Preface
High-strength aluminum alloys have high room temperature strength and good high temperature and ultra-low temperature performance. They are widely used in structural materials for aerospace, aerospace and other vehicles, such as liquid fuel tanks for launch vehicles, structural components for supersonic aircraft and automobiles, and lightweight The armor of the chariot, etc. The main welding methods currently used for aluminum alloy connections are: alternating tungsten argon arc welding (TIG) and direct current reverse polarity gas metal arc welding (MIG). Due to the use of alternating current, the TIG welding has a severe tungsten burning loss, which limits the welding current used. Moreover, this method has a weak penetration capability and is therefore only suitable for the welding of thin aluminum alloys. MIG welding includes continuous current welding and pulse current welding. When MIG welding, the welding wire is used as the anode, which can use a larger welding current than TIG welding. The arc power is large and the welding efficiency is high, so it is especially suitable for the welding of medium and heavy plate aluminum alloy. Experimental studies have found that pulsed current welding is superior to continuous current welding in aluminum alloy MIG welding, which improves the strength, plasticity and fatigue life of aluminum alloy weld metal. In order to further improve the stability of the arc, improve the weld formation and increase the penetration depth, and the efficient welding of thick aluminum alloy, in recent years, the development of monofilament composite pulsed MIG welding and double wire Tandem MIG welding method has been developed abroad. 7A52 medium and heavy plate high-strength aluminum alloy, research on monofilament single pulse, composite pulse and double wire Tandem MIG welding process, and applied in production.
1 Tandem double wire welding and monofilament composite pulse MIG welding principle
Tandem double wire welding is to place two wires in a specially designed welding torch at a certain angle. The two wires are respectively powered by separate power sources. In addition to the different wire feed speeds, other parameters, such as the material, diameter, and pulse of the wire, can be set independently of each other, thus ensuring that the arc works optimally. Compared with other twin wire welding techniques, since the arc of the two wires is burned in the same molten pool, the total welding current is increased, thereby increasing the welding efficiency and the welding speed. At the same time, since the two welding wires are alternately fed into the same molten pool, the molten pool has a stirring effect, thereby reducing the pore sensitivity and improving the quality of the weld.
A) Same phase with the same frequency (suitable for welding steel)
B) 180° difference in phase with the same frequency (suitable for welding aluminum)
C) Arbitrary frequency phase (suitable for welding steel)
The monofilament composite pulse welding process uses a low-frequency coordinated pulse to modulate the peak and time of another high-frequency pulse, so that the intensity of the pulse is periodically switched between strong and weak, and the periodic weak pulse group is obtained. The voltage waveform is shown in Figure 3. The modulated welding current changes the arc pressure acting on the molten pool, which not only improves the penetration of the weld, but also obtains a uniform and beautiful fish scale weld, and also enhances the impact vibration on the molten pool, reducing and eliminating welding. Slotted holes.
2 Experimental materials and experimental methods
The base material used in the experiment was 7A52 high-strength aluminum alloy. The test plate size was 300×150×30mm, the plate was butt-joined, the groove shape was X-shaped, and the angle was 70°. The welding equipment adopts GLC553MC3 monofilament and TANDEM double-wire MIG welder produced by CLOOS of Germany. The welding wire is ER5A56, the diameter is φ=1.6mm, and the shielding gas is argon gas with a purity of 99.99%. The chemical composition of the base metal and wire is shown in Table 1. Before welding, remove the oil on both sides of the groove with acetone, and then clean the groove with a stainless steel wire brush. Firstly, the single-wire welding machine is used for the welding experiment of single pulse and composite pulse, and then the double wire welding equipment is used for the double wire welding process experiment, and the swing and laser tracking are added at the same time. The welding process parameters are shown in Table 2. During the single wire welding, the front and back sides are welded 6 times, and the double wire welding is performed on the front and back sides respectively, and the interlayer temperature is ≤100 °C.
