The Viability of using a Gleeble for Physical Simulation of High Frequency Induction Welded TRIP 690 AHSS

Abstract

High Frequency Induction Welding (HFIW) is the predominant process for high volume production of small diameter tubes and pipes for hydroformed automotive and oil and gas applications. This process is well-established due to its high throughput and continuous nature which makes it ideal for industrial use. However, the HFIW process is also complicated, involving several physical phenomena occurring simultaneously such as mechanical deformation during the squeeze-out, phase transformations, large temperature gradients, high heating rates, and electromagnetic induction. These phenomena are difficult to decouple from one another, leading to gaps in the present understanding regarding how each individual phenomenon affects the formation of certain weld defects, such as oxide inclusions trapped within the bond line of the weld joint. With advances in automotive design, new high-Al TRIP steels are being used for automotive hydroforming applications, due to their capability to be used in high strength/light-weight designs. However, HFIW of these materials, such as TRIP 690, is susceptible to the formation of entrapped oxides containing aluminum (Al), manganese (Mn), and silicon (Si) within the bond line, reducing the operation window compared to other steels. In welds containing oxide inclusions, strength and ductility of the weld joint will be significantly decreased. During production, it is difficult to determine the formation of these oxides due to the dynamic and continuous nature of the HFIW process. Conducting mill trials for experimentation is not practical due to economic constraints as there are high operational costs to run a tube mill and trials result in high material usage. Thus, there is a need to be able to physically simulate the HFIW process at a laboratory scale to understand the effect of each of the individual process parameters on the formation of bond line oxide inclusions and weld quality. This study physically simulates the HFIW process in thin sheets of TRIP 690 AHSS using a Gleeble 3500 thermomechanical simulator. The results of this work demonstrated that the Gleeble could reproduce the microstructure across the bond line and heat affected zone of the HFIW produced welds. Mechanical characterization of the welds revealed a similar hardness distribution across both the Gleeble and HFIW welds. Notably, samples containing bond line oxide inclusions such as those found in HFIW welds were also recreated, and the effects of these inclusions on the tensile properties and fracture mechanism were determined. Through this study, the ideal conditions for producing oxide-free welds to ensure superior weld mechanical properties were determined.