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
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
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
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%.
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
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.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
To address the porosity, four variables were focused on:
- 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.
- Optimization of the stand-off distance of the filler wire.
- Application of a side shielding gas.
- 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.
This is a summary of a paper of the same title, authored by K. Májlinger, E. Kalácska, and P. Russo Spena, used by permission.M-65
Researchers at the Budapest University of Technology and Economics and the Free University of Bozen-Bolzano tested gas metal arc welding (GMAW) of dissimilar Advanced High-Strength Steel (AHSS) sheets.M-65 The test pieces were 100 x 50 mm samples of 1.4 mm TWIP (TWIP1000) and 0.9 mm TRIP (HCTC800T) sheet steels were welded in a lap joint configuration with 0.8 mm diameter AWS ER307Si austenitic stainless steel wire to determine appropriate GMAW parameters for good quality welds. Quality was determined by external appearance, microstructure, and mechanical properties. Good welds were achieved with linear heat inputs (Q) with ranges from 500-650 kJ/m. The only fractures that occurred appeared within the weld bead by ductile failure modes. The HAZ of the TWIP steel showed grain coarsening and the HAZ of the TRIP steel experienced microstructural changes relative to the distance from the fusion boundary. The ultimate tensile strength (UTS) varies between 73%-84% of the weaker of the two steels.
Welding was conducted with an automated linear drive system with pure Argon (99.996% Ar) shielding gas at 10L/min. Wire feed rates were approximately 3.5 m/min with Direct Current Electrode Positive (DCEP) polarity. Changes in current, voltage, weld speed, and the resulting linear energy are compared in Table 1.
Figure 1: Overview of Dissimilar AHSS GMAW Welding.M-65
Table 1: Results of the preliminary welding tests in terms of TWIP-TRIP joint quality.M-65
After welding, transverse sections were cut from the welds and etched to show the microstructure. Vickers hardness testing was conducted on the weld samples based on the ASTM E384 standard. Tensile tests were performed on the samples according to the EN ISO 6892-1 standard. Tests were also conducted on unwelded TWIP and TRIP steels for comparison. Scanning electron microscopy (SEM) examinations were made of fracture surfaces to determine failure modes and examine for microscopic weld defects.
The study concluded that dissimilar welds between AHSS steels with the GMAW process can be achieved with consistent results desired for automotive applications.
Figure 2: Vickers Hardness Across Weldment.M-65
Figure 3: Ductile Failure in the fragile zones (FZ).M-65
The unique physical characteristics of Advanced High Strength Steels (AHSS) present some challenges to welding and bonding processes. AHSS differ from mild steels by chemical composition and microstructure, and it’s important to note that their microstructures will change from welding operations. For example, intensive localized heat associated with some welding processes causes a significant change in the local microstructure, and hence affect properties. Due to fast Cooling Rates (CR) typical in welding, it is normal to see martensite and/or bainite microstructures in the weld metal and in the Heat-Affected Zone (HAZ).
When joining AHSS, production process control is important for successful assembly. Manufacturers with highly developed joining control methodology will experience no major change in their operations. Others may require additional checks and maintenance. In certain instances, modifications to equipment or processing methodologies may be required for successful joining of AHSS.
The coating methods for AHSS are similar to those for mild steels. Welding of either AHSS or mild steels with coatings will generate fumes. The amount and nature of fumes will depend on the coating thickness, coating composition, joining method, and fillers used to join these materials. The fumes may contain some pollutants. The chemical composition of fumes and the relevant exhaust equipment must meet appropriate regulatory standards. Thicker coatings and higher heat inputs cause more fumes. Additional exhaust systems should be installed. While welding AHSS (with or without metallic or organic coatings and oiled or not oiled) gases and weld fumes are created similar to mild steels. The allowed fumes or gases must comply with respective national rules and regulations.
Considering recent developments of hybrid approaches to welding, there are now over 100 types of welding processes available for the manufacturer or fabricator to choose from. The reason that there are so many processes is that each process has its list of advantages and disadvantages that make it more or less appropriate for a given application. Arc welding processes offer advantages such as portability and low cost but are relatively slow and use a considerable amount of heating to produce the weld. High energy density processes such as laser welding generally produce low heat inputs and fast welding speeds, but the equipment is very expensive and joint fit-up needs to be ideal. Solid-state welding processes avoid many of the weld discontinuities produced by those requiring melting (fusion), but they may be expensive and often are restricted to limited joint designs. Resistance welding processes are typically very fast and require no additional filler materials but are often limited to thin sheet applications or very high-production applications such as in the automotive industry.
Most welding processes produce a weld (metallic bond) using some combination of heat, time, and/or pressure. Those that rely on extreme heat at the source such as arc and high energy density processes generally need no pressure and relatively small-to-medium amounts of time. This section introduces joining processes that are applicable to HSS automotive applications, while describing unique process attributes.