Zhao Kunmin

Effect of pulse current on the mechanicalproperties and fracture behaviors of Al-Mg alloys

The influence of pulse electric current to aluminum’s flow stress and elongation under high electric energy density (0.150 J/mm3) and low energy density (0.105 J/mm3) are investigated. Since the temperatures generated by electric current are identical under same energy density with different electric conditions, non-Joule heating effect under electrically assisted forming can be proven. The results show that pulse current help to reduce aluminum’s flow stress and increase its elongation. Under the same energy density, as the density of pulse current increased, although the maximum flow stress remains unchanged, the instant stress drop due to pulse current increases as well as its elongation. Stress recovery model is proposed to estimate the stress drop under tension. The number of fracture dimple continues to decrease until completely disappearance as the density of pulse current increases; the increase of elongation results from the suppression of the nucleation and growth of voids by pulse current.

Laser confocal observation on the formation of martensite and test on the mechanical properties of steels treated by Q-P-T process

The advanced high strength steels(AHSS) for auto manufacturing have been developed to the 3rd generation. The typical one is steel treated by Q-P-T process, which consists of the prior martensite, fresh martensite and retained austenite. In this paper, the non-uniform distributive formation of martensite and untransformed austenite during the one-step quenching treatment was observed by laser scanning confocal microscope. For the Q-P-T process, the non-uniform distributions of untransformed austenite, prior martensite and segmented austenitic grains are the main reasons to form the multi-scale martenite. Moreover, the fresh martensite possesses three highlights, such as microstructure of finer laths, same orientation in a blocky area considered as a “packet” and higher mechanical properties. The research result of this paper reveals the microstructure evolution of Q-P-T treated steel, and provides the accurate theoretical basis for improving the Q-P-T process and the mechanical properties.

Process development and mechanism analysis for improving the formability of complex hot forming AHSS parts

Based on the analyses of how stress state affects the microstructure, thickness distribution and mechanical properties, rapid cooling pretreatment is proposed to solve the cracking issues of hot forming AHSS (Advanced High Strength Steel) parts with complex shapes. Rather than forming the part right after the blank is fully austenitized at around 900~950℃ as in the traditional hot forming process, the proposed process cools the blank rapidly to around 700℃ followed by forming. The rapid cooling pretreatment not only enables the material to have a higher hardening exponent n so as better formability in tension dominated areas, but also increases the probability of new nucleation of martensite in austenitic parent phase to obtain a dense microstructure and improved strength and toughness in compression dominated areas. Experiments show the rapid cooling prior to forming does help form AHSS parts without cracking. The process also leads to a microstructure with refined martensitic phase arrangement and uniformly distributed macro hardness of 460 HV or higher. The performance requirements of high strength and toughness are met. The rapid cooling method is thereby proved scientific and effective. It breaks through the bottleneck of the cracking problem in hot forming complex AHSS parts and establishes the theoretical basis of our own developed manufacturing process.