ORIGINAL RESEARCH PAPER Failure behaviour of laser spot welds of TRIP800 steel sheets under coach–peel loading S. Daneshpour 1 , S. Riekehr 1 , M. Koc ¸ak 1 , V. Ventzke 1 and A. I. Koruk 2 This study is carried out to investigate the laser spot welds (LSWs) of advanced high strength steel sheets of 1?0 mm thickness for application to the automotive industry. Mechanical properties and failure behaviour of LSWs for transformation induced plasticity steel sheets (TRIP800) subjected to monotonic coach–peel loading are investigated by experimental and finite element (FE) simulation methods. Microstucture of LSW and fracture surfaces of the welded specimens were examined by scanning electron microscopy (SEM) to describe the microstructural features and to clarify the crack initiation mechanism respectively. Based on simulation solutions equivalent plastic strain was obtained to describe local deformation of LSW joints. Experimental results revealed a ‘plug type’ of failure mode of the circular LSW joints under ‘peel–coach’ loading condition. This type of ductile failure is the most common failure mode for spot welds used in the automotive industry. Numerical simulation of damage process was compared with experimental results and this revealed that fracture path was successfully predicted. Keywords: TRIP steels, Coach–peel testing, Laser spot welding, Failure analysis, Finite element analysis, Mechanical properties Introduction In the automotive industry, increasing demands regard- ing weight, safety and cost have led to the introduction of the so called advanced high strength steels (AHSS). 1 These new steel grades with their high strength, good formability and weldability challenge the use of the light metals and the composites in car body due to cost advantages. Ranked in order of increasing strength, the most common AHSSs include dual phase (DP) steels, complex phase (CP) steels, transformation – induced plasticity (TRIP) steels and martensitic (MART) steels. 1 The mechanical properties of TRIP steels depend on the volume fraction, morphology and distribution of the retained austenite, along with the volume fractions and properties of the bainitic ferrite and equiaxed ferrite constituents. 2 Efforts to implement these steels have, however, been accompanied by welding related challenges. Currently, the body is largely assembled by the use of conventional resistance spot welding (RSW) and hence, significant amount of investigations carried out have been asso- ciated with that joining process. Many of the problems of the RSW process have related to the formation of martensites in the weld nugget. 3 Martensite, particularly with increasing carbon contents, results in high hardness values in the weld zones, which leads to a brittle fracture under external loading. Similar observations have also been made when laser beam welding LBW was used on these high strength steel grades. 3 The hardness levels achieved in these steels is directly associated with the cooling rates implicit in these joining processes. 3 Conventional resistance spot welding of TRIP steels has been considered to be critical because of the frequent occurrence of weld interfacial fractures. Basically, two types of weld joint failures are observed for such joints, namely, interfacial and plug type. The plug type or button pullout type joint failures are representing ductile (stable) fracture, which are preferred compared to the brittle fracture type described as ‘interfacial’ fracture. A simple and improved formula for predicting the transi- tion of failure mode from pull out to interfacial is suggested by Chao. 4 Significant amount of work is devoted to develop advanced RSW processes to be able to obtain plug type (button pullout) fractures on these steels. 5–8 For this purpose, numbers of options such as pulsed welding process and longer weld times aim to influence weld cooling rates, and thus reduce weld hardening. 7 However, the RSW process and its deriva- tives are not fully capable to provide satisfactory solutions to this issue. Recent developments in high power lasers and robotic control have accelerated the application of LBW to car body fabrication and assembly. LBW has the advan- tages of high welding speed and precision while providing consistent weld integrity and low heat process which yields reduced distortion. Unlike RSW, laser spot welding can be used as a single side, non-contact 1 GKSS Research Centre, Institute of Materials Research, Materials Mechanics Section, Joining and Assessment, D-21502 Geesthacht, Germany 2 Arcelor tailored blank senica, Dlha ´ , Senica, Slovakia *Corresponding author, email shahrokh.daneshpour@gkss.de ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 19 February 2007; accepted 22 May 2007 DOI 10.1179/174329307X213855 Science and Technology of Welding and Joining 2007 VOL 12 NO 6 508