Effects of weld microstructure on static and impact performance of resistance spot welded joints in advanced high strength steels

Abstract

Evaluating the impact performance of resistance spot welded joints in advanced high strength steels (AHSS) is critical for their continued integration into the automotive architecture. The effect of strain rate on the joint strength and failure mode is an important consideration in the design of welded structures. Recent results suggest that the failure mode is dependent upon the strength, chemistry, and processing of AHSS. Current literature, however, does not explain the effects of weld microstructure and a comprehensive comparison has yet to be conducted. The present study details the fracture paths within the joint microstructure of spot welded AHSS, including dual phase (DP), transformation induced plasticity (TRIP) and ferritic-bainitic (FB), in comparison to new high strength low alloy steels. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes were detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties were developed using observed microstructures in the fusion zone and heat affected zone. Results showed that good impact performance was obtained in DP780 and TRIP780 grades in relation to DP600, 590R and conventional high strength low alloy.

Characterization of magnesium spot welds under tensile and cyclic loadings

“…The same failure modes have been observed in aluminum spot welds [48,56], while partially interfacial and "button pullout" are the most common failure modes in well-prepared steel spot welds under monotonic tensile-shear loading [48,[57][58]. Button pullout is a failure mode in which one coupon tears around the HAZ, resulting in a plug on one coupon and a hole on the other coupon, but unlike partially interfacial failure there is no nugget shear-off in this mode.…”

Phase transformations contributing to the properties of modern steels

“…There are the so-called TRIP-assisted steels [33] in which solid-state phase transformation occurs under the influence of external forces, and the change in shape caused by the transformation enhances ductility via a number of mechanisms [34,35]. The ultimate tensile strength of these alloys is in the range 500-800 MPa, and stronger steels tend to have problems with weldability [36]. Furthermore, affordable steels which are beyond this range of strength tend to be difficult to form into shape at ambient temperatures for two reasons.…”

Investigation of the Failure of Advanced High Strength Steels Heterogeneous Spot Welds

“…Numerous authors use them directly to express the spot weld strength [5][6][7][8][9]. Others insist also on the role of the steel microstructure in the spot weld zone [10][11][12]. The weld nugget size is in fact the main varying parameter with the increase of welding current intensity during the determination of the welding range for a given sheet configuration.…”