Mechanism Of Atom Diffusion Under The Heat Treatment By Big Pulse Electric Current

Pulse electric current heat treatment (PECHT) developed by Sodick, Ltd., of Japan, is a recently developed material processing method which consists of spark plasma sintering and welding, plasma activated sintering and welding, big pulse electric current (BPEC) diffusion welding etc. The following are basic merits of PECHT: rapid heating and cooling; short sintering or welding time; lowering sintering or welding temperature. This process has been successfully used to sinter FGM, MMC, FRC and so on, and joining difficult-to-weld materials.Why PECHT is so quick is a hot topic and many scholars are trying to explain it by proving plasma generating. As a matter of fact, PECHT is achieved by atom diffusing. Thus atom diffusion is a'key factor to PECHT. How do atoms diffuse under BPEC heating? And is it the same as under radiation heating? Do the special electric and magnetic fields produced by BPEC accelerate atom diffusion? Understanding the above questions fully is a key to expose the mechanism of PECHT. In this paper, three unique models were designed to research atom diffusion at interface, contacting line and joining point under BPEC heating and compare with it under radiation heating in order to find out if BPEC speed atom diffusion.The first model was aimed to research atom diffusion at interface under BPEC heating. Experiment condition of BPEC diffusion welding sheet Cu and Ni : direct diffusion welding, at a pressure of l0MPa, welded at 750⁹00℃, with heat rate of 200⁴00℃/min for 10mm, ON/OFF of 6/l⁴8/8, with die or not, vacuum of 6Pa. The samples were welded at 750℃, 800℃, 850℃ and 900 ℃.The diffusion coefficients (D) of Cu and Ni at interface increased with the increase of welding temperature. And comparing with radiation heating, atom diffusion velocity at interface quickened. The samples were joined with heat rate of 200℃/min, 260℃/min, 370℃/min. DCu and DNJ reached a maxim at rate ofaround 260℃/min. The study on ON/OFF indicated that ON/OFF had an effecton D_cu and D_Ni- Two different methods of samples placed in the die were also researched: with die or not. D_cu and D_Ni were higher without die than with die.In the second model, wirelike Cu/Ni was welded directly by BPEC heating. The following is the Experiment condition: at a pressure of l0MPa, welded at 800 and 900℃, with heat rate of 200℃/min forlOmin, ON/OFF of 12/2, vacuum of 6Pa. The samples were welded at 800℃, 900℃. D_cu and Dni at contacting line increased with increase of welding temperature. And atom diffusion at contacting line welded by BPEC heating was much quicker than by radiation heating.The third model was designed to investigate atom diffusion at contacting point. Fe was sintered at 500℃, 550℃ > 600℃ , 650℃ > 700℃ under BPEC and radiation heating. The test results of image processing indicated that the average pore size took on wavelike with temperature under BPEC heating. While sintered by radiation heating, the average pore size decreased slowly with increase of temperature at first, and then fell sharply, showing that the PECHT technique could achieve consolidated structure at relative low temperature. Fe was sintered at ON/OFF of 6/1,12/2,24/4,48/8. Measured by the Archimedes method, the high relative density could achieve at ON/OFF of 12/2. The test results of SEM indicated that contacting area sintered by BPEC was bigger than by radiaton heating, demonstrating that material transferring under BPEC heating was quicker than under radiation heating. Comparing shrinkage under BPEC and conventional heating, we can conclude that obvious shrinkage occurred at relative lower temperature and velocity of shrinkage was greater under BPEC heating.