Sensitivity of dissimilar aluminum to steel resistance spot welds to weld gun deflection

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Abstract

This paper describes an experimental study in which a traditional resistance spot weld process with Multi-Ring Domed (MRD) welding electrodes was used to join AA5754-O to low carbon steel sheets in a coach peel configuration. A linear relationship was identified for the resistance spot weld C-gun stationary arm deflection with force applied by the movable gun arm. Due to the relative rotation of the electrodes during deflection, the point of electrode/coupon surface contact moved away from the original centerline to a point inboard towards the C-gun arm. This led to asymmetric features in aluminum/steel resistance spot weld morphology, intermetallic compound (IMC) characteristics, weld defect distribution, and aluminum hardness. The aluminum weld nugget solidified closer to the weld gun with respect to the centerline of the MRD electrode imprint and exhibited a softer thermo-mechanically affected zone (TMAZ) and expulsion which produced a thinner IMC layer and fewer oxide film defects. This in turn resulted in strong button pull-out strength and the welds were able to absorb 36% more energy under an applied directional load in comparison to aluminum weld nuggets that solidified further away from the weld gun with respect to the electrode imprint centerline. The mechanism responsible for these weld asymmetries was further revealed through analysis of the variation in contact condition between electrodes and workpieces due to weld gun deflection.

Introduction

The integration of lightweight metals into a traditional steel automotive body in white structure enables flexibility in vehicle design to achieve reduced automotive emission targets [1,2]. As a high-efficiency and low-cost joining method, resistance spot welding (RSW) has been widely applied to manufacturing of wholly steel and wholly aluminum body structures. There is significant motivation to join dissimilar aluminum-steel body structures using these existing production lines. Considerable efforts have been exerted to produce a high-quality Al-steel weld. Recent research on RSW of Al-steel has primarily focused on the characteristics and formation mechanism of Fesingle bondAl intermetallic compounds (IMC) as well as their influence on joint mechanical performance.

Bouche et al. [3] found that the Al-steel RSW was connected by an IMC layer comprised of two layers. A thin needle-like layer adjacent to the aluminum sheet was identified as FeAl3 and a thick, tongue-like layer adjacent to the steel sheet was Fe2Al5. Wan et al. [4] studied the formation mechanism of IMC in Al-steel RSW based upon the thermal history at the aluminum and steel faying interface. It was concluded that Al atoms diffused into solid steel gradually and reacted with iron to produce Fe2Al5; meanwhile, Fe atoms in contact with liquid Al were concentrated near the weld interface and formed FeAl3. Naoi et al. [5] studied the growth kinetic coefficient of IMC performing a solid-state reactive diffusion experiment between Fe and Al. They found that the growth rate of Fe2Al5, the main component of the IMC layer, was much larger than that of FeAl3. Yin et al. [6] studied the effect of Si on the growth of IMC during the reaction of solid iron and molten aluminum. Results demonstrated that as the Si content increased from 0% to 3%, Si atoms could occupy vacancies in the C-axis direction of the Fe2Al5 phase structure, hindering the diffusion of Al atoms into the steel and inhibiting the growth of the Fe2Al5 phase. Shi et al. [7] studied the effect of ZnNi coating on IMC growth in 6XXX Al-steel RSW and concluded that the large atomic radius of Ni will delay the diffusion of aluminum, resulting in the formation of a thinner IMC layer. Zhang [8] added an intermediate Cu layer between aluminum and steel sheets. They found the IMC thickness decreased significantly due to Cu atom accumulation on the IMC layer which blocked the diffusion of Al atoms into the steel matrix during the RSW process. Hwang et al. [9] reported that increasing the Mg content of the aluminum alloy from 0 to 3.66% resulted in an increase in IMC thickness from 0.7 μm to 2.9 μm during the Al-steel RSW process. Oikawa et al. [10] and Zhang et al. [11] support this and stated that the Mg alloying element in aluminum displaced the Al atoms in the IMC which promoted the reaction of aluminum and steel during the welding process. The crystal structure of Fe2Al5 is orthorhombic [12]. A unique feature of the Fe2Al5 phase is the anisotropic network of partial occupancy aluminum sites that run parallel to the c-axis [3]. The sites are similar in energy and can allow for greater diffusivity of aluminum along the c-direction [3]. Changes to the c-axis, such as substitution of aluminum atoms by larger magnesium atoms, may be able to contribute to changes in diffusion and overall growth of Fe2Al5.

