Study On The Mechanism And Process Of Pulse Current Hot Joining Of Alloys

Pulse Current Hot (PCH) is an advanced technique for materials synthesis and processing with the characteristics of lower temperature and faster speediness. It was applied widely to the sintering of materials. However, there is a few reports on PCH joining and on the diffusion of the joining surfaces, properties of the joined materials, temperature distribution during the joining and stress distribution of the joined materials, etc. In the present dissertation, PCH was adopted to the joining of Cu/Fe and Ti-6Al-4V/Ti-6Al-4Vsystems.PCH was used to join Cu/Fe. With the increases of joining temperature and holding time, the width of transition layer near the contacting surfaces become larger. Diffusion coefficients of Cu and Fe elements to PCH and Radiation Hot ( RH) joining methods were calculated. The coefficients to PCH were larger by three orders of magnitude than those of RH. Based on the diffusion joining dynamics and thermodynamics analysis, the following reasons were found to the difference between the two methods: temperature distribution gradually conduced by the special heating pattern, the electric and magnetic fields resulted by the pulse current during PCH joining, make the activated energy decrease, probability of element migration increase and driving force of diffusion enhance, then make the diffusion quicker and the diffusion coefficient larger than those of the RH.Finite element method (FEM) was used to calculated temperature and stress distributions during the PCH joining. The results indicated that the temperature distributed gradually and the highest temperature existed near the contacting surfaces, which were consistent with those of the results during the experiment processes. The temperature distribution pattern is favorable for high effective diffusion joining, because that the parts at far away the contacting surfaces with lower temperature avoid the damage of heat and the contacting surfaces needing higher temperatureattain the highest. The internal stress of the joined materials was only several MPa according to the result of calculation.Effects of graphite die on the properties and microstructures of Ti-6A1-4V joined by PCH were investigated. The results of tensile strength testing of joined samples showed that joining temperatures needed by the pattern of without graphite die were lower than those of the with graphite die under same pressure and holding time. Joint ratio increased with the increase of joining temperature and holding time to the two patterns. Higher joining temperature has little influence on microshardness of samples joined by PCH without graphite die. The microstructure observation near joint of the sample joined by PCH indicated that grains and phases had stridden over the joining surfaces and made the two parts into an integer.PCH and RH joining were used to join Ti-6A1-4V with the pattern of without graphite die. Comparing tensile strengths of joined samples, the results showed that the former was more optimal than the latter. The difference of heating patterns was thought to be main reason. Samples are heated through radiation to RP process, so that temperature of the whole sample is uniform. However, samples are heated directly by pulse direct current, which is to say by Joule heat to PCH joining without graphite die. Higher local temperature and electromagnetic field resulting from the heating pattern will accelerate diffusion, lower joining temperature and promote efficiency.According to the above conclusions, PCH joining is a temperature difference joining technique with faster speediness, lower temperature and higher efficiency.