii * dt. If the total energy is inside the tolerance band, it is determined that a correct heat input has been applied. If the total energy is above the tolerance band, it is determined that the duration of the current pulse is excessive and / or that the current level is excessive and / or there is overheating or even burning. If the total energy is below the tolerance band, it means that the current pulse duration is insufficient and / or that the current level is insufficient.

[0069] In an embodiment of the invention the method further comprises calculating the maximum value of the first derivative of the voltage curve of the measured voltage on the sheets during the application of the current pulse and comparing it to a tolerance band.

[0070] The maximum value of the first derivative of the voltage curve is calculated as Umax = [dV I dt]max. It is used to detect material expulsion during the current pulse for coating removal. Particularly, material expulsion, which is not desired because it weakens the weld, is associated to high values of V' (sudden voltage drops). Electrode sticking is also associated to high values of V' (sudden voltage changes due to secondary short-circuiting).

[0071] In an embodiment of the invention, if the maximum value of the first derivative of the voltage curve is inside a tolerance band, it is determined that there is no material expulsion and no electrode sticking. If the maximum value of the first derivative of the voltage curve is outsidethe tolerance band, it is determined that there is material expulsion and / or insufficient electrode force and / or excessive current level and / or electrode sticking.

[0072] In an embodiment of the invention the method further comprises calculating the dynamic resistance ripple during the current pulse and comparing it to a tolerance band.

[0073] The dynamic resistance ripple during the current pulse for coating removal is associated with contaminated electrodes with the coating of the sheets, electrode wear or electrode misalignment during the coating removal process. The dynamic resistance ripple is calculated as the absolute value of the difference between the moving average and the instantaneous value of the dynamic resistance, and it represents the instability of the signal, in this case of the dynamic resistance.

[0074] In an embodiment of the invention, if the dynamic resistance ripple is inside a tolerance band it is determined that there is no electrode wear and no electrode misalignment. If the dynamic resistance ripple is outside the tolerance band, it is determined that there is electrode / workpiece contamination and / or there is electro wear and / or there is electrode misalignment.

[0075] In an embodiment of the invention the method further comprises calculating current parameters during the current pulse and comparing them to a tolerance band.

[0076] The current parameters during the applied current pulse associated with the coating removal process are used to detect variability with respect to nominal conditions and detect changes in the coating removal process parameters.

[0077] In an embodiment of the invention the current parameters are selected from: current pulse levels (kA), current pulse transition positions (ms) and current pulse transition durations (ms).

[0078] If the current parameters are inside a tolerance band, it is determined that the current pulse is within nominal current values. If the current parameters are outside the tolerance band it is determined that the current pulse levels, pulse transition position or pulse transition durations are out of limits.With the application of the already disclosed current pulse, the AlSi coating on the spot of the contact surfaces at which the steel sheets are going to be welded, is substantially removed. The steel sheets are therefore ready to be welded on that spot.

[0079] In an embodiment of the invention, the method further comprises applying a second current pulse with a certain period on that spot to weld the steel sheets.

[0080] For example, for two steel sheets of llsibor 1500 AS150 (1mm thickness) a second current pulse of 300ms and 6.5kA may be applied. For example, for two metal sheets of llsibor 1500 AS150 (1mm thickness) a 500 ms and 7.5kA may be applied. For example, for three metal sheets of different materials, a first one of Usibor 1500 AS150 (2,3mm thickness) a second sheet of DP980 Gl Z100 (1 ,8mm thickness) and a third sheet of E270 Z140 (0,7mm thickness) a second current pulse of 300ms and 9kA may be applied. For all of these examples, a force of 4.5kN may be applied with the electrodes.

