 Schedule 1 (MRD/ballnose), Sigler et al. (Refs. 10, 11) used an MRD electrode on the aluminum side only and a ballnose electrode on the steel side (asymmetric or hybrid electrodes), while in Schedule 2, MRD electrodes were used on both the aluminum and steel sides of the stackup (symmetric electrodes). The weld schedules were based on the multiple solidification welding template developed by Sigler et al. (Refs. 7,12). In Stage 1, a large weld nugget was created with considerable penetration into the aluminum alloy sheet. The solidification of the weld nugget also solidified defects along the intermetallic layer that formed between the aluminum and steel. These defects included gas porosity, shrinkage porosity, and microcracking from remnants of oxide film incorporated into the weld nugget. After solidification, current flow was restarted and a relatively thin layer of aluminum melted adjacent to the intermetallic layer incorporating the defects previously described. Upon resolidification, porosity was consolidated close to the nugget center while the oxide film defects were broken up and dispersed within the nugget.
The two aluminum-steel welding schedules were designed to work in combination with the chosen electrodes. Under load, stresses are highest at the notch root of a resist-ance spot weld. These weld schedule and electrode combinations were designed to create quality weld nuggets with a thin intermetallic layer along the weld nugget periphery.
As a comparison for tensile shear testing, the 1.2-mm AA6022-T4 sheet material was welded to itself using MRD electrodes, and a weld schedule was developed specifically for spot welding aluminum (Ref. 13). This aluminum weld schedule was designed to initially heat the workpieces to seat the electrodes (conditioning), form the aluminum weld nugget at the faying interface (chaping), and grow the aluminum weld nugget to the desired size (sizing). Weld size was targeted at 5.5 to 6.0 mm, which would represent typical weld sizes in production. After welding, half of the specimens were baked to WELDING RESEARCH MARCH 2020 / WELDING JOURNAL 69-s

Metallographic Analysis
Metallography, optical, and scanning electron microscopy, along with FIB and TEM, were used to reveal the structure of the intermetallic layer at the interface. In addition, microhardness measurements and quasistatic tensile shear testing were performed. The details of these experiments were described in previous studies by Kang

Mini-Shear Tests and Analysis
A novel mini-shear test specimen was designed and presented in Fig. 1. The test was developed to ensure that shear fracture occurred at the interface of the weld nugget without tensile fracture occurring in either the aluminum alloy weld nugget or the heat-affected zone (HAZ), which are the weak locations, i.e., where L, W, t Al nugget,  IMC, and  Al nugget are the length and width of the shear zone, thickness of the aluminum weld nugget, shear strength of the intermetallic layer, and tensile strength of the aluminum nugget, respectively.
Assuming isotropy and von Mises yield criterion for the intermetallic layer and rearranging Equation 1 gives where  = 0.10 -0.15 is the indentation ratio, i.e., the thickness reduction in the aluminum side for the MRD electrode,  IMC is the tensile strength of the intermetallic layer, and  Al nugget / IMC is usually greater than 1, i.e., the tensile strength of the aluminum weld nugget is greater than that of the intermetallic layer. The strength of the aluminum weld nugget is dependent, in part, on the thickness of the aluminum nugget. During spot welding, the aluminum deforms and "indents" as heat is applied, reducing the aluminum thickness. This indentation was approximated as . Thus, the final result in Equation 2, (1 -) √3 t Al, gives a more conservative estimation of the shear zone length L.
Based on Equation 2, W = 2 mm and L = 1 mm were chosen for the shear zone size of all the mini-shear test specimens used in the present study where t Al is 1.2 mm. The mini-shear specimens were prepared using wire electrical discharge ma-

