Process And Mechanism Of Plasma Arc "In-situ" Welding Of SiCp/6061Al MMCs

SiC particle reinforced aluminum metal matrix composites(SiCp/Al MMCs), which exhibit excellent combinations of high specific strength, high specific stiffness, low coefficient of thermal expansion and excellent wear resistance, have wide application in aerospace-flight, aviation structure, and heat resistant-wearable parts of engine, and thus represent the mainstream of contemporary research and development of metal matrix composites. However, the special microstructure of SiCp/Al MMCs, which lead to the poor weldability and the difficulty in obtaining high strength welded joint, severely hinder the application of the composites in welded construction. Therefore, a systematic study on the weldability of aluminum matrix composites is imminent.In this paper, SiCp/Al MMCs were selected as research objects, and process and mechanism of SiCp/6061Al MMCs by plasma arc welding(Ar and N2 as ionized gas, Ar as fielded gas) weresystematically studied. OM(optics microscopy), SEM(scanning microscopy) and XRD(X-ray diffraction) were used to observe the microstructure and changing of compositions of fusion welded joints of composites. Mechanical properties of fusion welded joints were measured by microhardness and tensile tests.The analysis on PAW for SiCp/6061Al matrix composites without fillers showed that there were a lot of needle-like reaction phase Al₄C₃ and also many defects such as cracks, gas pores, incomplete fusion in the weld, which decreased the tensile strength and general properties.When PAW "in-situ" welding for SiCp/6061Al matrix composites with 0.8mm thick titanium plate as filler was conducted, the interfacial reaction was controlled, the formation of Al₄C₃ was effectively prohibited, a new composite reinforced by TiN and TiC was produced in the weld, consequently ensuring the quality of joint. The mechanical testing results show that tensile strengths of samples welded by PAW without fillers was only 31.24% to that of the parent metal, and 50.89% to that by adding 0.8mm thick titanium. The SEM observations showed that the fracture surface of welded joint exhibited hybrid tough fracture.When Ti-Si,Ti-Mg and Ti-Cu mixed powder was employed as fillers to PAW "in-situ" welding of SiCp/6061Al MMC, The results showed that the joints welded with Ti-Cu mixed powder was not ideal, needle-like crisp phase Al₄C₃ and a lot of porous were formed in the weld, which severely affected the mechanical properties. However the dense metallurgical joining with obscure interface could be acquired by adding Ti-Si and Ti-Mg mixed powder into the poor, and the needle-like phases was not found too. Corresponding to the three kinds of mixed powders, which were Ti-Si filler,Ti-Mg filler and Ti-Cu filler, the maximum strengths obtained were 232.3MPa, 204.7MPa and 187.5MPa respectively.The effect of nitrogen addition into the plasma gases on the appearance of weld, microstructure and mechanical properties was also investigated. The results showed that the nitrogen volume content increased, the weld penetration also correspondingly increased. The thermal cycle state and metallurgy response of the molten poor were also improved by adding the nitrogen. A new composite reinforced by TiN, AlN was produced in the weld, effectively suppressing the formation of Al₄C₃, consequently ensuring the quality of joints. The mechanical testing results showed that the microhardness and the tensile strengths of joins welded by adding the nitrogen were both increased. The tensile strengths of joints maximized 233.5MPa when the nitrogen volume content was approximately 15%.