[0081] A second aspect of the invention is an industrial installation comprising:

[0082] a resistance spot welding module, configured to apply a current pulse with a certain period to two or more steel sheets disposed one on another, such that a surface of one of the steel sheets becomes in contact with another surface of the other steel sheet, wherein at least one of the contact surfaces of the steel sheets has a coating,

[0083] sensing means, configured to measure on the steel sheets a voltage and a current applied by the current pulse, at the beginning of the current pulse,

[0084] a computing means configured to:

[0085] - calculate a dynamic resistance at the beginning of the applied current pulse with the measured voltage and current, and

[0086] - evaluate the thickness of the interdiffusion layer of the contact surfaces of the steel sheets from the calculated dynamic resistance.

[0087] The sensing means may be integrated in the resistance spot welding module (also referred to as welding machine) or may be external sensors.

[0088] In some embodiments, the industrial installation is further configured to measure other parameters and compute other calculations, as generally disclosed in the first aspect.The invention enables the optimization of welding time, reducing energy costs and increasing productivity as it provides a method to control the quality of all spot welds in real-time, based on tolerances of the critical supervision parameters. It also provides a method to improve the weldability of coated hot stamped steel.

[0089] BRIEF DESCRIPTION OF THE DRAWINGS

[0090] In order to complete the description and to provide a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

[0091] Figure 1 schematically shows a resistance spot welding process.

[0092] Figure 2 illustrates the application of a welding current to two overlapping metal sheets and a graph schematically showing values of electric resistance at different points of the metal sheets.

[0093] Figure 3 illustrates a computing means communicatively coupled to a resistance spot welding machine according to embodiments of the invention.

[0094] Figure 4 shows a graph of a current measured on two steel sheets when two current pulses are applied to the two steel sheets in a resistance spot welding process according to embodiments of the invention.

[0095] Figure 5 shows a graph of a voltage measured on two steel sheets when two current pulses are applied to the two steel sheets according to embodiments of the invention.

[0096] Figure 6 shows a graph in which an initial dynamic resistance value and a final dynamic resistance value of a pulse for coating removal according to embodiments of the invention is depicted.

[0097] Figure 7 shows a graph of the total energy during a current pulse for coating removal according to embodiments of the invention.Figure 8 shows a graph of the voltage first derivative obtained when a current pulse for coating removal is applied according to embodiments of the invention.

[0098] Figure 9 shows a graph of the dynamic resistance ripple during a current pulse for coating removal according to embodiments of the invention.

[0099] Figure 10 shows a graph of current parameters during a current pulse for coating removal according to embodiments of the invention.

[0100] Figures 11 to 15 show a flow diagram of the method according to embodiments of the invention.

[0101] DESCRIPTION OF A WAY OF CARRYING OUT THE INVENTION

[0102] The following description is not to be taken in a limiting sense but is given solely for the purpose of describing the broad principles of the invention. Next embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings, showing apparatuses and results according to the invention.

[0103] Figure 1 illustrates a resistance spot welding process wherein un upper sheet 1 and a lower sheet 2 are positioned in contact with each other. The sheets 1, 2 are coated with an AlSi coating. In particular, one of the surfaces of one of the sheets 1 is in contact, at least partially, with one of the surfaces of the other sheet 2. A current is applied in the form of a current pulse, to remove the coating of the sheets 1, 2 and a second pulse is applied to weld the sheets 1, 2 together. Particularly, two electrodes 3 apply force and current in the spot that is going to be welded. Thus, since the electrodes 3 apply force and current through surfaces opposite to the contact surfaces of the sheets 1, 2 and the current flows from the surfaces opposite to the contact surfaces, the metal sheets 1, 2 melt through the contact surfaces. As shown in the figure, once the coating has been at least partially removed, a nugget 4 is formed, where the sheets 1, 2 start to melt. Then, the electrodes 3 stop applying current, but they apply some additional force, during welding nugget solidification while the sheets 1, 2 are cooling. The last step is the retraction of the electrodes 3. In view of the above, it is of interest to analyze the removal of the AlSi coating to determine if the sheets 1, 2 meet certain quality requirements, whether the nugget 4 will be formed properly and whether the weld will therefore be of good quality.In a first exemplary embodiment, metal sheets 1, 2 made of USIBOR 1500 (boron steel) are used, with a 1.2mm thickness both. They are covered with an AS150 coating of 35 n and soaked for 200 s in an oven. The estimated IDL thickness is less than 16 n. The electrodes used are 20mm-8mm.

