Resistance Spot Welding: 5T Dissimilar Steel Stack-ups for Automotive Applications

Resistance Spot Welding: 5T Dissimilar Steel Stack-ups for Automotive Applications

Urbanization and waning interest in vehicle ownership point to new transport opportunities in megacities around the world. Mobility as a Service (MaaS) – characterized by autonomous, ride-sharing-friendly EVs – can be the comfortable, economical, sustainable transport solution of choice thanks to the benefits that today’s steel offers.

The WorldAutoSteel organization is working on the Steel E-Motive program, which delivers autonomous ride-sharing vehicle concepts enabled by Advanced High-Strength Steel (AHSS) products and technologies.

The Body structure design for this vehicle is shown in Figure 1. It also indicates the specific joint configuration of 5 layers AHSS sheet stack-up as shown in Table 1. Resistance spot welding parameters were developed to allow this joint to be made by a single weld. (The previous solution for this welded joint is to create one spot weld with the bottom 3 sheets indicated in the table and a second weld to join the top 2 sheets, combining the two-layer groups to 5T stack-up.)

NOTE: Click this link to read a previous AHSS Insights blog that summarizes development work and recommendations for resistance spot welding 3T and 4T AHSS stack-ups: https://bit.ly/42Alib8

 

 

Table 1. Provided materials organized in stack-up formation showing part number, name, grade, gauge in mm, and coating type. Total thickness = 6.8 mm

 

The same approach of utilizing multiple current pulses with short cool time in between the pulses was shown to be most effective in this case of 5T stack-up. It is important to note that in some cases, the application of a secondary force was shown to be beneficial, however, it was not used in this example.

To establish initial welding parameters simulations were conducted using the Simufact software by Hexagon. As shown in Figure 2, the final setup included a set of welding electrodes that clamped the 5-layer AHSS stack-up. Several simulations were created with a designated set of welding parameters of current, time, number of pulses, and electrode force.

Figure 2. Example of simulation and experimental results showing acceptable 5T resistance spot weld (Meets AWS Automotive specifications)

 

 

Thanks is given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University and Technical Editor – Joining, AHSS Application Guidelines, for this article.

 

Resistance Spot Welding: 3T and 4T Dissimilar Steel Stack-ups for Automotive Applications

Resistance Spot Welding: 3T and 4T Dissimilar Steel Stack-ups for Automotive Applications

Car body-in-white (BIW) structures, such as pillars and rails, are increasingly made of complex stack-ups of advanced high-strength steels (AHSS) for vehicle lightweighting to achieve improved fuel efficiency and crashworthiness. Complex stack-ups comprise more than two sheets with similar/dissimilar steels and non-equal sheet thicknesses. 

Resistance spot welding (RSW) of complex stack-ups can be challenging, especially when a thin sheet of low-strength steel is attached to multiple thick AHSS sheets with a thickness ratio of five or higher (thickness ratio = total thickness of the stack-up/thickness of the thinnest sheet). In such a case, the heat loss is much faster on the thin sheet side than on the thick sheet side, and consequently, obtaining sufficient penetration into the thin sheet without expulsion on the thick sheet side can be challenging.

An example of two automotive applications involving complex AHSS steel stack-ups is shown below.

 

automotive applications involving complex AHSS steel stack-ups

Examples of automotive applications involving complex AHSS steel stack-ups

 

For welding 2T steel stack-ups, the weld schedule may be relatively simple and utilize just one current pulse with a specific weld time. However, typical RSW machines and controllers can customize and precisely control each parameter indicated in Figure 1.  

Spot Welding Schedule/Cycle

Figure 1: General Description of Resistance Spot Welding Schedule

 

For RSW 3T and 4T applications, more advanced schedules are needed to achieve good weld nugget penetration through all the interfaces in the stack-up. To achieve this objective,  the use of multiple current pulses with short cool time in between the pulses showed to be most effective, and in some cases, the application of a secondary force showed to be beneficial.

Figure 2 describes a method for joining the 3T stack-up using two current pulses. The first one is a short-time pulse that does not allow enough time for the electrode cooling to dominate at the top sheet, so a weld can easily form between the top and middle sheet. Once that nugget has formed, the second pulse utilizes a lower current and longer time to form the second nugget, which then grows into the first nugget to form a single weld. 

This approach can be also used with electrode force variation during the welding cycle to provide additional control of the contact resistances, but of course, it is limited to machines that are capable of varying force during the weld cycle.

