Similar and dissimilar resistance spot welding of advanced high strength steels: welding and heat treatment procedures, structure and mechanical properties

Abstract

Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel and a 980 MPa transformation induced plasticity (TRIP) steel were resistance spot welded under different welding and heat treatment parameters. The microstructure and mechanical properties of spot welds were evaluated using metallographic technique, microhardness and tensile shear tests. The results showed that the tendency to fail in the pullout mode increased in the order of DP/ DP, TRIP/TRIP and DP/TRIP welds, which was caused by the different hardness distribution, carbon equivalent and susceptibility to shrinkage void formation of spot welds for different combinations. In the study of the effects of heat treatments on the DP/TRIP welds, the pre-heating procedure improved the splash of welding to some extent. When the cooling time was larger than or equal to 1000 ms, the post-heating procedure improved the mechanical properties of spot welds owing to the temper of spot weld microstructure.

Welding metallurgy of stainless steels during resistance spot welding Part II –heat affected zone and mechanical performance

“…1,[9][10][11][12][13][14][15] Fusion zone size is controlled by heat generation and heat dissipation, which in turn are governed by the welding parameters and physical properties of the BM.…”

“…martensite formation in FZ and HAZ and martensite tempering in HAZ) have been studied in reasonable details. [5][6][7][8][9][10][11][12][13] However, the welding metallurgy of stainless steels during RSW is yet to be addressed. There are limited works on the resistance spot weldability of stainless steels.…”

“…Generally, spot welds fail at four different modes ( Fig. 1): 1,[10][11][12][13][14][15][16][17] (i) interfacial failure mode in which fracture propagates through the FZ (ii) pullout failure (PF) mode in which failure occurs via withdrawal of the weld nugget from one sheet; in this mode, fracture may initiate in the BM, HAZ or HAZ/FZ depending on the metallurgical and geometrical characteristics of the weld zone and the loading conditions (iii) partial interfacial failure mode in which the fracture first propagates in the FZ and is then redirected towards the thickness direction (iv) partial thickness-partial pullout (PT-PP) mode in which fracture initiates in a manner similar to PF mode; however, some part of mating sheet thickness is removed by a slant crack through the FZ during final crack propagation around the circumference of the weld nugget. Generally, the pullout mode is the preferred failure mode due to its higher associated plastic deformation and energy absorption.…”

“…1,[5][6][7] The energy absorption capability of the spot welds, which is an important parameter in vehicle crashworthiness, is controlled by weld physical attributes [especially fusion zone (FZ) size and electrode indentation size] and weld metallurgical attributes [microstructure of the base metal (BM), FZ and heat affected zone (HAZ)]. [4][5][6][7][8][9][10] The failure mode of spot welds (i.e. the fracture path of spot welds during mechanical loading) is a qualitative measure of weld performance.…”

Problems in the Welding of Automotive Alloys

“…It therefore becomes essential to develop joining techniques for dissimilar metals, 18,19 and even between different classes of alloy systems. [20][21][22][23] Magnesium in many automotive applications requires to be joined to aluminium, and there are important problems with the formation of brittle intermetallic compounds at the junctions. 24 There clearly are joining related issues that need to be the focus of research before claiming success in the development of materials for automotive applications.…”