With regard to the factors affecting the quality of Al-steel RSW welds under different loading conditions, Chen et al. [13] observed that the fracture mode and strength of welds subjected to cross tension testing were dependent upon IMC thickness, dendritic shrinkage within the Al nugget, Al nugget diameter, and the Al sheet thinning. As would be expected, welds with thinner IMC and larger nugget diameters produced the greatest peak force and energy absorption. Chen et al. [14] also evaluated the tensile shear properties of Al-steel spot welds and concluded that 3 μm was the critical IMC thickness which determined weld facture mode. The preferred button pull-out or partial thickness fracture occurred when the thickness of the IMC layer was less than 3 μm. Sigler et al. [15] observed micro-defects near the Al-steel interface, which were identified as oxygen and magnesium rich features using electron probe micro-analysis (EPMA). Sigler et al. concluded that these micro-cracks, designated as oxide film defects, were remnants of the original oxide film layer present on the aluminum sheet prior to the RSW process. The oxide film defects and pores decreased coach peel strength of the Al-steel welds and resulted in unwanted interfacial fracture. Chen et al. [16] studied the robustness of the Al-steel RSW process under different working conditions including gaps between metal sheets and angles between the welding electrodes and metal sheets. They concluded that the presence of a gap between the aluminum and steel sheets increased the risk of expulsion which can reduce weld nugget size and thereby lower coach peel strength. In addition, the introduction of angles between the welding electrodes and metal sheets was observed to produce excessive internal expulsion which produced an asymmetric Al nugget with a sharper notch angle which acted in tandem to reduce coach peel strength.

Existing literature demonstrates that IMC thickness, Al nugget geometry, and defects within the Al nugget significantly influence Al-steel weld performance. This weld behavior can be further exacerbated by variations of welding equipment in production. Tang et al. [17] found that a stiffer weld gun can better constrain expansion in the molten pool of the weld and thereby limit welding expulsion. Tang et al. [18] also welded bare steel to bare steel and AA5754 to AA5754 using welders with different stiffness which exhibited a 3% improvement in average tensile shear peak loads using the high stiffness welder albeit with overlapping data ranges. Ding et al. [19] studied the structure of servo weld guns used in body in white assembly. They stated that a 3° weld electrode inclination could influence the weld strength in hot formed steel welds and continuous welding joints. Servohydraulic weld guns with a C-frame are extensively used in automotive production lines and the deformation of a C-frame under electrode force of RSW process is inevitable [20], which has been shown to produce asymmetric welding conditions in electrical, thermal, and mechanical aspects for steel to steel welding. However, there is no literature investigating these effects upon the Al-steel RSW process and the resultant weld properties. Thus, this work explores the effect of weld gun deflection in Al-Steel RSW and address the gap in the existing literature.

In the present study, Al-steel resistance spot welds were produced using a servo-hydraulic actuated weld gun with a C-frame. The weld gun's deflection during the welding process was measured using a high-speed camera. Following this, the weld morphology, IMC characteristics, defect distribution, and Al hardness were measured to explore the influence of gun deflection on weld structure and properties at both macro and microscopic levels. Finally, mechanisms are provided which explain the influence of weld gun deflection on dissimilar Al-steel RSW coach peel behavior.

Section snippets

Materials

1.2 mm thick aluminum alloy AA5754-O and 2.0 mm thick hot-dipped galvanized low carbon steel (HDG LCS) sheets were used in this study. The chemical compositions of the two materials are provided in Table 1. All sheets were cleaned by ethanol prior to welding to remove any organic contaminants on the as-received material surface [21]. The Zn coating on the steel sheets has a lower melting point than aluminum, it was readily melted and squeezed out of the weld region early during the welding

Deformation of the weld gun

Electrodes deflection are inevitable which is actually part of electrode misalignment, either axial or angular [26] when the movable electrode applied force to the stationary electrode. Fig. 5(a) presented the weld gun deflection in the vertical direction ∆y, skidding in the horizontal direction ∆x and an angular misalignment θ without applying a welding current. The deflections at several electrode forces were produced using the FANUC R-2000i industry robot controller. As can be observed in

Conclusions

This body of work investigated the influence of weld gun deformation on AA5754 to low carbon steel resistance spot welds using MRD electrodes and multiple solidification weld schedules. The weld nugget morphology, nugget location, IMC thickness distribution, microstructural defect distribution, and weld hardness exhibited asymmetries that corresponded to the weld gun deflection. These structural asymmetries translated to significant variation in mechanical performance. The conclusions drawn

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge the financial support of the GM Research and Development Center, the National Natural Science Foundation of China (Grant Nos. 52025058 and U1764251) and the State Key Laboratory of Mechanical System and Vibration (Grant No. MSVZD202111). The authors also thank Chad Clark (Fusion Welding Solutions) and David Sigler (General Motors, retired) for their welding expertise and valuable discussions.

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