When the metal matrix composite is welded by conventional fusion welding, since the composite reinforcement has a long contact time with the molten base metal, it is easy to accelerate the chemical reaction between the reinforcement and the matrix, which often leads to serious diffusion and enhancement between the two. Decomposition of the body, or even complete destruction. In addition, large pores often appear in the weld zone, which reduces the strength of the joint.
Therefore, these welding methods are not suitable for the welding of structures. Other joining techniques such as diffusion welding, friction welding, electron beam welding and electric resistance welding, although proven to be effective joining methods, may require complex special equipment, or require special joint forms, or butt weldments. Due to the high structural requirements, there are many restrictions in practical applications. Mechanical joining is often an effective method, however, such joints can cause catastrophic failure due to poor toughness and the tendency to form stress concentrations.
Although laser welding has many advantages such as low total heat input, high energy density, high welding speed, small deformation and small heat affected zone, there is still a strong interface when used for welding sic reinforced aluminum matrix composites. The reaction forms a brittle phase of al4c3 and the joint performance is deteriorated. In order to solve this problem, at home and abroad, the laser parameters are mainly used to slow down the interface reaction, or the aluminum matrix composite with a high Si content (such as a356, 6061) is used to suppress the interfacial reaction. However, these two methods cannot The harmful reaction product al4c3 between the sic and the base metal (al) is completely eliminated.
Test conditions and methods
The material used for the test was 2124al+20vol% sicp aluminum-based composite material, and the heat treatment state was “solution treatment + artificial agingâ€, and the average diameter of the sic particles of the reinforcement was 3 μm, and the metallographic structure was as shown in FIG. 1 .
The laser used for welding is an nd:yag pulsed solid state laser. The laser parameters are: the wavelength is 1.06μm, the average power is less than 100w, the maximum single pulse energy is 20j, the pulse frequency is 10 times/second, the pulse width is 2.5ms, the divergence angle is <6mrad, and the focus position is on the surface of the sample.
The welding method used is: 1 conventional laser welding without filler; 2 laser induced reaction welding - in order to eliminate the addition of other elements to increase the complexity of reactive welding, only pure titanium is added to the weld. The test piece size is 40 mm × 10 mm × 2 mm. The joint form is butt weld.
The phase structure analysis of the substrate and the weld is carried out on a Japanese d/max-ra target x-ray diffractometer with cu as the target and graphite as the monochromator. The voltage and current vary with the sample.
The conventional laser welding head melting zone is mainly composed of al4c3 and gray block particles si, and al4c3 is needle-like and brittle, which will reduce the mechanical properties of the metal matrix composite. The size and number of al4c3 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.
Although the sic particles in the laser-induced reaction welds with titanium added disappeared, the needle-like al4c3 phase was not found, and instead, fine tic particles were formed, and its morphology was as shown in Fig. 6. In addition, phase analysis shows that alumg and al7cu3mg6 are also produced 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 a matrix, and a very small amount of titanium aluminum compound may be present, but no titanium aluminum compound is found in the phase analysis.
From the obtained welds, there is no sic particle redistribution zone found in the literature. This is mainly because the sic particles in the material used in this test are very small, and the average diameter is only 3 μm, while the average diameter of the sic particles in the literature is 10 μm. The smaller the sic particle is, the larger the surface area is, and the easier it is to completely react with the liquid aluminum to disappear. In the literature, the matrix material of mmcs is a356, its si content is very high (about 7%), and there is free si. According to reaction formula (1), si can inhibit the formation of al4c3, so al4c3 only has temperature in the melting zone. Formed in higher areas. However, the content of si in the 2124 matrix is ​​extremely low, and no free si exists. Therefore, the formation of al4c3 is not inhibited, and al4c3 can be formed in the entire melting zone.
The phases involved in conventional laser welding and laser induced reactive welding are mainly al, sic, ti. Under the action of a high-energy laser, sic melts or melts to produce c. Therefore, the chemical reactions that may occur during the welding process are:
4al+3sic=al4c3+3si(1)
Δgt=-11 ​​260+10.83t
Ti+sic=tic+si(2)
Δgt=-28 500+t
Al4c3+3ti=3tic+4al(3)
Δgt=-74 120-7.83t
Ti+c=tic(4)
Δgt=-44 100+2.902t
4al+3c=al4c3(5)
Δgt=-58 180+9.936t
In laser welding of sic particle reinforced 2124 aluminum matrix composite, al4c3 is formed by reaction formula (1), and since al4c3 easily reacts with water, it often causes the joint to become brittle. Conversely, if tic is formed in the joint instead of al4c3, the joint performance may increase because the thermal stability of tic is extremely high, melting at 3343k but not decomposing (al4c3 is completely decomposed at this temperature), and its Both density and hardness are higher than sic and al4c3.
It can be known from the reaction free 焓δg 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 and al may react to form al4c3 during the welding process. However, the newly formed al4c3 will immediately react with ti (3) to form tic. The use of the reaction element ti as an interfacial filler can increase the surface energy and can improve the wetting property 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 brittle phase of al4c3 and form a stable tic phase in the melting zone, which can improve the joint performance of the composite.
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