As a high purity copper material, oxygen-free copper (OFC) has excellent thermal conductivity, electrical conductivity, corrosion resistance, processing and weldability. Based on the extensive application of copper in various fields, its welding performance has been paid more and more attention. However, copper in the welding process will appear the following problems:

 (1) The weld is difficult to fuse, the forming is bad;

 (2)Easy to occur welding deformation, the tendency of hot cracking; 

(3)The large heat input leads to the coarse grain size and the decrease of the performance of the welded joint. Solution: High Electron beam welding efficiency, energy concentration, small heat input, make weld metal cooling fast, can effectively avoid coarse grains. In recent years, some scholars have made some researches on the welding properties of copper and copper alloys with dissimilar metals. The main welding methods are friction stir welding, laser welding, Argon arc welding, brazing and so on, there has been little research on oxygen free copper Electron beam welding. In this paper, the microstructure and mechanical properties of the welded joints of large thickness oxygen-free copper plates were studied by using reasonable Electron beam welding process, which can provide reference for the application of copper and copper alloy Electron beam welding.

1.Sample preparation and test methods

Test Material: Domestic annealed oxygen-free Copper Sheet with Tu 1, chemical composition (WT%) : 99.97 cu + Ag, 0.002P, 0.004Fe, 0.003Pb, 0.003Zn, 0.002O, 0.004S. Test Equipment: High Voltage vacuum electron beam welder ZD150-15A, Leca DM6000M optical microscope, DMH-2 microhardness tester, Zeiss Supra 55 scanning electron microscope, Shimadzu AG-100kNG electronic universal testing machine process mode: critical penetration welding, welding parameters for Accelerated Service Standard: GB/T 228.1-2010 metallic materials-tensile testing-part 1: Test Methods at room temperature

2.Test results and discussion

(1)microstructure

As can be seen from Fig. 1, the cross-sectional surface morphology of the welded joint of oxygen-free copper thick plate is good without obvious defects. The Weld is a pin-shaped weld with a large depth-to-width ratio. The maximum width is about 1mm. The microstructure of the weld and the base metal is clearly demarcated, while the heat affected zone on both sides of the weld is narrow and the base metal is not clearly demarcated.

As can be seen from Fig. 2, the base metal is a single-phase-Cu structure, due to the low stacking fault energy of copper, a large number of annealing twins are formed in the crystal, a large number of equiaxed grains are formed in the weld pool during solidification, and the heat affected zone is affected by welding heat, the closer the base metal is, the smaller the undercooling degree and cooling rate, the coarser the grain size and the microstructure of the weld and the base metal are-Cu phase.

As can be seen from Fig. 3, the equiaxed grain size of the weld decreases with the increase of the distance from the upper surface of the weld. 2. Microhardness

As can be seen from Fig. 4, the hardness at all 3 positions is 50 ~ 60 HV0.025. For welded joints in the same position, there is no significant difference in hardness between the weld line, heat affected zone and base metal, there is no significant difference in microhardness with different grain sizes, that is, the resistance to local deformation is the same at the upper, middle and lower positions. The purity of oxygen-free copper is very high. There is almost no difference in composition between inside and outside the weld pool during welding. Although the grain size of the microstructure in the upper and lower parts of the weld is quite different, the microhardness of the material has not changed much.

(3)tensile properties

As can be seen from Table 1, the tensile strength of the Electron beam welding joint is slightly lower than that of the base metal, about 97% of that of the base metal, and the yield strength is equivalent to that of the base metal. Compared with the base metal, the elongation after fracture and the shrinkage of the section show that the elongation after fracture of the joint is slightly lower than that of the base metal, but the shrinkage of the section is almost the same, which shows that the plasticity of the welded joint is not obviously lower, to the extent required for use.

Fig. 5 shows the microstructure of the Longitudinal section of the welded joint broken at the Base metal and the weld, respectively. It can be seen that before the fracture of the base metal, the grains are elongated in the form of fibers, and a large number of cracks appear in the grains, this also explains why the average elongation of welded joints after fracture is slightly lower than that of the base metal.

Figure 6 is the SEM micrograph of the tensile fracture of the base metal and the welded joint. It can be seen that the tensile fracture of both the oxygen-free Copper Base metal and the welded joint smooth specimen are ductile fracture with obvious necking. The fracture shape is cup-cone shape, the cone is 45 ° from the principal stress. The fracture surface of the base metal is relatively smooth, and a large number of equiaxed dimples are uniformly distributed on the surface. A large number of snake-like sliding patterns appear on the side of the fracture surface, which is the result of the interaction of several sliding systems during stretching and opening. The fracture surface of the weld is uneven, the distribution of dimples is uneven, and there are elongate dimples formed by local shear.

3.Conclusion

(1)Due to the concentration of Electron beam welding energy, low heat input and high thermal conductivity of Oxygen Free Copper, the weld seam of the welded joint is narrow and there is no obvious defect on the surface. The microstructure of the weld zone is as-cast equiaxed microstructure of G C, that of the base metal is annealing twin microstructure of g Cu, and that of the heat affected zone is coarse grain growth microstructure of the base metal. 

(2)There is no obvious change in the microhardness of the welded joint from the weld to the base metal, which indicates that the ability of the weld and the base metal to resist microzone deformation is equal.

 (3)The results show that the tensile strength and plasticity of the welded joints are similar to that of the Base metal and the weld. 

Source: "physical and chemical examination-physical volume, " wechat public number, from: "physical and chemical examination-physical volume, " vol. 54,2018.3 by Ding Xun, assistant engineer, China Aviation Manufacturing Technology Research Institute