A comparative study of fatigue behaviour of MAG and laser welded components using reliability analysis

Fatigue is one of the main causes of failure of structures and mechanical components, occurring due to the progressive weakening of their strength that reduces significantly their lifetime, when subjected to cyclic stresses over time. In welded components, the joints are the zones most susceptible to crack by fatigue.Therefore, the base of this study are the Metal Inert Gas/Metal Active Gas (MIG/MAG) and LASER welding manufacturing processes, focused in three main areas involved in an automotive metallic system under dynamic loads: Fatigue testing in order to prevent structural collapse; Heat Affected Zones (HAZ) characterization to evaluate the material properties modification originated by those different technologies; Reliability analysis in order to analyse the performance of the samples and to select the best connection in terms of product life cycle. For this purpose, samples representative of industrial automotive applications (long welds) have been selected to carry out this work. Two types of connected specimens were manufactured, consisting of two steel plates of different thicknesses, overlapping and welded by the MAG process (type A) or the Laser process (type B). Metallographic characterization was performed for both typologies, namely macrostructural and microstructural characterization of the weld joint, and respective HAZ. Mechanical properties were inferred by measuring and mapping microhardness variation on the neighbour of the weld joint. Fatigue tests were carried out for specimens type A and type B, using 15 samples of each type that were tested under 3 levels of stress amplitude. The samples manufactured by the Laser process show better fatigue behaviour when compared to the samples manufactured by MAG welding. The better weld joint solution is proposed in accordance with the reliability analysis of the obtained fatigue test results.