[0104] In a second exemplary embodiment metal sheets 1, 2 made of IISIBOR 1500 are used, with a 1 ,2mm thickness both. They are covered with an AS80 coating of 21 pm and soaked for 200s in an oven. The estimated IDL thickness is less than 12 pm .The electrodes used are 20mm-8mm.

[0105] In a third exemplary embodiment metal sheets 1, 2 made of IISIBOR 1500 are used, with a 1.2mm thickness both. They are covered with an AS80 coating of 21 pm and soaked for 600 s in an oven. The estimated IDL thickness is less than 12 pm. The electrodes used are 20mm-8mm.

[0106] In all three exemplary embodiments the electrodes used are type B tapered electrodes. 20mm refers to the diameter of the base of the electrode and 8mm refers to the diameters of the surface in contact with the steel.

[0107] The difference between the first exemplary embodiment and the second exemplary embodiment is the type of coating. The difference between the second exemplary embodiment and third exemplary embodiment is the oven time. In the third exemplary embodiment, as the oven time is much longer, the interdiffusion layer (IDL) layer is increased.

[0108] A current pulse of 70ms is used in the three exemplary embodiments. Other steel sheets may require different intensity and duration of current pulse.

[0109] Figure 2 illustrates a diagram showing the distribution of the electrical resistance at different points in the resistance spot welding circuit. In particular, at different points along the electrical path from the point on the surfaces of the metal sheets 1, 2 on which the pulses are applied, to the point on the opposite surfaces of the metal sheets 1, 2, which are the points on which the sheets 1 , 2 are going to be welded. Points a and g represent the internal resistance of the electrodes 3 and the cables to the power supply. Points b and f illustrate contact resistance between the electrodes 3 and the surface of the sheets 1 , 2. Points c and e illustrate the internal resistance of the upper and bottom sheets 1, 2. Finally, point d represents the interfacial contact resistance between the upper and bottom sheets 1, 2. This is the point with themaximum resistance, where the nugget 4 is formed with the highest temperature due to Joule’s law of heating.

[0110] As previously described, hot stamped steel with an AlSi coating faces challenges during resistance spot welding due to the formation of an interdiffusion layer (IDL). This IDL forms during the hot stamping process as a result of the high temperatures and diffusion between the coating and the steel substrate.

[0111] The proposed method analyses the coating removal in resistance spot welding, to avoid bad quality welds due to the IDL. Figures 11 to 15 show flow diagrams of the steps of the method and figures 4 to 10 illustrate some parameters measured and variables calculated and used to analyze the resistance spot welding process. Figure 3 illustrates an industrial installation 8 for resistance spot welding and computing means 7 connected to said industrial installation, wherein the computing means execute the calculations of the proposed method.

[0112] Figure 4 illustrates a graph of the current applied to the two sheets 1, 2, wherein a current pulse is applied with the electrodes 3 to the sheets 1, 2. The current pulse is applied to strip the coating from the sheets 1, 2 in a spot where they are going to be weld. Then, a second current pulse is applied to weld the two sheets 1, 2 together.

[0113] Figures 11 to 15 show flow diagrams of different embodiments of the method of monitoring a resistance spot welding process and more particularly of monitoring the AlSi coating removal of hot stamped steel during resistance spot welding.

[0114] In any of these embodiments, a first step of the method, illustrated in figures 11 to 15, is applying a current pulse with RSWwith DC current 12 to two metal sheets 1, 2, wherein the metal sheets 1, 2 are press hardened steel 11 coated with AlSi and obtaining the raw signals 13, in this case voltage and current. Those signals can be obtained from external current sensors 16, or they can be captured by internal sensors of the RSW industrial installation and stored in internal files 14 or they can be obtained from the industrial installation’s analog outputs 15. The measured voltage and current signals 5 are preferably conditioned and processed 17 and then several variables of interest are calculated 18, which will be explained in detail below.When external current sensors 16 are used, they should be placed as close as possible to the electrodes 3 to get more accurate measurements, avoiding errors with current leakages to ground.