Typical pulse times are 50 – 350 ms with cool times of 20 – 35 ms and current levels between 8 – 15 KA, depending on materials type and thickness. 

 

RSW Schedule for Joining 3T Stack-Up Using 2 Current Pulses

Figure 2: Example of RSW Schedule for Joining 3T Stack-Up Using 2 Current Pulses

 

A 4T stack-up example is shown in Figure 3. In this case, a similar approach was used with three current pulses applied during the weld cycle to produce a weld through all interfaces. 

The common theme in resistance spot welding all complex stack-ups is using a complex, multi-pulse weld cycle. These more complex schedules should be developed experimentally and potentially with computational modeling. Another consideration that may be beneficial in some cases is to vary the top and bottom electrode face diameter, such as that the smaller electrode face is on the thinner material side of the stack-up.

 

RSW Schedule for Joining 4T Stack-Up Using 3 Current Pulses

Figure 3: Example of an RSW Schedule for Joining 4T Stack-Up Using 3 Current Pulses

 

 

 

Thanks is given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University and Technical Editor – Joining, AHSS Application Guidelines, for this article.

Process, Microstructure and Fracture Mode of Thick Stack-Ups of Aluminum Alloy to AHSS Dissimilar Spot Joints

Process, Microstructure and Fracture Mode of Thick Stack-Ups of Aluminum Alloy to AHSS Dissimilar Spot Joints

This article summarizes a paper entitled, “Process, Microstructure and Fracture Mode of Thick Stack-Ups of Aluminum Alloy to AHSS Dissimilar Metal Spot Joints”, by Luke Walker, Colleen Hilla, Menachem Kimchi, and Wei Zhang, Department of Materials Science and Engineering, The Ohio State University.W-9

Researchers at The Ohio State University studied the effects of adding a stainless steel (SS) insert to a dissimilar Advanced High-Strength Steel (AHSS) to aluminum (Al) resistance spot weld (RSW). The SS insert was ultrasonically welded to the Al sheet prior to the RSW being performed. The purpose of the SS is to reduce the intermetallic layer that forms when welding steel to aluminum. This process increases the strength and toughness of the weld. In this study, the process is applied to three sheet (3T) stack up that contains one Al sheet and two 1.2 mm thick Press Hardened (PH) 1500 sheets. The joint strength is measured in lap shear testing and the intermetallic thickness/ morphology is studied after cross sectioning the welds.

During the microstructure evaluation it was noted that Al 6022  contained a larger nugget diameter as compared to the Al 5052 welds. A few potential reasons for the hotter welds were proposed including cleanliness of the electrodes, surface oxides, and thickness of the different alloys. The welds on the Al 5052 stack ups were made first on clean electrodes whereas the Al 6022 was made on potentially dirty electrodes that increased the contact resistance. The effects of different surface oxides are not likely given the SS sheet is ultrasonically welded but could still add to the higher heat input in the RSW. The Al 6022  is 0.2 mm thicker, which could increase the bulk resistance and decrease the cooling effect from the electrodes.

The 3T welds likely had much lower strength and toughness due to cracks that formed at the Al-SS insert interface. These can be attributed to an increase in intermetallic compound (IMC) thickness and the residual stress caused by the forge force. The IMC thickness was measured two ways:  The first measurement was of the continuous IMC layer and the second was from the Al-Fe interface to the end of the IMC dendrites (Figure 1, 2 and Table 1). The Al 5052 observed the thickest continuous IMC layer but Al 6022 was close to the Al 5052 thickness. This can be attributed to the increased Si content of Al 6022 which has been shown to decrease the growth of Fe-Al intermetallics.

Figure 10: IMC in the Al Alloy 5052 to Stainless-Steel Weld.W-9

Figure 1: IMC in the Al Alloy 5052 to Stainless-Steel Weld.W-9

 

Figure 2: IMC in the Al Alloy 6022 to Stainless-Steel Weld.W-9

Figure 2: IMC in the Al Alloy 6022 to Stainless-Steel Weld.W-9

 

Table 1: IMC Thickness of Both the 5052 Weld and the 6022 Weld.W-9

Table 1: IMC Thickness of Both the 5052 Weld and the 6022 Weld.W-9

 

 

Referencing Figure 3, the 2T stack-up has a higher tensile strength as well as significantly higher fracture energy absorbed compared to the 3T stack-ups. This is mainly attributed to the failure mode observed in the different stack-ups. The 2T welds had button pullout failure while 3T stack-ups had interfacial Failure.

