Characterization of Microstructure and Mechanical Properties of Resistance Spot Welded DP600 Steel

Ali Ramazani,Krishnendu Mukherjee,Aydemir Abdurakhmanov,Ulrich Prahl,Mahmoud Abbasi

doi:10.3390/met5031704

Ali Ramazani, Krishnendu Mukherjee + Show 3 more

Open Access

https://doi.org/10.3390/met5031704

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Abstract

Resistance spot welding (RSW) as a predominant welding technique used for joining steels in automotive applications needs to be studied carefully in order to improve the mechanical properties of the spot welds. The objectives of the present work are to characterize the resistance spot weldment of DP600 sheet steels. The mechanical properties of the welded joints were evaluated using tensile-shear and cross-tensile tests. The time-temperature evolution during the welding cycle was measured. The microstructures observed in different sites of the welds were correlated to thermal history recorded by thermocouples in the corresponding areas. It was found that cracks initiated in the periphery region of weld nuggets with a martensitic microstructure and a pull-out failure mode was observed. It was also concluded that tempering during RSW was the main reason for hardness decrease in HAZ.

Highlights

The main focus of the automotive industry is to reduce the weight of car, enhance safety and crashworthiness while the cost is low [1,2,3]
In the microstructure of the BM zone near HAZ (Figure 5c) is the presence of some carbide phases distributed in ferrite matrix is recognizable
The fusion zone in the Resistance spot welding (RSW) weld consisted of a hard martensitic columnar microstructure with hardness of about 400 HV

Summary

Introduction

The main focus of the automotive industry is to reduce the weight of car, enhance safety and crashworthiness while the cost is low [1,2,3]. DP steels are attractive to automakers as they can offer safety, affordability, fuel efficiency, and environmental responsibility [6,7,8]. Wang et al [9] studied the high strain rate behavior of high strength DP steels They reported that for high strain rate tests, comparable to crash situations, the DP steels fail in ductile mode. A well-balanced ratio between ferrite and martensite volume fractions has been reported to be the most important factor affecting the mechanical properties of DP steels [10,11]. The other factors affecting the mechanical behavior includes: the morphology of the martensite islands, carbon content in the martensite, and the deformation condition of ferrite, i.e., dislocation density [12,13]

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