Fracture Behavior of Resistance Spot Welded Joints (FB RSW) JOIN4+ 1.2

Project: Research project

Description

The steelmaking industry continues to place great emphasis on the development of higher-strength steel grades in order to meet the demands of light-weight design and the increasing demands in the automotive industry with regard to material stiffness and crash performance. Advanced high-strength steels (AHSS) include multi-phase grades such as dual-phase, complex-phase, TRIP and martensitic steels which contain varying combinations of the phase components ferrite, martensite, tempered martensite, bainite and retained austenite in order to achieve a very diversified spectrum of material properties. Press-hardened grades comprise another group of high-strength steels that are produced by means of conversion hardening (austenite-martensite) during or subsequently to the forming process. These new steel grades meet the highest crash performance demands in very thin gauges. The research has focused on high-strength multi-phase steels ranging between 800 MPa and 1200 MPa minimum tensile strength. The use of resistance spot welding is predominant in automotive bodies in white. Resistance spot welds must meet specific requirements in order to effectively achieve the benefits of the AHSS grades. Optimized resistance spot welding is characterized by a sufficiently broad welding range, a sufficiently long electrode life and the achievement of force transition under quasi-static, cyclic and dynamic (crash) loads. Although the fracture behavior in resistance spot welded joints (plug fracture, partial interfacial fracture, interfacial fracture) does not necessarily result in different transmissible loads, some users interpret interfacial fracture to be the determining criterion in the chisel and cross-tension test. Some are of the opinion that composite fractures have a negative effect on crash behavior as a result of low energy absorption in tensile testing. While the fractures in mild deep-drawing steel grades are almost exclusively of the plug fracture type, high-strength steel grades tend to yield undesirable partial interfacial and interfacial fracture in chisel and cross-tension tests. The objective of this project is an improved understanding of influences and fracture mechanisms of resistance spot welded joints of advanced high strength steels, enabling a definition of rules for alloying and processing concepts to avoid composite fractures in cross-tension loads. The fracture behavior results from ongoing and completed research and development projects at voestalpine will be included in this study
StatusFinished
Effective start/end date1/03/101/03/14