Figure 3: Failure Load and Fracture Energy [(A) Al to steel (Al-Us) welds and (B) steel to steel (Us-Us) welds (the 2T 6022 results are from previous work(10))]W-9

Figure 3: Failure Load and Fracture Energy [(A) Al to steel (Al-Us) welds and (B) steel to steel (Us-Us) welds (the 2T 6022 results are from previous work(10))]W-9

The Al 6022 contains higher Si content which likely decreased the growth of the continuous IMC layer but not the overall IMC layer (as seen in Figure 4 and Figure 5) due to higher weld temperatures. The joint strength of the welds in the 3T stack-ups were closer to the expected weld strength unless there was expulsion that caused a 5-kN drop in strength.

Figure 12: EDS Line Scan of the IMC in Location 2 on the 5052 3T Sample (SS stands for austenitic stainless steel 316).W-9

Figure 4: EDS Line Scan of the IMC in Location 2 on the 5052 3T Sample (SS stands for austenitic stainless steel 316).W-9

 

Figure 13. EDS Line Scan of the Intermetallic Layer at Location 1 on the 6022 3T Sample (SS stands for austenitic stainless steel 316).W-9

Figure 5: EDS Line Scan of the Intermetallic Layer at Location 1 on the 6022 3T Sample (SS stands for austenitic stainless steel 316).W-9

 

 

Improvement of Delayed Cracking in Laser Weld of AHSS and 980 3rd Gen AHSS

Improvement of Delayed Cracking in Laser Weld of AHSS and 980 3rd Gen AHSS

This article is a summary of the paper entitled, “IMPROVEMENT OF DELAYED CRACKING IN LASER WELD OF AHSS AND 980GEN3 STEELS”, by Linlin Jiang, Kyle Kram, and Chonghua Jiang.L-62

Researchers from AET Integration, Inc. worked on developing welding techniques that mitigated delayed cracking issues when laser welding Advanced High-Strength Steels (AHSS) and 3rd Gen AHSS. Stitch welds are most likely to have delayed cracking issues therefore the test specimen were cross tension samples with stitch welds. The test was run with ten specimens for each material with the same parameters. The specimen were visually inspected after welding in intervals from 30 min to 12 hours for 48 hours. The specimen were then inspected using X-ray analysis, cross-sectioning, and scanning electron microscope (SEM) analysis. Cracks were observed in the weld schedules used in Figure 1 and 2, and no cracking was observed in the welding schedules developed by researchers as seen in Figure 3 and 4 by altering parameters such as laser power, travel speed, beam oscillation, and defocusing techniques. The techniques are currently patent-pending and were not discussed in the research paper. Further validation tests on 3rd Gen steels is expected in the future.

Figure 1: Delayed Crack in Weld of Material A. (Top) Crack from top surface, (Bottom) Cross Section of Crack.L-62

Figure 1: Delayed Crack in Weld of Material A. (Top) Crack from top surface, (Bottom) Cross Section of Crack.L-62

 

Figure 6: Delayed Cracking in Weld of Material B, (Top) X-ray, (Bottom) Cross section of crack.L-62

Figure 6: Delayed Cracking in Weld of Material B, (Top) X-ray, (Bottom) Cross section of crack.L-62

 

Figure 3: Examples of Visual Inspection Photographs. (Top) Top surface, Material A, (Middle) Top surface, Material B, (Bottom) Top surface, Material C.L-62

Figure 3: Examples of Visual Inspection Photographs. (Top) Top surface, Material A, (Middle) Top surface, Material B, (Bottom) Top surface, Material C.L-62

 

Figure 4: Examples of Cross Section Photographs. (Top) Cross section of weld center, Material A; (Bottom) Cross section of weld crater, Material A.L-62

Figure 4: Examples of Cross Section Photographs. (Top) Cross section of weld center, Material A; (Bottom) Cross section of weld crater, Material A.L-62

 

 

 

Hot cracking investigation in HSS laser welding with multi-scale modelling approach

Hot cracking investigation in HSS laser welding with multi-scale modelling approach

This article summarizes the findings of a paper entitled, “Hot cracking investigation during laser welding of high-strength steels with multi-scale modelling approach”, by H. Gao, G. Agarwal, M. Amirthalingam, M. J. M. Hermans.G-4

Researchers at Delft University of Technology (TU Delft) in The Netherlands and Indian Institute of Technology Madras in India attempted to model Hot Cracking susceptibility in TRIP and DP steels. For this experiment, TRIP and DP steels are laser welded and the temperatures experienced are recorded with thermocouples at three positions. Temperatures experienced during welding are measured and used to validate a finite element model which is then used to extract the thermal gradient and cooling rate to be used as boundary conditions in a phase field model. The phase field model is used to simulate microstructural evolution during welding and specifically during solidification. The simulation and experimental data had good agreement with max temperature deviation below 4%.