[0115] Figure 4 illustrates the current value measured 5 on the sheets 1 , 2 when a first pulse is applied for removing the coating from the sheets 1, 2 and the current value measured 6 on the sheets 1, 2 when a second pulse is applied for welding the sheets 1, 2 together. From now on, we refer to the first pulse. The various parameters and variables that will be calculated are associated to this first current pulse. Similarly, figure 5 illustrates the voltage value measured on the sheets 1 , 2 when the first pulse is applied and when the second pulse is applied.

[0116] The next step of the method, as illustrated in figure 11 to 15, is calculating the dynamic resistance at the beginning of the first current pulse 19 with the measured values of current and voltage applied for removing the coating. The values of current and voltage measured at the beginning of the first pulse refer to the values reached in the metal sheets 1, 2 at the beginning of the first current pulse. The dynamic resistance is illustrated in figure 6 and its first value is the one marked on the left. In particular, figure 6 represents an enlargement of the first pulse of figure 3, i.e. of the current measured on the sheets 1 , 2 when the first current pulse is applied.

[0117] Once the initial dynamic resistance has been calculated, that is the dynamic resistance at the beginning of the first pulse, its value is compared with a tolerance band 20, to estimate if the initial thickness of the IDL is correct. If the initial dynamic resistance is outside its tolerance band 22, it is determined that the IDL thickness is not adequate 22, probably due to the soaking temperature or heating out of range during the manufacturing of the sheets 1, 2. In this case a defective batch of sheets is detected. If the initial dynamic resistance is inside the tolerance band 21, it is determined that the initial IDL thickness is adequate and in principle the coating will be removed correctly. Therefore, by measuring the current and the tension in the metal sheets 1, 2 at the beginning of the first current pulse applied, the thickness of the IDL of the two metal sheets 1 , 2 can be evaluated.

[0118] When using the steel sheets 1, 2 of the first exemplary embodiment (USIBOR 1500, 1.2 mm thickness, AS150 coating, 200 s in the oven) an average current level of 5 kA and an average voltage level of 2 V are used for the first pulse. The sampling frequency is 1.1kHz. At the beginning of the pulse, a first value of the dynamic resistance of 2217 pOhm is obtained. When using the steel sheets 1, 2 of the second exemplary embodiment (USIBOR 1500, 1.2mmthickness, AS80 coating, 200 s in the oven) an average current level of 4,92 kA and an average voltage level of 2 V are used for the first pulse. At the beginning of the pulse, a first value of the dynamic resistance of 2250 pOhm is obtained. When using the steel sheets 1, 2 of the third exemplary embodiment (USIBOR 1500, 1.2 mm thickness, AS80 coating, 600 s in the oven) an average current level of 5 kA and an average voltage level of 2.2 V are used. At the beginning of the pulse, a first value of the dynamic resistance of 2750 pOhm is obtained.

[0119] The third example is the one with a thicker IDL layer due to the longer oven time. In addition, as it can be seen, the dynamic resistance at the beginning of the pulse is higher (2750pOhm versus 2250 pOhm).

[0120] Other variables can subsequently be calculated to assess other properties of the RSW process. For example, following the diagram of figure 12, apart from the value of the dynamic resistance at the beginning of the current pulse, a value of the dynamic resistance at the end of the current pulse 23 is calculated. Said value is also illustrated in figure 6, on the right side. The last value of the dynamic resistance is calculated using the current and voltage values measured on the sheets 1, 2, just before the first current pulse descends to zero. The last value of the dynamic resistance is then compared with its corresponding tolerance band 24.

