Phosphorous and Boron Segregation During Resistance Spot Welding of Advanced High Strength Steels

The mechanical properties of advanced high strength steels (AHSS) for automotive applications are sensitive to their multi-phase microstructures. Phosphorous is added in AHSS mainly as a solid solution strengthener and to suppress the formation of cementite thereby enhancing the room temperature stabilisation of austenite in TRIP and Quench and Partitioning steels. However, the weldability of phosphorous containing steels is poorer than conventional low carbon steels due to the grain boundary embrittlement resulting from the segregation of phosphorus to the grain boundaries during the solidification of the weld pool. In order to reduce the grain boundary embrittlement, the addition of boron has been suggested. However, the addition of boron also increases hardenability and the ratio of boron to other alloying elements present in the steels should be properly controlled to achieve the required mechanical properties of the welds.

In this work, the partitioning behaviour of phosphorous and boron during the solidification of weld pools under various weld thermal cycles has been studied using experimental simulations and a Dictra^TM based diffusional model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to various linear and resistance spot welding thermal cycles. Mechanical properties of the welds were evaluated by hardness, cross tension tests and correlated with weld microstructure. Diffusion calculations show that the phosphorus concentration of the last solidified area in the weld pool can reach about 0.38 wt. % for a steel with a bulk phosphorous concentration of 0.08 wt. %. Elemental analysis indicated that with the absence of boron, the grain boundaries of laminar grains in the weld pool are decorated with phosphorous. As a result, intergranular failure was found to occur along the grain boundaries during cross tension tests. With the addition of boron, apart from an increase in weld strength, the failure mode turns to transgranular.