Joining in automobile construction part 2

The welding parameters used to face the primary standards on the welding spot vary greatly depending on the automotive manufacturers. Those standards are the minimum weld nugget diameter, the shear and tensile strength, the spatter propensity, and the electrode wear. Another frequent request is hybrid joints, a combination of adhesive bonding and welding. For this purpose, the adapted welding parameters must fulfill the quality standards with adhesive between the upper and lower sheets.

In comparison to the RSW of steel sheets, the high electrical and thermal conductivity, the considerable melting range, and the thick and robust oxide layer of the aluminium alloys are massive challenges  and reaching high welding quality with a low level of electrode wear. The primary sequence of the RSW process is depicted in Figure 1. In the first step, welding force F is provided by the welding electrodes onto the sheets, followed by applying the welding current. After melting both sheets at the contact surface, the current is switched off, and the force remains until the weld nugget is solidified.

The effects of the surface conditions on the RSW have, so far, rarely been investigated. In the automotive industry, untreated, pickled, or pickled surfaces with conversion coating are used with or without forming oil or dry lubrication. The large number of influences and the limited technical literature regarding the correlations, confront not least the aluminium producer with major challenges in order to provide sheet metal for an optimal end product. Therefore, this work focuses on the correlation between the surface conditions of 5xxx and 6xxx aluminium alloys and the hybrid RSW welding quality. The main criterion is the process stability portrayed by the spatter propensity and the electrode wear. To cover almost all material impacts on the weldability and welding quality, the first test series investigated the effect of the surface roughness of 5182 sheets in the heat treatment O and 6016 sheets in the T4 - FH condition. For that, sheets with different roughnesses were welded with and without adhesive. The welds without adhesive demonstrate the impact on the RSW weldability and represent a reference for welds with adhesive. The second test series addresses the consequences of pickling, conversion coating and dry lubrication on the hybrid weldability. For both test series, two very different sets of process parameters were used to cover the vast variations of the manufacturer’s parameters.

Experimental extent and sample preparation:

The chemical compositions of both alloys are defined in the standard EN 573-3 and summarized in Table 1.During the process, sample sheets were cut from stationary parts of the coils at several different process steps, as shown in Table 2. The 5182 sheet thickness is 1.5 mm, and the 6016 sheet thickness is 1.0 mm. The pickling and conversion coating were carried out in the exact same way as the standard pickling process and the Ti/Zr conversion coating for the automotive industry aluminium sheets.From the sample sheets, welding samples were prepared with the dimensions of 500x60 mm to statistically determine the weld nugget diameter, scatter propensity, and electrode wear. In addition, samples with 105 mm length and 45 mm width were cut for quasistatic shear tensile tests. For hybrid joints, the upper and lower sheets were pressed together after applying the adhesive bead, reaching a final adhesive thickness of 0.3 mm. Thus, this adhesive thickness matches the automotive manufacturer´s specifications. Before the welding trials, no sample surfaces were cleaned to simulate the most extreme process conditions.All welds were conducted with similar sheet thickness.

Welding parameters:

In automotive production, many different parameter sets are used for each alloy, sheet thickness, and hybrid joints, massively varying between the manufacturers. Based on that, the two welding parameter variations used for this work cover the whole parameter range. Variation V1 for the 5182 alloy has a short preheating pulse of 50 ms with 8 kA followed by an equally short welding pulse with a welding current between 28 and 35 kA. In contrast, variation V2 shows a very long and high preheating pulse of 450 ms with 12 kA. The welding pulse was kept the same as for V1, just as the welding force of 7 kN and the total welding time of 800 ms (Figure 2).Increasing the welding current to 40 - 45 kA for the 6016 alloy is necessary to reach the required weld nugget diameter due to the lack of alloying elements, resulting in a very high electrical conductivity compared to the 5182 alloy. The manufacturer’s specifications for the welding process are a minimum weld nugget diameter of d > 5√t (t equals the thickness of the sheets), a minimum electrode durability of 60 welding spots without aluminium pick-ups, and a spatter propensity lower than 10 %. The electrode wear has to be pictorially documented after every tenth welding spot. A section preparation of these spots must be free of cracks and demonstrate the weld nugget´s quality.

Mechanical testing:

The spot diameter is evaluated as described in the standard DIN EN 10447 using a manual tool to split the welding joints. The spatter propensity and electrode wear are visually assessed, and the shear strength is tested by applying the standard DIN EN 14273.