External factors influencing the root crack of 6 series aluminum alloy fillet welding

With the rapid development of China's high-speed railway and the continuous improvement of aluminum alloy welding technology, aluminum alloy car body has also been widely used in high-speed rail projects. As one of the welding quality control items, the quality of aluminum alloy fillet welds is also one of the key control contents in high-speed rail projects.

     Relatively easy to carry outRT,UTFor butt welds with equal volume nondestructive testing, the quality of butt welds is easy to judge and control. The fillet weld, due to its structural factors, in the case of non-destructive, can not be internal volume class testing, such.RTWait. To better understand the post-weld quality of fillet welds, it can only be verified by macroscopic section or fracture tests. The actual welding macro sample shows that the root crack of aluminum alloy fillet welding is a key quality problem, which often plagues aluminum alloy manufacturers.

1Test process
   In order to study the impact of various external factors on6The quality of the aluminum alloy fillet weld is affected by the two most common thicknesses of the product.4mm,10mmof6005AThe profiles were subjected to different tests. Chemical composition of base material as shown in Table1Welding shielding gas is99.999%High purity argon. The test adopts the melting electrode argon arc welding process, and the welding adopts the imported Olymcon brand.ER5087welding wire with a diameter1.2mm, detailed chemical composition as shown in the table2.

Table1  6005AChemical composition of aluminum alloy (%）

Table2  ER5087Chemical composition of welding wire (%）

 
      The test starts with the change of welding torch angle and the change of plate thickness combination during welding. Under the same conditions, only one influencing factor is changed at a time to verify the effect of the crack at the root of the diagonal weld. See Table for specific welding parameters3.

Table3  6005AAluminum alloy fillet welding parameters

 
2Influence of welding gun angle during welding
      Figure1shown, welding with different gun angles and the wire pointing to the center of the weld. Through the macroscopic metallographic, it can be seen that the angle of the welding torch is40°~45º, the weld forming is more beautiful than other welding gun angles, and the vertical plate and the bottom plate are fused well on both sides, as shown in the figure.2b; and the torch angle is45º~50 degreeWithin the interval, the fusion of the weld bottom plate is less. Through microscopic metallographic analysis, it can also be found that the angle of the welding torch is greater50 degreeIn the case of, there will be a contraction groove extending into the weld through the fusion line at the root of the weld. At the front end of the contraction groove, there are fine cracks extending to the center of the weld, as shown in the figure.3.
              

      
Figure1 Schematic Diagram of Angle Test of Welding Torch

             Figure2 Macroscopic metallographic diagrams under various torch angles

Figure3 Torch angle greater50 degreemicro-metallographic appearance of welding seam

         
3Effect of arc pointing
During welding, the direction of the arc can usually be biased to the vertical plate.1~1.5mm, pointing to the center, biased to the bottom plate1~1.5mmOne of the three ways, as shown in4. When verifying the influence of arc pointing, the above verification welding operation angle is better35 degrees~45ºLocation.
 

Figure4 Schematic diagram of arc direction during welding
           
Figure5 Various arc points to the lower weld macro and micro metallographic
 

Figure5It is the macroscopic metallographic phase of the weld and the root of the weld toe under three different arc directions. Through metallographic observation, it can be found that when the arc points slightly to the vertical plate or to the center of the weld toe, microscopic cracks will appear at the root of the weld, while when the arc points slightly to the bottom plate, the microscopic defects at the root of the weld will disappear. The reason is mainly because the fillet weld is a three-dimensional heat dissipation mode, there are two directions of heat conduction on the bottom plate, and only one direction of heat conduction on the vertical plate. When the arc points to the vertical plate or to the center of the welding toe, the bottom plate dissipates heat faster than the vertical plate due to heat conduction in two directions, so the molten pool on the bottom plate in the root area of the welding toe will condense first, while the molten pool in the direction of the vertical plate will condense later. under the action of shrinkage tensile stress, the root of the welding toe will be found as shown in the figure5a,bWhen the arc points slightly to the bottom plate, the condensation rate of the molten pool at the root of the weld toe in the three-dimensional direction is equivalent, and the force in the three-dimensional direction of the molten pool is balanced, so the root of the weld toe is not easy to appear micro-cracks.

In addition, the arc observation during the welding process: when the arc points slightly to the vertical plate, the arc will actively expand and stretch to the bottom plate, and in the process of the arc extending and stretching to the bottom plate, the arc will often jump, and the unstable tendency of the welding process is relatively larger than the other two welding methods. The beating of the arc in the welding process also increases the probability of welding defects.

4Influence of vertical plate and bottom plate thickness
      In the actual production of aluminum alloy high-speed train, it is often encountered that the thickness of the vertical plate and the bottom plate is not equal to the thickness of the fillet welding. In order to explore the similarities and differences in the quality of the fillet weld when the vertical plate and the bottom plate are not thick, and combined with the actual characteristics of the aluminum alloy high-speed train product structure, the choice4mm,10mmTwo kinds of common thickness of the profiles are respectively bottom plate and vertical plate, welding test. The torch angle and arc direction used in the trial welding are the best parameters based on the above relevant test results. Please refer to the figure for details.6.

Figure6 Schematic diagram of vertical plate and bottom plate combination with different thickness
 

Figure7 Microscopic metallographic of fillet weld root

Figure7for the base plate4mm, and the standing board is10mmMicroscopic metallographic analysis of welds. There are small micro cracks in the root, and extend from the root to the center of the weld. When the fillet weld is welded, the molten pool dissipates heat in three directions. Because the bottom plate is thin and dissipates heat in two directions, its heat conduction speed is faster than that of the thicker vertical plate, so the cooling speed of the molten pool side of the bottom plate is faster than that of the vertical plate. Due to the difference in cooling rate on both sides of the molten pool during the welding process, the newly formed solid-state weld is subjected to tensile stress, which in turn produces microcracks.

5Conclusion
By6005AWelding test of aluminum alloy fillet weld, can draw the following conclusions:
5.1  6When welding aluminum alloy fillet, the welding gun operating angle that is most conducive to ensuring the internal quality of the weld is35 degrees~45º;
5.2 In order to reduce6The tendency of micro-cracks at the root of fillet weld of aluminum alloy, the welding wire should be directed slightly to the bottom plate, about a diameter distance of the welding wire;
5.3 Welding6In the case of aluminum alloy fillet welds, the thickness of the base plate should be as much as possible not thinner than the vertical plate, so as to avoid microcracks at the root of the weld.

From WeChat Public Number<Welding process>