Elena Koleva and Georgi Mladenov *
E LECTRON B EAM W ELDING OF D ISSIMILAR M ATERIALS
Nevertheless that the copper could be used to braze steel, the joining of these dissimilar metals by fusion welding is difficult. The copper and steel are not very compatible components for mixing in a weld. Often explosive or friction welding was applied [21,22] for that joints, but use of these methods are highly dependent of configuration of the components.
Electron beam welding (EBW) process has been found to be especially well suited in this area. In aerospace applications, nuclear and scientific devices design various joints of these metals, such as heat exchanger tubes [23], copper cavities and copper beam lines with Conflate stainless-steel flanges [24] are done by EBW. Selection of the appropriate welding conditions and parameters needs [25-27] thorough investigations. In this paper are given results of a study of welding conditions and obtained welds of these dissimilar metals.
The EBW is done using a conventional 60 kV electron beam welder. The vacuum chamber volume is about 300 l and the vacuum pressure during welding is 10-4 Torr. The gun cathode is from tungsten sharp with width 1 mm. The experiments are performed with plates, placed horizontally on the manipulator in the vacuum chamber of EBW machine. They are weld together using a vertical electron beam. Below the joint welded a thin copper plate is placed (with thickness of about 2 mm) in the case of 10 mm copper plate thickness or by
Process Parameter Optimization and Quality Improvement at Electron Beam Welding 113 machining a 2 mm sub-plate under the joint was formed in the case of 12 mm copper plate.
After the EBW the welded plates were cut to pieces with width 12 mm and then machined in order to make narrower central parallel length (with width 10 cm) of the specimen for the tensile test. The welding was performed for one pass without preheating. Welding results at beam position on the steel or on the copper predominately are investigated. Oscillations of the beam are not used during the experiments. The accelerating voltage is 60 kV and the distance from the electron gun to the sample surface is 36 cm. The variations in the experimental conditions are given in Table 4.
Three standard types of copper were used during the experiments (see Table 4). The data for the copper used and the chemical composition of specimens are given in Table 5. The chemical composition of the stainless steel (SST) according the Bulgarian Standard (BDS) is presented in Table 6. These types of SST correspond closely to SST used in other standards (German, American Iron and Steel Institute, Russia):
BDS DIN 1.4541 AISI321 GOST
X18h9t X10crniti189 Ae30321 12x18h10t
BDS DIN 1.4501 AISI316 GOST
X18h10m21 X5crnimo1810 Sae30316 04x19h11m3
Table 4. Welding experimental conditions
№ Ib, mA v, cm/s If. mA Type of Cu1 Type of SST2 PBD3
1 70 0.5 501 b A SST
2 65 0.5 501 b A SST
3 75 0.7 495 b A SST
4 70 0.5 509 a A SST
5 80 0.7 501 b A Cu
6 85 0.7 501 b A Cu
7 82 0.5 478 c B Cu (65%)
8 90 0.5 485 c B Cu (90%)
1 types of Cu: a – M1; b – M3 grade I; c – M3 grade II
2 types of SST: A – X18H10M21; B- X18H9T
3PBD – predominant beam direction
Table 5. Analysis of the chemical composition (wt.%) of Cu plates observed by optical spectral method
Copper type Pb Sn Ni Fe As Sb Bi Zn
M1 (99.9%) 0.0006 0.0003 0.004 0.014 0.001 0.002 0.00004 0.0003 M3 grade I
(99.5%)
0.027 0.029 0.006 0.030 0.001 0.001 0.001 0.049 M3 grade II
(99.5%)
0.0063 0.0085 0.0024 0.0031 0.001 0.001 0.001 0.011
Table 6. Standard chemical composition according BDS of SST in weight % (max) or (from-to)
Type SST C Si Mn P S Mo Cr Ni Ti
X18H9T 0.12 0.8 2.0 0.035 0.025 0.3 17-19 8-10 5xC%-0.8 X18H10M21 0.15 1.5 2.0 0.04 0.04 2-2.5 17-19 9-11 -
The strength of the welds is tested using Instron 1195 Testing machine and Alfred J, Amsler & Co testing machine. Extensio-meter model G 51 12 M with length L=25 mm is used in the case of Instron machine, while the extensometer at A.A & Co machine is with length L=50 mm. The measurements of the strength are performed at room temperature.
Figure 13. The relationship of the force and the relative extension vs. time for the weld 1 (see Table 4)
Figure 14. Tensile profile for weld 1 (see Table 4)
Process Parameter Optimization and Quality Improvement at Electron Beam Welding 115 The placed in the jaws piece was stretched at a rate of 0.05 per min. During this time the force on the machine set of jaws increases. At the use of the Instron testing machine the relationship of the force and the relative extension vs. time as well as the force vs. relative extension or the force vs. displacement (extension measured in length units). The results, obtained for weld 1, when the beam was preliminary directed toward SST, are given on Figures 13 and 14 (see Table 4 for the welding parameters).
