Dissimilar spot welding of dual phase steel/ferritic stainless steel: phase transformations

Abstract

Phase transformations in dissimilar resistance spot welds of dual phase steel and ferritic stainless steel are analysed. In contrast to a full martensitic microstructure predicted by the Schaeffler and Balmforth diagrams, a ferrite-martensite microstructure was observed in the fusion zone. The formation of ferrite phase in the fusion zone can be attributed to the rapid cooling rate of resistance spot welding, which suppresses the post-solidification ferrite-austenite transformation. The grain growth and martensite formation were main metallurgical features of the heat affected zone of ferritic stainless steel side. Microstructure gradient of heat affected zone in dual phase steel side was dictated by martensitic transformation. The effect of weld thermal cycle on the mechanical performance of the joint is discussed.

Investigating the impact of a selective laser melting process on Ti6Al4V alloy hybrid powders with spherical and irregular shapes

“…This is in complete agreement with Ref [9]. The lath martensite in alloy steels is analogous to this hierarchical structure of α′ martensites in the Ti6Al4V alloy [53,54].…”

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

“…There are limited works on the resistance spot weldability of stainless steels. [15][16][17]25 Part I of this work 26 addressed the solidification and post-solidification phase transformations in the FZ of three types of stainless steels, including austenitic, ferritic and duplex steels. Part II is focused on the phase transformations in the HAZ and mechanical properties of stainless steels spot welds.…”

“…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.…”

Welding metallurgy of stainless steels during resistance spot welding Part I: fusion zone

“…17,18 However, there are limited publications on resistance spot weldability of stainless steels. [22][23][24][25][26][27] The current understanding of the processstructure-property relationships is limited for RSW of stainless steels. Therefore, improving the knowledge regarding the microstructural characteristics and failure behaviour of stainless steels is a priority for the successful implementation of new design in vehicle applications.…”