Welding Metallurgy of

Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included.

Critical review of automotive steels spot welding: process, structure and properties

https://www.research.manchester.ac.uk/portal/files/65058553/OLEN_5119_edit_report.pdf

Comparison of microstructure and mechanical properties of ultra-narrow gap laser and gas-metal-arc welded S960 high strength steel

Literature presents extensive experimental evidence of large deformation and ductile fracture behaviour of lath martensite in martensitic and multi-phase high strength steels under quasi-static, uniaxial loading conditions. The physical origin of this apparent ductile behaviour of martensite is not clear, since martensite generally provides a high material strength. The presence of thin films of interlath retained austenite may trigger the observed apparent martensite ductility. The present contribution investigates the role played by interlath retained austenite on the mechanics of lath martensite by means of crystal plasticity simulations. It is shown that independently from the interlath retained austenite volume fraction and the exact lath morphology, localized shearing along the lath habit plane occurs as long as there are enough carriers for plasticity. The austenite film acts like a 'greasy' plane on which the stiffer laths can slide. The shearing mechanism is not a mere consequence of the lower flow stress in the austenitic phase, but it is largely due to the orientation relationship between the retained austenite face centred cubic lattice and the body centred cubic lath crystals.

https://iopscience.iop.org/article/10.1088/0965-0393/22/4/045011/pdf

On the role of interlath retained austenite in the deformation of lath martensite

The paper is focused on the strength and fracture toughness of AISI420 martensitic stainless steel resistance spot welds under the tensile-shear loading The failure behavior of AISI420 spot welds was featured by quasi-cleavage interfacial failure with low load bearing capacity and weak energy absorption capability which was a function of the weld fusion microstructure, predominately carbon and chromium rich martensite plus δ-ferrite Fracture toughness of the fusion zone proved to be the most important factor controlling the peak load of the spot welds made on AISI420 failed in interfacial mode A geometry-independent fracture toughness of the weld nugget (ca 23 MPam05) was determined using fracture mechanics concept Through modeling it was found that there is a critical fracture toughness which beyond that the pullout failure mode can be obtained which is a function of sheet thickness and tensile strength of the base metal

Fracture toughness of martensitic stainless steel resistance spot welds

Fusion zone of three TRIP steels, categorized as AT: C–Mn–Al, AST: C–Mn–Al–Si and ST: C–Mn–Si, in resistance spot welding was characterized with respect to microstructure, phase analysis, and hardness. The fusion zone microstructure was found to depend on chemistry: (i) AT steel contained ferrite phase surrounded by bainite and martensite regions, (ii) AST steel showed a bainite structures along with martensite laths and interlath retained austenite, whereas (iii) ST steel constituted single phase martensite laths with interlath austenite. X-ray diffraction study indicated that retained austenite fraction in the fusion zone increases with increase in Si content in it. The AST fusion zone hardness lies between those of the AT and ST steels; the ST fusion zone hardness was higher than that of AT steel because of the single phase martensite microstructure. Comparison of fusion zone microstructure and hardness to earlier study on laser welding of the TRIP steels with similar chemistries revealed that higher cooling rate in resistance spot welding led to higher fusion zone hardness compared to laser welding; which was attributed either to decrease in softer ferrite phase (AT steel) in the microstructure or refinement of martensite laths (ST steel).

Microstructure–hardness relationship in the fusion zone of TRIP steel welds

Resistance spot welds (RSW) in transformation-induced plasticity (TRIP) steel sheets were locally postweld heat treated by applying a second pulse of current in the welding schedule. The evolution of the fusion zone (FZ) microstructure by optical and scanning electron microscopy, microhardness, and standardized quasi-static lap-shear tensile testing was investigated for all the pulse current conditions. The most important result of this study is that improved mechanical properties with desirable pullout failure mode is accomplished when the FZ microstructure consists of a recrystallized structure of martensite, which was achieved in the medium level of the second pulse current postweld heat treatment. Even though a considerable reduction in FZ hardness was observed at the lowest value of the second pulse current due to the presence of plate-like ferrite and tempered martensite structure, the failure mode and load-bearing capacity of the weldment was not improved significantly. At higher current levels of the second pulse condition, remelting occurred with consequent formation of a new weld nugget comprising columnar structure but larger nugget size giving mixed failure modes. The fusion zone microstructures and microhardness results correlated well with the simulated welding thermal history of the postweld local heat treatments.

Second Pulse Current in Resistance Spot Welded TRIP Steel — Effects on the Microstructure and Mechanical Behavior The second pulse current improved mechanical properties when the fusion zone microstructure consisted of a recrystallized structure of martensite

Y. Okita

Papers

Microstructure–hardness relationship in the fusion zone of TRIP steel welds

Second Pulse Current in Resistance Spot Welded TRIP Steel — Effects on the Microstructure and Mechanical Behavior The second pulse current improved mechanical properties when the fusion zone microstructure consisted of a recrystallized structure of martensite