Figures 15 and 16 present results obtained for welds 5 and 8 (Table 4), when the beam was directed preliminary on Cu. The test in Figure 16 was stopped before reaching the breaking point.
The ultimate strength (UTS) and the proportional limit (PL) values are presented at Figure 17 for all the experimental conditions.
Figure 15. Tensile profile for weld 5 (see Table 4)
Figure 16. Tensile profile for weld 8 (see Table 4)
Figure 17. Ultimate strength (UTS) and the proportional limit (PL) for the welds, obtained at 8 experimental conditions (Table 4)
a) b)
Figure 18. Micrographs of weld 1, weld 8 (5)
On Figure 18 and 19 are shown the micrographs of the cross-sections of the welds performed under the conditions of the weld 1 and weld 8, using different level of enlargement. The first weld corresponds to preliminary beam direction during welding on SST (Figure 18a), while at the second weld – the preliminary beam direction is toward Cu (Figure 18b).
On Figure 19 can be seen the mixing of the welded materials in the interface zone.
Hardness distributions of these welds (1 and 8) are shown in Figures 20 and 21. They are measured in each cross-section using Vickers hardness tester with 10 kg load. The welds have satisfactory hardness (not very high). In the other cases of measurements of wider welds intermediate hardness is observed. When copper of type M3 grade I is used a decrease of the Vickers hardness in the thermally affected Cu zone during the process of welding is observed.
The use of SST: X18H9T and Cu: M3 grade II is better then SST: X18H10M2 and Cu: M3 grade I from the hardness point of view.
A brief attempt for scanning electron microscope testing was done using SEM JEOL JSM 35 CF electron microscope analyzer (using TRACOR NORTNERN TN 2000 energy dispersion system).
Process Parameter Optimization and Quality Improvement at Electron Beam Welding 117
a) b) c)
Figure 19. a) Microstructure of weld 1: X12H10M2 150;
b) microstructure of weld 1: interface (150);
c) microstructure of weld 8: interface weld-copper (125)
Figure 20. Hardness distribution of weld 1.
In Table 7 and Figure 22 are given the results of analysis along a line in the middle of the cross-section of two welds. It can be seen that in the used tensile tests copper content in the weld does not affect considerably the weld strength. According to the analysis of the welded metal it seems that there is a little vaporization of alloy components. In the given micrograph small SST and Cu drops in the metal can be seen. From our experience in investigating SST composition changes in such drops can be concluded that only Mn in SST drops has the ability to dissipate for a short time in the welding bath.
From some electron microscope examinations and from direct measuring with magnetometer (Ferritehaltmesser 1054, made by institute Dr. Forster, Reutlinen, Germany) small ferrite phase in the welds is observed. For the cross-section of the seam, produced at welding conditions of weld 1, this phase was at the top part of the weld. For the cross-section of weld performed at conditions of weld 11 (weld 10), the phase was at the root part.
Figure 21. Hardness distribution of weld 8
Table 7. EDS analysis of weld 1 (wt%) averaged on the analyzing spot 0.80.8 mm2
Points Components
Cu Fe Al Si Mo Cr Mn Ni Co
1 99.5
2 60.08 23.30 0.88 0.92 0.86 6.62 0.87 4.46 3 48.71 33.99 0.72 0.72 1.01 8.91 0.9 5.05
4 66.36 0.27 0.45 2.3 16.86 2.31 10.68 0.72
Figure 22. EDS analysis of weld 2 (wt%) averaged on the analyzing spot 0.20.2 mm2 (in 8 points) This result is important for the special use of designed calorimeter working in magnetic field. The quantity of this phase is small: from 1% to 5% in the part of the weld.
Process Parameter Optimization and Quality Improvement at Electron Beam Welding 119 The tensile tested specimens of electron beam welded joints of SST (X18H10M21 and X18H9T) and copper (M1 and M3) showed sound properties and were mostly fractured in the base metal. The absorbed energy of the weld metal in the case of beam predominately directed on Cu exceeds the assorted energy of the case of beam directed predominately on SST. The welding bath (and the surface) is more of the weld situated predominately in SST.
The increase of copper contaminant concentration causes vaporization, boiling spattering and splashing of the welded material.
An important conclusion in the working conditions was that welding deep must be on full sample thickness to have guaranties that stress concentration shall be avoided.
The investigation made was directed mainly toward the increase of the knowledge about the process, rather than making technological instructions. Due to the difference in the weld depth in the case of Cu and SST and difficult control of the exact beam shift towards the position of the component contact before the welding in the real conditions, the choice of the welding with beam direction on Cu must be recommended.