AHSS, Blog, homepage-featured-top, Joining, Joining Dissimilar Materials, main-blog, Steel Grades
The discussions relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid formed.
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous factors. This blog covers some important considerations related to welding these materials for automotive applications. Most important is the discussion on Resistance Spot Welding (RSW) as it is the dominating process in automotive manufacturing.
Setting Correct Welding Parameters for Resistance Spot Welding
Specific welding parameters need to be developed for each combination of material type and thickness. In general, the Hot Press (HP) steels require more demanding process conditions. One important factor is electrode force which should be higher for the HP steel than for cold press type steel of the same thickness. The actual recommended force will depend on the strength level, and the thickness of the steel. Of course, this will affect the welding machine/welding gun force capability requirement.
Another important variable is the welding current level and even more important is the current range at which acceptable welds can be made. The current range is weldability measurement, and the best indicator of the welding process robustness in the manufacturing environment and sometime called proceed window. Note the relative range of current for different steel types. A smaller process window may require more frequent weld quality evaluation such as for weld size.
Relative Current Range (process windows) for Different Steel Types
The Effect of Coating Type on Weldability
In all cases of resistance spot welding coated steels, it is imperative to move the coating away from the weld area during and in the beginning of the weld cycle to allow a steel-to-steel weld to occur. The combination of welding current, weld time and electrode force are responsible for this coating displacement.
For all the coated steels, the ability of the coating to flow is a function of the coating type and properties, such as electrical resistivity and melting point, as well as the coating thickness.
An example of cross sectioned spot welds made on Hot Press Steel with Aluminum -Silicon coating is shown below. It shows two coating thicknesses and the displaced coating at the periphery of weld. This figure also shows the difference in current range for the different coating thickness. The thicker coating shows a smaller current range. In addition, the Al-Si coating has a much higher melting point than the zinc coatings on the cold stamped steels, making it more difficult to displace from the weld area.
Hot Press Steel with Aluminum -Silicon
Liquid Metal Embrittlement and Resistance Spot Welding
Cold-formable, coated, Advance High Strength Steels such as the 3rd Generation Advanced High Strength Steels are being widely used in automotive applications. One welding issue these materials encounter is the increased hardness in the weld area, that sometime results in brittle fracture of the weld.
Another issue is their sensitivity to Liquid Metal Embrittlement (LME) cracking. These two issues are discussed in detail on the WorldAutoSteel AHSS Guidelines website and our recently released Phase 2 Report on LME.
Resistance Spot Welding Using Current Pulsation
The most effective solution for the issues described above is using current pulsation during the welding cycle. A schematic description is shown below.
The pulsation allows much better control of the heat generation and the weld nugget development. The pulsation variables include the number of pulses (typically 2-4), the current level and time for each pulse, and the cool time between the pulses.
In summery, pulsation (and sometime current upslope) in Resistance Spot Welding proved to be beneficial for the following applications:
- Coated Cold Stamped steels
- Cold stamped Advance High Strength Steels
- Multi materials stack-ups – As described in our articles here on 3T/4T and 5T Stack-Ups
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.
Blog, homepage-featured-top, Joining, Joining Dissimilar Materials, main-blog, News, Resistance Spot Welding, Resistance Welding Processes, RSW Modelling and Performance, RSW of Dissimilar Steel
This blog is a short summary of a published comprehensive research work titled: “Peculiar Roles of Nickel Diffusion in Intermetallic Compound Formation at the Dissimilar Metal Interface of Magnesium to Steel Spot Welds” Authored by Luke Walker, Carolin Fink, Colleen Hilla, Ying Lu, and Wei Zhang; Department of Materials Science and Engineering, The Ohio State University
*****
There is an increased need to join magnesium alloys to high-strength steels to create multi-material lightweight body structures for fuel-efficient vehicles. Lightweight vehicle structures are essential for not only improving the fuel economy of internal combustion engine automobiles but also increasing the driving range of electric vehicles by offsetting the weight of power systems like batteries.
To create these structures, lightweight metals, such as magnesium (Mg) alloys, have been incorporated into vehicle designs where they are joined to high strength steels. It is desirable to produce a metallurgical bond between Mg alloys and steels using welding. However, many dissimilar metal joints form intermetallic compounds (IMCs) that are detrimental to joint ductility and strength. Ultrasonic interlayered resistance spot welding (Ulti-RSW) is a newly developed process that has been used to create strong dissimilar joints between aluminum alloys and high-strength steels. It is a two-step process where the light metal (e.g., Al or Mg alloy) is first welded to an interlayer (or insert) material by ultrasonic spot welding (USW). Ultrasonic vibration removes surface oxides and other contaminates, producing metal-to-metal contact and, consequently, a metallurgical bond between the dissimilar metals. In the second step, the insert side of the light metal is welded to steel by the standard resistance spot welding (RSW) process.
Cross-section View Schematics of Ulti-RSW Process Development
For resistance spot welding of interlayered Mg to steel, the initial schedule attempted was a simple single pulse weld schedule that was based on what was used in our previous study for Ulti-RSW of aluminum alloy to steel . However, this single pulse weld schedule was unable to create a weld between the steel sheet and the insert when joining to Mg. Two alternative schedules were then attempted; both were aimed at increasing the heat generation at the steel-insert interface. The first alternative schedule utilized two current pulses with Pulse 1, high current displacing surface coating and oxides and Pulse 2 growing the nugget. The other pulsation schedule had two equal current pulses in terms of current and welding time.
Lap shear tensile testing was used to evaluate the joint strength using the stack-up schematically, shown below. Note the images of Mg and steel sides of a weld produced by Ulti-RSW.
Lap Shear Tensile Test Geometry and the Resultant Weld Nuggets
An example of a welded sample showed a distinct feature of the weld that is comprised of two nuggets separated by the insert: the steel nugget formed from the melting of steel and insert and the Mg nugget brazed onto the unmelted insert. This feature is the same as that of the Al-steel weld produced by Ulti-RSW in our previous work. Although the steel nugget has a smaller diameter than the Mg nugget, it is stronger than the latter, so the failure occurred on the Mg sheet side.
Joint strength depends on several factors, including base metal strength, sheet thickness, and nugget size, making it difficult to compare how strong a weld truly is from one process to another. To better compare the dissimilar joints created by different processes, joint efficiency, a “normalized” quantity was calculated for various processes used for dissimilar joining of Mg alloys to steels in the literature, and those results, along with the efficiencies of Ulti-RSW with inserts, are shown together below. Most of the literature studies also used AZ31 as the magnesium base metal. The ones with high joint efficiency (about 53%) in the literature are resistance element welding (REW) and friction stir spot welding (FSSW). In our study, Ulti-RSW with SS316 insert was able to reach an excellent joint efficiency of 71.3%, almost 20% higher than other processes.
Process Evaluation and Comparison
Thanks are given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University, and Technical Editor – Joining, AHSS Application Guidelines, for this article.
AHSS, Blog, homepage-featured-top, Joining, main-blog, News, Resistance Spot Welding, RSW of Dissimilar Steel, Steel Grades, Tool & Die Professionals
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.
Automotive Welding Process Comparison, Blog, Joining
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.
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.
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.
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.
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.
Resistance Welding Steel to Aluminium
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 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
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](https://ahssinsights.org/wp-content/uploads/2021/12/fig4-process.jpg)
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 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 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
Laser Welding
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 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 4: Examples of Cross Section Photographs. (Top) Cross section of weld center, Material A; (Bottom) Cross section of weld crater, Material A.L-62
