Study on the Electron Beam Repair Welding of 5000 Aluminum Alloys

Article information

J Weld Join. 2024;42(5):530-538

Publication date (electronic) : 2024 October 31

doi : https://doi.org/10.5781/JWJ.2024.42.5.10

Insung Choi ^*

, Heon min Seo ^**

, In-Chul Jung ^**

, Jungsoo Choi ^*

, In-Duck Park ^*

, Jeong Suh ^*^,

* Department of Industrial Laser Technology, KIMM, Busan, 46744, Korea

** Research Institute, Halla E. B. Technology Inc., Busan, 46748, Korea

†Corresponding author: jsuh@kimm.re.kr

Received 2024 August 15; Revised 2024 September 10; Accepted 2024 October 7.

Abstract

Electron beam welding (EBW) has been attracted due to its high energy density, high depth-to-width ratio, low thermal strain, compared with arc and laser welding. In addition, it ensures high quality weld joint in structural metals in a wide range of thickness from 0.025 mm to 300 mm. Therefore, EBW is widely used in industries such as automotive, aviation, nuclear power plant, etc. Although a lot of studies on EBW for several decades have been presented, EB based repair welding with supplement of filler metal has less reported. In this work, EB repair welding for 5083 aluminum alloys was investigated because the aluminum alloys are becoming more important as a lightweight material in mobility industries. Material characterization and mechanical properties of the repair-welded specimens were evaluated, compared with characteristics of base metal and bead welding without repair. The optimized experimental conditions showed tensile strength of 294 MPa, which is a similar to base metal. Our study indicates that EB repair welding can be considered for the candidate of repair welding of aluminum alloys.

Keywords: Electron-beam; Repair welding; Aluminum alloys; Filler wire; Al5083; Al5356

1. Introduction

Electron beam welding (EBW) technology using the heat generated during the collision of electrons with the materials has a unique benefit compared to competing technologies. In particular, it can weld ultra-thick plates at one pass without a filler metal using high energy density (e.g., 10⁷ W/cm²), and the ratio of the depth of the weld bead to its width may exceed 20. According to references, EBW can be applied to a thickness of 0.01 to 250 mm for steel and a thickness of up to 500 mm for aluminum1,2). Compared to laser beam welding technology, it is applicable regardless of the reflectivity of the material, and exhibits low residual stress and a small heat affected zone through low heat input. It can also exclude such effects as oxide film formation because the process is performed in a vacuum chamber. On the other hand, the size of the weldable specimen is limited by the size of the vacuum chamber, and it is difficult to apply EBW technology to magnetic materials. In addition, precise machining of the weld zone and the specimen preparation process close to the zero gap (e.g., 0.1 mm or less) are essentially required. Considering these benefits and shortcomings properly, EBW technology has been used as welding technology for mechanical parts that require high precision and high quality in major industries, such as automobiles, aviation, nuclear power, and defense3-5).

In recent years, material and component production technologies to apply non-ferrous materials have been developed for weight reduction in the field of transport machinery. Aluminum alloys, which are representative lightweight metals, have low density, high specific strength, excellent corrosion resistance, and low temperature characteristics, and the types of alloys are distinguished according to the heat treatment process and chemical composition. The main elements of aluminum alloys are Cu, Mn, Si, Mg, and Zn. Aluminum alloys are classified into 3000 (Al-Mn), 4000 systems (Al-Si), and 5000 (Al-Mg) non-heat treated alloys as well as 2000 (Al-Cu), 6000 (Al-Mg-Si), and 7000 (Al-Zn-Mg) heat treated alloys according to the heat treatment process. Although many studies have been conducted on arc welding and laser welding, welding defects, such as pore formation, high-temperature cracks, and lack of penetration, frequently occur during welding due to the unique characteristics of aluminum materials6-12). The EBW, on the other hand, can minimize the welding defects caused by hydrogen (H), such as pores, because it uses high energy density under vacuum condition13-16). Such defects as underfill, however, may occur on the weld surface, and they can be improved through repair welding. There are few studies on electron beam repair welding due to difficulty in supplying filler wires inside a high vacuum chamber.

