Solidification cracking during laser beam welding

Abstract

Third-generation advanced high strength steels (AHSS) are a new class of steels that offer superior functional properties and significant weight savings in the body-in-white (BIW) structure of a car. Weight savings directly translate to reduced CO2 emissions from cars which aids automotive manufactures to meet the vehicle emission guidelines put forth by regulatory bodies around the world. Further increase in weight savings and productivity can be realised when BIW components are fabricated using laser beam welding (LBW). However, the phenomenon of solidification cracking of AHSS during LBW poses challenges to not only its application in the automotive industry, and also in the production lines of steel manufacturers. From the body of literature pertaining to solidification cracking two fundamental conditions can be identified that result in solidification cracking in alloys. First is the development of thermo-mechanical stresses/strains during liquid melt solidification and second, is the formation of a crack susceptible microstructure. In addition to this, the welding conditions can influence the susceptibility of alloys to solidification cracking. The objective of the present study is to understand the influence of variable processing conditions during LBW on solidification cracking tendency of AHSS and how to control these conditions to minimise it. In particular, an attempt is made to understand the effect of keyhole configuration, welding speed and laser beam spot size on solidification cracking. Furthermore, finite element analysis is used to predict the size and shape of the weld pool during LBW, and to determine the net process efficiency which in turn is compared with the calculated process efficiency from the existing analytical model. Bead-on-plate LBW following the testing procedure of the VDEh (German Steel Institute) standard hot cracking test was performed on three third-generation AHSS at two LBW facilities with different beam quality. Due to this, the keyhole during the tests at the two facilities was identified to exist in the closed keyhole configuration and more towards the open keyhole configuration. The susceptibility to solidification cracking was found to increase when the keyhole prevailed in the closed keyhole configuration during LBW. The keyhole configuration was varied by altering the process parameters (welding speed and spot size of the laser beam) in comparison to the parameters corresponding to the closed keyhole configuration. Reducing the welding speed, while keeping the laser power and spot size constant, resulted in the open keyhole configuration and subsequent reduction in the solidification cracking tendency but, until a limit. Similarly, reducing the spot size, while keeping the welding speed and laser power constant, also reduced the solidification cracking tendency as the open keyhole configuration was enforced. The macroscopic area of the fusion zone (weld size) was found to corroborate with the solidification cracking tendency of the alloys. Consequently, the spot size of the laser beam was varied to determine the critical spot size which resulted in a critical weld size at which solidification cracking did not occur. Corresponding to this, the critical process efficiency was determined which is representative of the critical heat input above which solidification cracking occurs. However, the magnitude of the critical spot size, weld size and process efficiency is dependent on the solidification cracking susceptibility of the alloy in question.