ELECTRICALLY ASSISTED RAPID HEAT TREATMENTS IN SHEET METAL FORMING

Abstract

Electrically assisted (EA) rapid heat treatment is a promising alternative heat treatment method, in which the microstructure and mechanical properties of a metal alloy are modified by simply applying pulsed electric current for a short duration in sheet metal forming process. The effectiveness of EA rapid heat treatment can be explained by the combination of the athermal effect of electricity and the thermal effect of rapid resistance heating. Even though the performance of EA rapid heat treatment in sheet metal forming process surpasses that of conventional heat treating, the mechanism of electrical effect is not clearly identified yet. Also, the quantitative effect of electric current on the microstructure and resultant mechanical behavior has not been evaluated yet. The purpose of this study is to provide a precise processing technique in sheet metal forming (i.e., EA manufacturing), and to elucidate the post-EA rapid heating microstructure and mechanical behavior of the metal alloy during EA manufacturing process. Firstly, a modified EA rapid heating of Al-Si coated hot stamping steel is suggested. The intermetallic evolution in the coating during heating is experimentally investigated. In the modified EA rapid heating, a continuous electric current for a suitable duration is applied to a specimen to heat it to a temperature slightly below the melting temperature of the coating. The temperature of the specimen is then kept constant for specified dwell time. The result of the microstructural analysis shows that the modified EA rapid heating could effectively increase the thickness of the intermetallic layer between the coating and steel substrate much faster than conventional furnace heating and induction heating. The effectiveness of EA rapid heating may be due to the athermal effect of the electric current on the mobility of atoms, in addition to the well-known resistance heating effect. EA rapid heating also provides a technical advantage in that partial austenization can be easily achieved by properly placing the electrodes, as demonstrated in the present study. Secondly, through the understanding of the mechanism of electricity effect during EA rapid heating, the effect of EA annealing on the mechanical behaviors of pre-strained two different aluminum alloys during EA dual-stage forming are experimentally investigated. First, a specimen is deformed to a specific pre-strain by uniaxial tension and then automatically unloaded. After that, the pre-strained specimen is subjected to EA annealing by electric current with a fixed subsecond duration. Finally, the specimen is reloaded until fracture. Experimental results show that application of electric current with a subsecond duration induces EA annealing to both pre-strained aluminum alloys. The electric current also increases total achievable elongation until fracture during EA dual-stage forming for both aluminum alloys. However, analysis of the stress-strain behavior during reloading and microstructural observations suggests that the quantitative effects of electric current on the post-EA annealing mechanical behavior and resultant microstructure are strongly dependent on the type of aluminum alloy. Together, our findings indicate that while EA annealing is effective at improving the productivity of dual stage forming of an aluminum alloy, the composition of the aluminum alloy should be carefully considered in the design of the forming process utilizing the concept of EA annealing. From this study, the concept of EA rapid heat treatment in sheet metal forming is suggested. Also, the intermetallic evolution of Al-Si coated hot stamping steel, and the mechanical behavior of pre-strained two different aluminum alloys during EA rapid heating are investigated clearly. The optimal condition to achieve the efficiency of the process is suggested considering both microstructure and mechanical behavior aspects. Through the athermal effect of electric current on the mobility of atoms, in addition to the well-known resistance heating effect, EA rapid heating is a feasible technique to apply in the real industry with the significant benefits in cycle time reduction.