Controlling infiltration when brazing P/M parts and during manufacture of aluminium metal matrix composites

Abstract: Infiltration is used in the production of several different material groups as electric contact materials, copper infiltrated sintered steels and metal matrix composites. The mechanism of infiltration causes also unwanted difficulties in processing as brazing of porous sintered compacts. The common question, in this work has been, how is it possible to control infiltration in different materials processing techniques? In joining of powder metallurgically (P/M) produced porous compacts, by brazing, the inherent porosity of the compacts causes the melt filler metal to infiltrate the interconnected pore channels of the P/M parts, by capillary forces. This will result in high penetration depths and filler metal consumption and a limited amount of filler metal will be available for the joint. In the production of metal matrix composites (MMC:s), the difference in surface energies between the metallic and ceramic components prohibits a spontaneous infiltration of the metallic phase into the ceramic porous body. This work includes a general analyse of the different physical and mechanical methods to control infiltration in brazing of porous compacts and processing of MMC:s respectively. The experimental part of the work concentrates on the study of physical methods for the infiltration control. Brazing of porous sintered compacts has been studied experimentally through different thermal treatments. A special (Cu-Ni-Mn-Si) filler-metal, developed by others in order to facilitate alloying between iron in P/M- compacts and the elements in the filler-metal, has been used and the results has been studied by optical and scanning electron microscopy and the mechanical strength and hardness has been measured. In the work on MMC:s a method for processing of aluminium matrix-alumina reinforced composites by spontaneous infiltration has been studied by wetting and in- situ high temperature X-ray experiments. The investigation of brazing shows that the filler metal starts to melt already at 930 oC and a two phase alloy is developed in the joint. The diffusion of elements from the filler metal and the sintered compact causes a development of an alloy with high melting temperature in the surface area of the sintered compact that blocks the surface pores from continued infiltration. The wetting experiments show that the spontaneous infiltration in production of MMC:s is enabled by chemical reactions in the system concerned. The in-situ X-ray experiments show that the formation of magnesium-nitride appears below 600 oC. The formation of AlN as a second reaction product in the spontaneous infiltration has been detected for compact tested after a processing cycle with increased pressure of nitrogen- gas in the processing furnace. The in-situ X-ray study of the spontaneous infiltration has shown that the formation of magnesium nitride could be detected. The results show also that it is possible to study chemical reactions at and above the melting temperature of the metallic constituent of the system. The results show also that it would be possible to create alumina-aluminium MMC with different hardness levels.