In this study, electron beam repair welding was performed on the underfill-containing weld surface of the 5083 aluminum plate, which has the highest strength and excellent low-temperature characteristics among non-heat treated alloys, and mechanical properties were compared and analyzed with the specimen without repair welding. First, specimens with underfill were prepared through electron beam bead welding, and repair welding was performed using the 5356 aluminum wire with high weldability by optimizing the supply speed and electron beam current. Considering the diameter of the wire and the width of the underfill, the test specimens were prepared by varying the diameter of the electron beam from 1 to 3 mm, and a tensile test and a microhardness test were conducted. In addition, the microstructure, chemical composition, and texture of the repair weld metal (RWM) were analyzed, and the actual applicability of electron beam repair welding technology was examined.

2. Used Materials

2.1 Used Materials

Table 1 shows the chemical composition of the base metal (BM) and filler wire of 5000 aluminum alloys used for electron beam repair welding. The components of BM were measured using the optical emission spectroscopy method, and the mill sheet provided by the manufacturer was referred to for the components of the filler wire. The 5083 aluminum plate is an aluminum- magnesium alloy with a magnesium content of approximately 4.5%, which has the highest strength among non-heat treated alloys. It can be utilized in industries, such as pressure vessels, marine structures, and aviation, due to high weldability, high sea water resistance, and excellent low-temperature characteristics. The 5356 aluminum wire is a filler metal with high weldability with 5000 plates, high strength (maximum tensile strength: 290 MPa), and high corrosion resistance. In this study, the 5083 plate (250 mm (width) × 150 mm (length)) with a thickness of 8 mm and the 5356 wire with a diameter of 1 mm were used.

Table 1

Chemical composition of materials (wt%)

3. Experimental Method

3.1 Electron beam repair welding

In this study, bead welding and repair welding specimens were prepared using a 120 kV electron gun. First, the underfill specimens in the longitudinal direction (150 mm in length) were prepared. Next, the wire feeder was installed in the high vacuum chamber, and the process parameters for repair welding were investigated by supplying the wire along the underfill line. Key experimental variables include the electron beam current, weld speed, scanning diameter, wire supply speed, and supply angle. Table 2 summarizes the process parameters derived through this study. Bead welding specimen A with no defect was prepared to compare material properties with the repair welding specimen, and repair welding specimens B, C, and D were prepared by varying the electron beam diameter. In this study, the optimized current was 18 mA, which is converted into a power of 2.16 kW.

Table 2

Experimental condition of electron beam welding

3.2 Material analysis of repair welding specimens

KEYENCE’s VHX-7100 model was used as a digital optical microscope for the macro images of the cross section of electron beam repair welding specimens and millimeter-scale length measurement. Field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS) were performed using HITACHI’s SU5000 model to identify the chemical composition of the microstructure and elements. In addition, OXFORD’s C-Nano model was used as electron backscatter diffraction (EBSD) to analyze the texture of RWM.

3.3 Mechanical property testing

A microhardness test and a tensile test were conducted to examine the mechanical properties of RWM. MITUTOYO’s HM210D model was used for the microhardness test. Test conditions were a load of 0.1 kgf, a duration of 15 s, and a measurement interval of 100 ㎛. The tensile test complied with the code of ASME IX QW-150:2021, and SHIMADZU’s UH-F500kNI model was used. The bead welding specimen with no defect and the BM specimen were also subjected to the tensile test to compare their mechanical properties with those of the repair welding specimens.

4. Discussion

4.1 Material analysis of electron beam repair welding specimens

Fig. 1 shows the cross-sectional images of the bead welding of the 5000 aluminum plate with a thickness of 8 mm (Fig. 1(a)) and the repair welding specimens (Fig. 1(b) to 1(d)). The bead welding specimen (A) used an electron beam diameter of 0.5 mm and a current of 18 mA, and welding defects, such as underfill, pores, and cracks, were not found. The specimens with underfill were artificially created by increasing the electron beam current, and the width and depth of underfill were 2 mm and 0.65 mm, respectively. In this study, repair welding characteristics were analyzed by fixing the electron beam current, scanning speed, and wire supply speed while varying the electron beam diameter (1 to 3 mm) among the repair welding-related process parameters. For reference, the wire supply speed, an important parameter, was set to four times the scanning speed of the electron beam by referring to a previous study17). Fig. 1(b) to 1(d) show the macro-scale cross-sectional images of repair welding. Remelting and fusion with the filler wire were occurred, during repair welding, at the 3 to 4 mm weld zone in the depth direction. Welding defects, such as pores and cracks, were not found.