[0121] If the final dynamic resistance value calculated is inside the tolerance band 25, it is determined that the IDL thickness is adequate after the coating removing pulse, the first current pulse. It is also determined that the duration of the first current pulse is adequate. If the last value of the dynamic resistance is outside the tolerance band, it is determined if it is above the tolerance band 26. In this case it is determined that the IDL layer has been insufficiently removed 27 due to insufficient first current pulse duration and / or due to insufficient current level. If the dynamic resistance is below the tolerance band 28, it is determined that the first current pulse duration is excessive and / or that the current level of the first current pulse is excessive.

[0122] If the last value of the dynamic resistance is inside the tolerance band, then the second current pulse can be applied to weld the two sheets 1 , 2. If the last value of the dynamic resistance is outside the tolerance band, the second current pulse is not applied or must be corrected, to avoid a non-quality weld.

[0123] When using the steel sheets 1, 2 of the first exemplary embodiment (USIBOR 1500, 1.2mm thickness, AS150 coating, 200s in the oven) a dynamic resistance at the end of the current pulse of 370uOhm is obtained. When using the steel sheets 1, 2 of the second exemplaryembodiment (IISIBOR 1500, 1.2mm thickness, AS80 coating, 200s in the oven) a dynamic resistance at the end of the pulse of 371 Ohm is obtained. When using the steel sheets 1, 2 of the third exemplary embodiment (IISIBOR 1500, 1.2mm thickness, AS80 coating, 600s in the oven) a dynamic resistance at the pulse of 403pOhm is obtained.

[0124] The dynamic resistance at the end of the current pulse is very similar in all cases (370 pOhm, 371 pOhm and 403 pOhm). This means that the coating removing step has been successful in all cases, even though the IDL layer is much higher in one of them.

[0125] Following the diagram of figure 13, another parameter that can be calculated is the total energy during the first current pulse 29. Figure 7 illustrates said energy. If the total energy calculated is inside its corresponding tolerance band 30, it is determined that the heat input applied by the first current pulse is correct 31. If the total energy is above the tolerance band 32, it is determined that the duration of the first current pulse for the coating removal may be excessive and / or or the current level of the first current pulse may be excessive and / or there may be overheating or even burning 33. If the total energy is below the tolerance band, it is determined that the duration of the coating removal first current pulse may be insufficient and / or the current level may be insufficient 34.

[0126] When using the steel sheets 1, 2 of the first exemplary embodiment (IISIBOR 1500, 1.2mm thickness, AS150 coating, 200s in the oven) a total energy during the current pulse of 0.7kJ is obtained. When using the steel sheets 1, 2 of the second exemplary embodiment (IISIBOR 1500, 1.2mm thickness, AS80 coating, 200s in the oven) a total energy during the current pulse of 0.7kJ is obtained. When using the steel sheets 1, 2 of the third exemplary embodiment (IISIBOR 1500, 1.2mm thickness, AS80 coating, 600s in the oven) a total energy during the current pulse of 0.77kJ is obtained.

[0127] Following the diagram of figure 13, another variable that can be obtained is the maximum dV / dt value during the first current pulse 35, which is illustrated in figure 8. If the value is inside its tolerance band 36, it is determined that there is no material expulsion and no electrode sticking 37. If the value is not inside the tolerance band it is determined that there is material expulsion and / or there is insufficient electrode force and / or there is excessive current level or there is electrode sticking 38.

[0128] Another variable of interest, as illustrated in figure 14, is the dynamic resistance ripple during the first current pulse 39, which is illustrated in figure 9. If it is inside its tolerance band 40 it isdetermined that there is no electrode 3 wear and no electrode 3 misalignment 41. If it is outside the tolerance band, it is determined that there may be electrode / workpiece contamination, electrode 3 wear or electrode 3 misalignment 42.