3 Experimental results and discussion
It is found that the weld microstructure of the composite pulse welding is relatively small, and the eutectic structure distributed between the grain boundaries and the dendrites is uniformly distributed in the form of spheres or fine films. In contrast, the weld microstructure of single-pulse welding is relatively large, and the continuity of eutectic structure along the grain boundary and dendrite increases, which will affect the strength and plasticity of the weld metal. The composite pulse welding causes further refinement of the weld structure, which is caused by the periodic change of the arc current caused by the periodic change of the peak current, causing the vibration of the molten pool liquid and causing the molten pool liquid to agitate. In single-pulse welding, since the peak current of the pulse is constant and the pulse frequency is high, the arc pressure changes little, the amplitude of the liquid vibration on the surface of the molten pool is small, and the stirring effect of the molten pool is weak. In the composite pulse welding, due to the superposition of a low-frequency coordinated pulse, the peak current of the pulse changes continuously according to the frequency of the low-frequency pulse, and accordingly the arc pressure also changes greatly. When the peak current is high, the arc pressure is large, and the liquid on the surface of the molten pool is concave; when the peak current is low, the arc pressure is small, and the liquid on the surface of the molten pool is convex, thereby causing the upper and lower vibration of the liquid on the surface of the molten pool, causing Stirring of the molten pool liquid. The agitation of the molten pool liquid on the one hand increases the original convection in the molten pool, increases the liquid flow, reduces the temperature gradient, and expands the supercooled region of the front edge of the solid-liquid interface; on the other hand, the partially melted grains The nucleation core is added by leaving the side wall of the molten pool and entering the molten pool. In addition, the strong convection generated by the composite pulse can promote the crystallization of the grains detached from the side wall of the molten pool and the nucleation sites precipitated in the molten pool, such as TiAl3, ZrAl3, etc., to the supercooling zone of the front edge of the solid-liquid interface. The heterogeneous nucleation of α(Al) thus refines the weld structure. The above organizational changes are determined by the characteristics of the twin wire welding process. (1) When two wires are welded, the two wires are arranged side by side, so that the volume of the molten pool increases, the residence time of the high temperature becomes longer, and the cooling rate becomes slower. (2) The total heat input of the double wire welding is slightly higher than that of the single wire (monofilament) The heat input of welding and double wire welding is 8KJ/cm and 12KJ/cm respectively, the liquid metal in the molten pool is not overheated, and the burning loss of alloying elements is not much; (3) the stirring of the second pulse and the back wire arc The effect is to significantly reduce the tendency of the pores.
The composite pulse welded weld has the highest strength and plasticity, the joint coefficient of the weld metal is close to 70%, the weld of the single pulse weld is the second, and the weld strength and plasticity of the double wire weld are the lowest, which corresponds to their welding. The texture of the seam is consistent. The weld structure of the composite pulse welding is the smallest, and the grain boundary eutectic phase is uniformly distributed in the form of spherical and fine film, so the weld has the highest strength and the best plasticity. The double wire welding is not very large due to the total heat input. Although the coarse equiaxed dendrite structure is obtained, and the number of eutectic phases distributed along the grain boundary is significantly more than that of monofilament welding, the strength and plasticity are slightly reduced. . The single-pulse welding is coarser than the composite pulse welding, but it is obviously thinner than the double-wire welding, so the strength and plasticity are obviously better than the double-wire welding, which is slightly lower than the weld of the composite pulse welding. From the experimental results of this study, although the strength and plasticity of the weld metal of the double wire welding is reduced, the two welding wires are burned in the same molten pool during the double wire welding, which improves the total welding current and makes the welding The deposition efficiency is much higher than that of single wire welding, which can fully meet the performance requirements of welded joints, and is especially suitable for the welding of medium and heavy plate aluminum alloys.
4 Conclusion
(1) During the single-wire composite pulse welding, the strong mixing action of the molten pool liquid caused by the periodic change of the bee value current refines the weld structure and improves the strength and plasticity of the weld.
(2) The welding efficiency of double wire MIG is much higher than that of single wire welding, and the weld performance meets the requirements, which is especially suitable for the welding of medium and heavy plate aluminum alloy.
(3) Production application proves that monofilament composite pulse MIG welding and TANDEM double wire MIG welding are ideal methods for welding aluminum alloy.
Pipe Porn Tube,Seamless Steel Tube Gals,Alloy Steel Tube,Double Wall Steel Pipe
Steel Tubing,Stainless Pipe Co., Ltd. , http://www.czsteelpipes.com