Heat-affected zone liquation crack on resistance spot welded TWIP steels

Recently, in the automotive industry, Al-coated boron steel sheets (22MnB5) have been used for hot stamping, and the use of these sheets makes it possible to achieve a tensile strength of over 1,500 MPa, since a metallurgical transformation from austenite to martensite occurs during the process. In this study, resistance spot welding (RSW) experiments were performed in order to evaluate the weldability of single-lap joints between GA780DP and 22MnB5. The effect of the weld current on the nugget diameter and load-carrying capacity was evaluated by observing the nugget diameter and performing a tensile-shear test. Furthermore, the fracture behavior was evaluated by carrying out optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) observations. Ductile regions were observed on the interfacially fractured surface of the weld, and this implies that a high load-carrying capacity can be obtained even when interfacial fracture (IF) occurs. IF is caused by the stress concentration resulting from the presence of the sharp notch at the boundary of the nugget as well as by the high hardness and the brittle microstructure of the weld; the microstructure is brittle because of the high carbon equivalent (Ceq) and the penetration of Al in the weld.

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Evaluation of weldability for resistance spot welded single-lap joint between GA780DP and hot-stamped 22MnB5 steel sheets †

The failure of advanced high-strength steels’ spot welds is a critical issue for automotive crashworthiness. This paper deals with understanding the underlying factors of the tensile-shear strength of automotive steels’ resistance spot welds during interfacial failure. It was found that the ratio of the fracture toughness to the hardness of the fusion zone is the critical factor governing the interfacial failure mechanism: ductile shear failure (controlled by the fusion zone hardness) vs. cleavage crack propagation (controlled by the fracture toughness). This clarification could pave the way for more accurate modelling of interfacial failure of advanced steel resistance spot welds and shed light on the design of proper post-weld heat treatment for improving the weld mechanical performance.

Understanding the factors controlling the interfacial failure strength of advanced high-strength steel resistance spot welds: hardness vs. fracture toughness

The present work studied the influence of the geometric design of the electrode on Zn-assisted liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). LME cracking of the galvannealed transformation-induced plasticity (TRIP) steel welds, induced by two types of electrodes, a radius type with different radius of curvature (R), and a dome type with variable tip diameter (d), was studied both experimentally and by simulation. The current density decreased and the contact area at the electrode/sheet (E/S) interface increased with the increasing R, resulting in low temperatures and thermal stress, which subsequently led to decreased LME tendency. On the contrary, the current density decreased but the initial contact area at the E/S interface remained unchanged with increasing d, causing only a minor reduction in the temperature and hence less influence on LME cracking. These results suggested that R is the most critical design parameter of the electrode that controls LME cracking. Moreover, the radius type electrode displayed lower LME sensitivity as compared with the dome type electrode. This is attributed to the fact that the radius type electrode provides the benefits of increase in both R and d.

Critical design parameters of the electrode for liquid metal embrittlement cracking in resistance spot welding

Resistance spot weldability of galvannealed and bare DP600 steel

The automotive vehicle is made through the following processes such as press shop, welding shop, paint shop, and general assembly. Among them, the most important process to determine the quality of the car body is the welding process. Generally, more than 400 pressed panels are welded to make BIW (Body In White) by using the RSW (Resistance Spot Welding) and GMAW (Gas Metal Arc Welding). Recently, as the needs of light-weight material due to the CO2 emission issue and fuel efficiency, new joining technologies for aluminum, CFRP (Carbon Fiber Reinforced Plastic) and etc. are needed. Aluminum parts are assembled by the spot welding, clinching, and SPR (Self Piercing Rivet) and friction stir welding process. Structural adhesive boning is another main joining method for light-weight materials. For example, one piece aluminum shock absorber housing part is made by die casting process and is assembled with conventional steel part by SPR and adhesive bond. Another way to reduce the amount of the car body weight is to use AHSS (Advanced High Strength Steel) panel including hot stamping boron alloyed steel. As the new materials are introduced to car body joining, productivity and quality have become more critical. Productivity improvement technology and adaptive welding control are essential technology for the future manufacturing environment.

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Importance of Fundamental Manufacturing Technology in the Automotive Industry and the State of the Art Welding and Joining Technology

Estimation of the weldability of single-sided resistance spot welding

The weldability of resistance spot welding of TRIP1180 steels for automobile components investigated enhance in order to achieve understanding of weld fracture during tensile-shear strength (TSS) test. The main failure modes for spot welds of TRIP1180 steels were nugget pullout and interfacial fracture. The peak load to cause a weld interfacial failure was found to be related to fracture toughness of the weld and the weld diameter. Although interfacial fracture occurred in the spot welded samples, the load-carrying capacity of the weld was high and not significantly affected by the fracture mode. Substantial part of the weld exhibits the characteristic dimple (or elongated dimple) fractures on interfacial fractured surface also, dimple fracture areas were drawmatically increased with heat input which is propotional to the applied weld current. In spite of the high hardness values associated with the martensite microstructures due to high cooling rate. The high load-carrying ability of the weld is directly associated with the area of ductile fracture occurred in weld. Therefore, the judgment of the quality of resistance spot welds in TRIP1180 steels, the load-carrying capacity of the weld should be considered as an important factor than fracture mode.

Evaluation of Resistance Spot Weld Interfacial Fractures in Tensile-Shear Tests of TRIP 590 Steels

Both theoretical and experimental studies have been performed in order to investigate the feasibility of thermite sparking and the impact of rusted steel on aluminum coated steel. For dry rust, the exothermic reaction occurred at 900 °C while the wet rust ignited an exothermic reaction at 1000 °C. This delayed ignition was attributed to the consumption of energy by the water while in wet rust for phase transformation. The experimental study used a compressed-gas-powered impact-testing apparatus that showed that the dry rusted steel ignited sparking upon colliding with un-coated steel. It was also found that the presence of an aluminum coat generated a higher susceptibility to sparking than colliding with un-coated steel.

Yongjoon Cho

Papers

Importance of Fundamental Manufacturing Technology in the Automotive Industry and the State of the Art Welding and Joining Technology

Estimation of the weldability of single-sided resistance spot welding

Evaluation of Resistance Spot Weld Interfacial Fractures in Tensile-Shear Tests of TRIP 590 Steels

Study on the initiation of a thermite reaction in aluminum coated steel with a high-velocity impact