Search

The heat treatment process is a vital step for manufacturing high-speed railway spring fasteners. In this study, orthogonal experiments were carried out to obtain reliable optimised heat treatment parameters through a streamlined number of experiments. Results revealed that a better comprehensive mechanical performance could be obtained under the following combination of heat treatment parameters: quenching temperature of 850 °C, holding time of 35 min, medium of 12% polyalkylene glycol (PAG) aqueous solution, tempering temperature of 460 °C, and holding time of 60 min. As one of the most important testing criteria, fatigue performance would be improved with increasing strength. Additionally, a high ratio of martensite to ferrite is proven to improve the fatigue limit more significantly. After this heat treatment process, the metallographic microstructure and mechanical properties satisfy the technical requirements for the high-speed railway practical operation. These findings provide a valuable reference for the practical forming process of spring fasteners.

The demand for roll-formed high strength steel square hollow sections has extensively increased by automotive and construction industries due to their excellent structural performances and attractive appearances. However, due to their low ductility and demand for large forming tool loads, high-strength steels are challenging to be roll-formed at room temperature. Partial heating roll forming is a recently proposed manufacturing technique aiming to overcome these difficulties by applying temperature on bend areas. In this study, the finite element software LS-DYNA was utilized to predict effective plastic strain and stress developments and associated residual stresses induced during the partial heating roll forming method. Furthermore, comparisons were made between cold roll forming and partial heating roll forming methods. The results indicate that the effective stress value during the partial heating roll forming method was lower than the cold forming method. In addition, the product manufactured by the partial heating roll forming method had smaller and uniform residual stress distributions along the section perimeter and across the section thickness than the cold-formed counterpart. Further investigations on corner geometry showed that high strength steel square hollow sections with a minimum bend radius could be successfully manufactured using the partial heating roll forming method.

The wire-arc additive manufacturing (WAAM) process applies gas metal arc welding (GMAW) technology. The wire materials of aluminum alloy are melted to form metal droplet by the arc discharge and are accumulated layer-by-layer in this process. The arc voltage and arc acoustic emission (AE) signals were detected synchronously in the experiment. According to the signal feature of arc voltage and arc AE, the droplet transfer behaviors in various transfer modes were investigated based on the GMAW-WAAM process. Furthermore, significance of parameters was analyzed and computational models of droplet size and deposition rate were developed to evaluate the quality and efficiency of WAAM. The results showed that the identification of the characteristics of droplet transfer mode can be realized by means of arc signals analysis, which is an alternative method for the analysis of droplet transfer in the GMAW-WAAM process. The interaction of arc voltage and arc current (U × I) is the most important factor to influence the droplet transfer, and the cubic polynomial regression equation is a more ideal model to describe the relationship between the frequency of droplet transfer and arc power. The droplet radius decreases exponentially with the increase of frequency of droplet transfer, and the deposition rate increases with the increase of droplet transfer frequency by cubic polynomial rule.