[0129] A final variable that can be obtained is also illustrated in figure 14 and are the current parameters during the coating removal pulse 43, that is the first current pulse. These parameters are illustrated in figure 10 and are one or more of current pulse level 60, current pulse transition position 61 and current pulse transition duration 62, 63. If they are inside their respective tolerance bands 44, it is determined that the first current pulse is within nominal current values 45. If they are outside their tolerance bands, it is determined that the current pulse levels, the pulse transition positions and / or the pulse transition durations are outside the limits 46. Therefore, it is determined that the current values used for coating removal are not the nominal ones.

[0130] The tolerance bands previously mentioned can be obtained beforehand as illustrated in the flow diagram of figure 15. To do so, a first resistance spot welding is applied to two trial sheets 1, 2 under controlled conditions 50. These sheets 1, 2 have an IDL layer that is already known 51. Also, all the parameters of the weld are already known 52. The two sheets 1, 2 are spot welded under controlled conditions 53 and destructive and non-destructive testing is performed 54 to know whether the weld is correct. If it is correct 55, the parameters of interest (first value of the dynamic resistance, last value of the dynamic resistance, total energy, maximum dV / dt value, dynamic resistance ripple and current parameters) are calculated 56 from the current and voltage measured on the sheets 1, 2 during the application of the first current pulse and tolerance bands around reference values for all parameters are obtained 57. If the spot weld is not correct, another test is performed.

[0131] In the method of the invention, if any of the obtained parameters (first value of the dynamic resistance, last value of the dynamic resistance, total energy, maximum dV / dt value, dynamic resistance ripple and current parameters) is outside its corresponding tolerance band, an alert can be generated. This is to avoid proceeding with the application of the second current pulse and also proceeding with the welding, which in this case, given that some of the parameters are not within the tolerance range, would not comply with the quality criteria.

[0132] The numerical signs and corresponding components indicated in Fig. 1 to 15 are further listed below:

[0133] Upper sheet 1Lower sheet 2

[0134] Electrode 3

[0135] Nugget 4

[0136] First current pulse 5

[0137] Second current pulse 6

[0138] Computing means 7

[0139] Industrial installation 8

[0140] Coating 10

[0141] Press hardened steel 11

[0142] RSWwith DC current 12

[0143] Obtaining the raw signals 13

[0144] Welding machine files 14

[0145] Welding machine analog outputs 15

[0146] External sensors 16

[0147] Conditioning and processing 17

[0148] Calculation of variables of interest 18

[0149] Calculating the dynamic resistance at the beginning of the first current pulse 19 Comparing dynamic resistance at the beginning of the pulse with a tolerance band 20 IDL thickness is adequate 21

[0150] IDL thickness is not adequate 22

[0151] Calculation of dynamic resistance at the end of the current pulse 23

[0152] Comparing dynamic resistance at the end of the current pulse with its corresponding tolerance band 24.

[0153] The IDL thickness is adequate after the coating removing pulse 25

[0154] Determining if the dynamic resistance at the end of the current pulse is above the tolerance band 26

[0155] IDL layer insufficiently removed 27

[0156] Current pulse duration is excessive and / or that the current level of the first current pulse is excessive 28

[0157] Calculating total energy during the current pulse 29

[0158] Total energy inside its corresponding tolerance band 30

[0159] Heat input applied by the first current pulse is correct 31

[0160] Total energy is above the tolerance band 32

[0161] Duration of the first current pulse for the coating removal may be excessive and / or or the current level of the first current pulse may be excessive and / or there may be overheating or even burning 33Duration of the coating removal first current pulse may be insufficient and / or the current level may be insufficient 34

[0162] Calculating maximum dV / dt value during the first current pulse 35

[0163] Comparing dV / dt with its tolerance band 36

[0164] No material expulsion and no electrode sticking 37

[0165] Material expulsion and / or there is insufficient electrode force and / or there is excessive current level or there is electrode sticking 38

[0166] Calculating dynamic resistance ripple during the first current pulse 39

[0167] Comparing dynamic resistance ripple with its tolerance band 40

[0168] No electrode wear and no electrode misalignment 41

[0169] Electrode / workpiece contamination, electrode wear or electrode misalignment 42 Calculating the current parameters during the coating removal pulse 43