Fig. 1

Cross-sectional digital images of EBW specimens, (a) specimen A : bead welding, (b) specimen B : repair welding, (c) specimen C : repair welding, (d) specimen D : repair welding. (e) surface image of repair welding specimen, (f) schematic of repair welding process

Fig. 2(a) to 2(c) show the SEM images that magnified the cross section of the RWM of repair welding specimens B to D by 200 times. The scanning diameter of the electron beam was varied, but no difference in microstructure was found. Fig. 2(d) to 2(f) show the cross-sectional images of the interface between RWM and the weld metal (WM). The interface can be clearly identified due to grain refinement at the interface. It appears that the upper part of the interface was solidified due to the occurrence of remelting and mixing with the filler metal while the lower part of it had only the remelting of WM. Microstructure changes of RWM and WM were identified through the EBSD analysis of this study.

Fig. 2

Cross-sectional FE-SEM images (200×) of repair welding. Repair weld metal (RWM) of (a) specimen B, (b) specimen C, (c) specimen D. interface images between RWM and WM of (d) specimen B, (e) specimen C, (f) specimen D

EDS analysis was conducted to identify the chemical composition of RWM and WM. Fig. 3 shows the cross-sectional images (500× magnification) of RWM and WM, which are located at 2 and 5 mm, respectively, in the depth direction from the surface of specimen B, and the EDS energy spectrum analysis results. The contents of the elements of aluminum and magnesium confirm that the plate and wire are 5000 aluminum alloys. EDS analysis was also conducted at the same location for specimens A, C, and D. The results of analyzing the contents of Al, Mg, Si, Fe, Mn, Cr, and Ti are summarized in Table 3. It should be noted that the content of Mg at the RWM of repair welding specimens B, C, and D was more than 20 % less than the filler wire (4.6 % → 3.39 to 3.64 %), indicating the possibility of Mg evaporation during the repair welding process.

Fig. 3

Cross-sectional FE-SEM and EDS analysis of specimen B. (a) EDS analysis position of RWM (i) and WM (ii), (b) FE-SEM image (500×) and (c) EDS analysis spectrum of RWM (i), (d) FE-SEM image (500×) and (e) EDS analysis spectrum of WM (ii)

Table 3

EDS elemental analysis (wt%)

4.2 Mechanical property testing of electron beam repair welding specimens

The microhardness test was conducted at 2 and 5 mm from the specimen surface to evaluate the mechanical properties of the electron beam repair welding specimens. The location of 2 and 5 mm from the surface corresponds to RWM and WM area, respectively. Considering the small size of the weld bead, a load of 0.1 kgf was maintained for 15 seconds and the test was conducted at a total of 50 locations at 100 μm intervals. For reference, fine polishing was performed up to 1 μm before the test, and chemical etching was performed after the test to identify the location of the indentation. For a direct comparison, the bead welding specimen without repair welding (A) was also subjected to the hardness test under the same test conditions. Fig. 4 shows the microhardness test results for specimens A and B. Comparing Fig. 4(b) and 4(e) revealed that the hardness of the weld was slightly higher than that of BM for specimen A without repair welding while the hardness of RWM was slightly lower than that of BM for repair welding specimen B.

Fig. 4

Microhardness analysis. Specimen A, (a) optical images of indentation position, (b) microhardness at 2 mm from surface, (c) microhardness at 5 mm from surface. Specimen B, (d) Optical images of indentation position, (e) microhardness at 2 mm from surface, (f) microhardness at 5 mm from surface

The result that the hardness of RWM was slightly lower than that of BM was also similarly observed from the test of specimens C and D. Fig. 5(b) and 5(e) show the hardness values of specimens C and D at RWM, respectively. Overall, specimen D, which applied repair welding with an electron beam scanning diameter of 3 mm, showed lower hardness values than specimens B and C, which applied repair welding with a scanning diameter of 1 to 2 mm.

