It can be seen from Fig. 2 and Fig. 3 that the melting zone of the conventional laser welding head is mainly composed of Al 4 C 3 and gray massive particles Si , and the Al 4 C 3 is needle-like and brittle, which reduces the mechanical properties of the metal matrix composite.  . The size and number of Al 4 C 3 depends on the heat input of the laser, ie the degree of reaction between the reinforcing phase ( SiC ) of the composite and the matrix ( 2124Al ) is directly proportional to the laser energy. Therefore, proper control of laser parameters may reduce the formation of aluminum carbide.
It can be seen from Fig. 4 and Fig. 5 that although the SiC particles in the laser-induced reaction welding seam with titanium added are all disappeared, the needle-like Al 4 C 3 phase is not found, and the resulting SiC particles are formed in a small size. TiC particles, their morphology, as shown in Figure 6 . In addition, phase analysis shows that AlCuMg and Al 7 Cu 3 Mg 6 are also formed in the welded joints of conventional laser welding and laser induced reaction welding. Ti is mainly present in the weld in the form of TiC , and a small amount of Ti is dissolved in the Al matrix, and a very small amount of the titanium aluminum compound may be present, but no titanium aluminum compound is found in the phase analysis.
Figure 6 TiC morphology generated by laser induced reaction welding
From the welds obtained in Figure 2 , there is no redistribution zone of SiC particles found in the literature [ 6 ] . This is mainly because the material used in this test is very fine SiC particles, only an average diameter of 3 Î¼ m, and  of SiC particles having an average diameter of 10 Î¼ m. The smaller the SiC particles, the larger the surface area, and the easier it is to completely react with the liquid aluminum to disappear. In the literature [ 6 ], the matrix material of MMCs is A356 , its Si content is very high (about 7% ), and there is free Si . According to the reaction formula ( 1 ), Si can inhibit the formation of Al 4 C 3 , so, Al 4 C 3 is formed only in a region of higher temperature in the melting zone. However, in the 2124 matrix, the Si content is extremely low and free Si is not present, so the formation of Al 4 C 3 is not inhibited, and Al 4 C 3 can be formed throughout the melting zone.
The phases involved in conventional laser welding and laser induced reactive welding are mainly Al , SiC , Ti . Under the action of high-energy laser, SiC melted or melting [2,5] to produce C. Therefore, the chemical reactions that may occur during the welding process are:
4Al+3SiC=Al 4 C 3 +3Si ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ (1)
Î” G T =-11 260 + 10.83T ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€
Ti+SiC=TiC+Si ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ (2)
Î” G T = -28 500 + T ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€
Al 4 C 3 +3Ti=3TiC+4Al ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ (3)
Î” G T =-74 120 - 7.83T ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€
Ti + C = TiC ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ (4)
Î” G T =-44 100 + 2.902T ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€
4Al + 3C = Al 4 C 3 ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ã€€ (5)
Î” G T =-58 180 + 9.936T
The thermodynamic data in the formula is taken from the literature [ 7, 8 ].
The variation of free ç„“Î” G with temperature T in these reactions can be represented by Figure 7 .
Figure 7 The variation of Î” G with T
In the laser welding of SiC particle reinforced 2124 aluminum matrix composite, Al 4 C 3 is formed by the reaction formula ( 1 ), and since Al 4 C 3 easily reacts with water, the joint often becomes brittle. Conversely, if TiC is formed in the joint instead of Al 4 C 3 , the joint performance may increase because the thermal stability of TiC is extremely high, melting at 3343K but not decomposing (At this temperature, Al 4 C 3 is completely Decomposed), and its density and hardness are higher than SiC and Al 4 C 3 .
It can be known from the reaction free ç„“Î” G that after the addition of Ti to the weld, the reaction between SiC and Ti is easier than the reaction with Al , so the reaction is more likely to form TiC ; although some SiC may react with Al during the welding process. Al 4 C 3 , however, the newly formed Al 4 C 3 will immediately react with Ti ( 3 ) to form TiC . The use of the reactive element Ti as an interfacial filler can increase the surface energy and can improve the wetting properties of the matrix material by forming a stable TiC .
In summary, both theory and experiment have proved that the laser induced reaction welding method of silicon carbide reinforced aluminum matrix composite can completely eliminate the Al 4 C 3 brittle phase and form a stable TiC phase in the melting zone , which can improve the joint performance of the composite.
Fund Project:Project supported by the National Natural Science Foundation of China (59575069)
About the author: Xu Jiuhua, born in October 1964 , is an associate professor at the School of Mechanical and Electrical Engineering of Nanjing University of Aeronautics and Astronautics. Engaged in professional: mechanical manufacturing and automation. Research direction: cutting, forming and laser welding. Deputy Director of China Aviation Cutting Research Association, Deputy Director of Cold Processing Professional Committee of Jiangsu Aviation Society. He has won 5 provincial-level scientific and technological progress awards and published 30 academic papers.
Fan Yunan, born in September 1966 , is currently a Ph.D. student at the School of Mechanical and Electrical Engineering of Nanjing University of Aeronautics and Astronautics. He is mainly engaged in laser welding and anti-friction coatings.
Lin Xiangfeng male, born in March 1941 , is a professor of materials engineering at Nanjing University of Aeronautics and Astronautics. He is mainly engaged in research on welding and heat treatment. He has published more than ten academic papers in recent years.
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 , China
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