[0170] Comparing current parameters to their respective tolerance bands 44

[0171] First current pulse is within nominal current values 45

[0172] The current pulse levels, the pulse transition positions and / or the pulse transition durations are outside the limits 46

[0173] Controlled conditions 50

[0174] Known IDL layer 51

[0175] Known weld parameters 52

[0176] Spot welding under controlled conditions 53

[0177] Destructive and non-destructive testing is performed 54

[0178] Testing correct 55

[0179] Calculating variables of interest 56

[0180] Obtaining tolerance bands around reference values 57

[0181] Current pulse level 60

[0182] Current pulse transition position 61

[0183] Current pulse transition duration 62, 63.

Claims

24CLAIMS1.- A method for analysing a process of resistance spot welding, wherein the method comprises the steps of:-disposing two or more steel sheets (1, 2) one on another, such that a surface of one of the steel sheets (1) becomes in contact with another surface of the other steel sheet (2), wherein at least one of the contact surfaces of the steel sheets (1, 2) has an AlSi coating,- applying a current pulse with a certain period to the steel sheets (1, 2), thus removing the AlSi coating on a spot where the steel sheets (1, 2) are going to be welded, - measuring on the steel sheets (1, 2) a voltage and a current at the beginning of the current pulse applied,- from the measured voltage and the measured current, calculating a dynamic resistance at the beginning of the applied current pulse, and- evaluating the thickness of an interdiffusion layer of the contact surfaces of the steel sheets (1, 2) from the calculated dynamic resistance.2.- The method of claim 1, wherein to evaluate the thickness of the interdiffusion layer the calculated dynamic resistance at the beginning of the applied current pulse is compared to a predefined tolerance band.3.- The method of any of the previous claims, wherein the method further comprises:- measuring on the metal sheets (1, 2) a voltage and a current applied by the current pulse at the end of the current pulse,- from the voltage and the current at the end of the current pulse, calculating a dynamic resistance,- estimating the effectiveness of the coating removal from the calculated dynamic resistance at the end of the applied current pulse.4.- The method of claim 3, wherein to estimate the effectiveness of the AlSi coating removal the calculated dynamic resistance at the end of the applied current pulse is compared to a predefined tolerance band.5.- The method of any of the previous claims, wherein the method further comprises calculating a total energy applied during the current pulse with the voltage and the current measured and comparing it to a tolerance band.6.- The method of any of the previous claims wherein the method further comprises calculating the maximum value of the first derivative of the voltage curve of the measured voltage on the steel sheets (1, 2) during the application of the current pulse and comparing it to a tolerance band.7.- The method of any of the previous claims, wherein the method further comprises calculating the dynamic resistance ripple during the current pulse and comparing it to a tolerance band.8.- The method of any of the previous claims wherein the method further comprises calculating current parameters during the current pulse and comparing them to a tolerance band.9.- The method of any of the previous claims wherein the current parameters are selected from: current pulse levels (kA), current pulse transition positions (ms) and current pulse transition durations (ms).10.- The method of any of the previous claims wherein the method further comprises applying a second current pulse with a certain period to weld the steel sheets (1, 2).11.- An industrial installation comprising:a resistance spot welding module (8), configured to apply a current pulse with a certain period to two or more steel sheets (1 , 2) disposed one on another, such that a surface of one of the steel sheets (1) becomes in contact with a surface of another steel sheet (2), wherein at least one of the contact surfaces of the steel sheets (1 , 2) has an AlSi coating,sensor means, configured to measure on the steel sheets (1, 2) a voltage and a current applied by the current pulse, at the beginning of the current pulse,a computing means (7) configured to:- calculate a dynamic resistance at the beginning of the applied current pulse with the voltage and the current, and- evaluate the thickness of the interdiffusion layer of the contact surfaces of the steel sheets (1, 2) from the calculated dynamic resistance.

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