Summary

Referencing Figure 1 (Figure 6 in the original paper) which shows the microstructure where the dendritic tips meet the wel, centerline, it is observed that TRIP steels reach a solid fraction of 93.7% and DP steels reach a solid fraction of 96.3% meaning that TRIP steels have a larger solidification range than the DP steels. Figure 8 shows the phosphorus distribution where the dendritic tips reach the weld centerline. TRIP steels show a concentration of up to 0.55 wt-% where segregation occurs compared to the original composition of 0.089 wt-%. DP steels show a max of 0.06 wt-% which is significantly lower than the TRIP steels. In addition to phosphorus, Al is seen in high concentration in TRIP steels which contributes to the broder solidification range. A pressure drop is the last factor contributing to the Hot Cracking observed in TRIP steels(figure 2). The pressure drop is due to a lack of extra liquid feeding in the channels and forms a pressure difference from the dendrite tip to root. The pressure drop in TRIP steels is calculates to 941.2 kPa and 10.2 kPa in DP steels. The combination of element segregation, pressure drop, and thermal tensile stresses induced during laser welding results in a higher Hot Cracking susceptibility in TRIP steels as compared to DP steels.

Figure 1: Phase distributions in the TRIP and the DP steel when the dendritic tips reach the weld centreline.G-4

Figure 1: Phase distributions in the TRIP and the DP steel when the dendritic tips reach the weld centreline.G-4

 

Hybrid Laser-Arc Welding (HLAW) Pore Formation and Prevention

Hybrid Laser-Arc Welding (HLAW) Pore Formation and Prevention

This is a summary of a paper by M. Mazar Atabaki, et al, entitled, “Pore formation and its mitigation during hybrid laser/arc welding of advanced high strength steel”M-66, referenced by permission.

 

Hybrid laser-arc welding (HLAW) is a popular welding process for thick sections. HLAW combines the high penetration depth and speed of laser welding with the volume and high deposition of gas metal arc welding (Figure 1).

Figure 1: HLAW Diagram

Figure 1: HLAW Diagram.M-66

 

A common issue with this process is the formation of pores, which researchers from Southern Methodist University in Dallas addressed by optimizing various variables to reduce the presence of pores. HLAW was used to weld two coupons of Advanced High-Strength Steel (AHSS) with a carbon equivalent (CE) of 0.75 wt% (Figure 2). The pores appeared to be caused by two different sources. One type of pore appeared to form when the filling rate of the collapsing keyhole is slower than the solidification rate of the molten material. This left voids (pores) in the material. Other pores were caused by the trapping of shielding gas in the weld pool, a result of the fast solidification rate of laser welding.

Figure 2: Pores in weldment.M-66

Figure 2: Pores in weldment.M-66

 

To address the porosity, four variables were focused on:

  1. Increasing heat input in HLAW; creating a larger weld pool and slower solidification rates, giving more time for gases to escape from the weld pool.
  2. Optimization of the stand-off distance of the filler wire.
  3. Application of a side shielding gas.
  4. Use of a hot wire.

The researchers also found that the presence of certain alloying elements could affect the stability of the keyhole (alloys with a low boiling point). It was also important to find the optimum stand off distance for the wire. When the filler wire was too close, the arc got in the way of the laser and reduced power to the keyhole, causing instability. When a hot wire was used, the fusion zone microstructure consisted of martensite and bainite. Fracture testing showed brittle fracture surface with indications of gas entrapment, emphasizing the need to diffuse gases out of the molten weld pool.

The researchers found that the optimum stand-off distance between the laser and the arc was 8mm. At this distance, the molten weld pool was allowed to widen, which helped with degassing the liquid metal. When combined with a 92%Ar/8%CO side shielding gas, much of the porosity was alleviated. The use of a hot wire also improved the stability of the weld pool reducing some of the longitudinal porosity.