Fatigue analysis of laser welded hairpins in electric car motors

Abstract: Hairpins are copper filaments used in the windings in the stators of electric motors in modern times. For the motor to work, they have to be welded in pairs, normally with laser welding. Studying the fatigue response of the joint of hairpins welded with a set of values of laser welding process parameters is the main objective of this project. For this reason, a criterion to select laser welding parameters was found and hairpin specimens were welded by laser. To study the fatigue properties, a fatigue test was designed, as well as a method to prepare the test specimens. The experiments were performed by applying sinusoidally cyclic tensile loads to the specimens until they fracture. The results were the mean load and the amplitude and the number of cycles needed to break the specimens. Introducing the loads to a FEM model the stresses were obtained and S-N curves were constructed. A correlation between porosities and fatigue behaviour was found, the larger the pores, the shorter the fatigue life for the same parameter test. Some sources of imprecision that led to some invalid experiments and affected the results are discussed. One example is the difficulties to apply the predefined loads due to the imprecision of the machine.