A B C
chining. Since the high stress region is at the weld nugget periphery adjacent to the notch root, the shear zone (2 × 1 mm) was entirely positioned at that location for all the resistance spot welds tested.
The mini-shear test specimens were loaded in tension using a standard MTS LandMark® 370 load frame with a 5-kN load cell at a rate of 0.3 mm/min until specimen fracture occurred. After mini-shear testing, specimens were collected for fractography and energy dispersive spectroscopy (EDS) mapping completed on both the aluminum and steel sides using a field emission scanning electron microscope (FESEM). Figure 2A, B are macrostructure images and the associated microhardness measurements of AA6022-T4 to IF steel resistance spot welds using two different multiple solidification welding schedules. The welds in this study exhibited structures that are typical for aluminum-steel (Al-steel) resistance spot welds as described by Sigler and Carlson (Ref. 1). Al-steel resistance spotwelds exhibited a planar interface with a large aluminum weld nugget that wets the steel surface, which is consistent with the results shown in Sigler and Carlson (Ref. 1). Defects such as gas porosity, shrinkage porosity, or cracking were not observed within the aluminum weld nugget. Figure 3 is a macrostructure image of a joint produced by interrupting the full, multistage weld schedule, Schedule 1 (MRD/ballnose), following Stage 1 and before Stage 2 of the weld schedule. The arrows highlight the interfacial porosity at the faying interface after Stage 1. This porosity is not evident in the macrostructure presented in Fig. 2, which shows a weld structure after both Stage 1 and 2 of the weld schedule. This demonstrates the effectiveness of the complete two-stage multiple solidification weld schedule in consolidating and eliminating porosity near the faying interface. A thin, continuous layer of intermetallic compounds formed at the faying interface between the aluminum and steel sheets. From Fig. 2, the average nugget diameter at the faying interface was measured to be 8.6 and 8.3 mm for AA6022-T4 to IF steel resistance spot welds using Schedule 1 and 2, respectively.

Maximum Tensile Loads
The average maximum tensile loads are directly compared to the nominal shear strength in Fig. 4. To calculate the nominal shear strength, the average maximum tensile loads were normalized by the weld nugget area using the weld nugget diameter from tensile shear tests of the welded AA6022-T4 to IF steel RSW stackups. The Al-steel resistance spot welds exhibited greater average maximum tensile loads than the analogous Al resistance spot welds. This is due, in part, to the larger aluminum nugget diameters (8-9 mm) that develop in Al-steel welds as compared to the standard, smaller Al RSW nugget diameters (5-6 mm). It is difficult to grow Al resistance spot welds to the larger nugget diameters due to aluminum's increased electrical and thermal conductivities, which leads to an increased propensity for internal expulsion with increasing current and weld time. Furthermore, to meet requirements for fatigue and other configurations of static mechanical tests, i.e., coach peel and cross tension, Al-steel resistance spot welds require larger nugget diameters than Al resistance spot welds or even those outlined for steel resistance spot welds (d = 4√t) in Ref. 16. This is due to the fact that, in Al-steel resistance spot welds, the weld nugget is contained only within the aluminum sheet and is separated from the steel sheet by a layer of intermetallics; there is no mixing of metal between the aluminum and steel sheets. Thus, it is necessary to have a greater load bearing area in Al-steel resistance spot welds to achieve acceptable strengths. Growing large aluminum nuggets in Al-steel resistance spot welds is relatively easy due to the resistive heat that is developed within the steel sheet. Once normalized for weld diameter, the Al-steel and Al resistance spot welds exhibited approximately equivalent nominal shear strengths. The baked Al-steel resistance spot welds from Schedule 1 WELDING RESEARCH MARCH 2020 / WELDING JOURNAL 71-s   Figure 6 compares the measured shear strength of the intermetallic layer formed at the interface of the Al-steel welds under the two different welding conditions. These results were taken from the perimeter of the weld nugget (refer to Fig. 1). The results show the intermetallic layer had an average shear strength of 64 MPa irrespective of welding condition. The results of EDS maps of fracture surfaces of the tested mini-shear specimens (Fig. 7) confirmed that fracture occurred within the intermetallic layer; thus, the measured strength was indeed the strength of the intermetallic layers.

Advantages of Mini-Shear Specimens
There is presently no standard procedure to directly measure the