Fig. 5

Microhardness analysis. Specimen C, (a) optical images of indentation position, (b) microhardness at 2 mm from surface, (c) microhardness at 5 mm from surface. Specimen D, (d) optical images of indentation position, (e) microhardness at 2 mm from surface, (f) microhardness at 5 mm from surface

Fig. 6 shows the strain-stress curves of specimens A to D and BM after the tensile test. For each specimen, three specimens were prepared and the tensile test was conducted for the reliability of the test results. For specimen A without repair welding, the tensile strength was approximately 12% lower compared to BM. In the case of specimens B to D with repair welding, however, higher tensile strengths than specimen A could be confirmed. In particular, specimens B and C, which applied repair welding with a scanning diameter of 1 to 2 mm, exhibited tensile strengths similar to that of BM. Specimen D, which applied repair welding with a scanning diameter of 3 mm, showed slightly lower tensile strength. Fig. 6(f) shows the results of quantitatively comparing tensile strengths by referring to the stress- strain curve results. The average tensile strengths of specimens A to D were found to be 261, 294, 292, and 285 MPa, respectively, while that of BM was 296 MPa. The experiment results of this study confirmed that an electron beam scanning diameter of 1 to 2 mm led to similar mechanical properties while a scanning diameter of 3 mm caused slightly lower mechanical properties when a wire diameter of 1 mm was supplied.

Fig. 6

Strain-stress curves of specimen (a) A, (b) B, (c) C, (d) D, and (e) base metal, comparison graph of tensile strength test of (a) ~ (e)

4.3 Texture analysis of electron beam repair welding specimens

To analyze the phenomenon that the strength of EBW specimens decreases while it increases to a level similar to that of BM after repair welding, the grain boundary (GB) and kernel average misorientation (KAM) map were analyzed using the EBSD technique. Fig. 7 shows the GB map and KAM map analysis results at the center of the weld bead of specimen A (i) and the boundary with the heat affected zone. Dendritic structures in the vertical direction were observed in the approximately 200 μm area at the center of the weld, and high angle grain boundaries of more than 15 degrees were concentrated. In addition, strain was also relatively high at the center on the KAM map. For this reason, it is judged that the tensile strength of the weld was lower than that of BM.

Fig. 7

EBSD analysis of Specimen A. (a) Cross-sectional digital image, Analysis of (i), (b) Image quality-grain boundary (IQ-GB) map, (c) Kernel average misorientation (KAM) map, Analysis of (ii), (d) IQ-GB map, (e) KAM map, (f) Rotation angle of boundaries of (i) and (ii)

Fig. 8 shows the GB map and KAM map analysis results at the RWM of specimen B (i) and the interface between RWM and WM (ii). No dendritic structure was observed in the RWM area, but polygonal lamellar structures were identified. Overall, similar deformation occurred and high angle grain boundaries were dominant (approximately 85 %). At the interface, grain refinement and small deformation were observed in the RWM area. This appears to have improved tensile strength after repair welding. Phase map analysis was also conducted even though it was not included in the figures of this paper, and the FCC crystal structure of 100 % aluminum alloys was identified.

Fig. 8

EBSD analysis of Specimen B. (a) Cross-sectional digital image, Analysis of (i), (b) IQ-GB map, (c) KAM map, Analysis of (ii), (d) IQ-GB map, (e) KAM map, (f) Rotation angle of boundaries of (i) and (ii)

5. Conclusion

In this study, the electron beam repair welding characteristics of 5000 aluminum alloys, which have the highest strength among non-heat treated aluminum alloys, were analyzed. A tensile test and a microhardness test were conducted to examine the mechanical properties of the repair weld metal (RWM), and the results of bead welding specimens and the base metal (BM) were compared and analyzed. In addition, mechanical property testing and material analysis were carried out by varying the scanning diameter of the electron beam, and the following conclusions were drawn.

1) After the EBW of a 5000 aluminum alloy with a thickness of 8 mm, the tensile strength decreased by approximately 11% compared to BM. The tensile strength became similar to that of BM after repair welding that used the filler metal. This appears to be due to the microstructure change and grain refinement. Dendritic structures in the vertical direction were observed from the electron beam weld metal (WM), but lamellar structures were observed from RWM. In the lower part of the interface where the filler metal was not mixed, the existing dendritic structures partially formed lamellar structures due to remelting. In addition, the electron backscatter diffraction (EBSD) analysis confirmed that the average size of the grains of RWM decreased by approximately 20% compared to WM.

2) From a perspective of process optimization, since only surface melting is required for repair welding, it is judged that repair welding will be possible with lower than the power of 2.16 kW used in this study. In addition, the wire supply speed is important and it was optimized to be four times faster than the travel speed of the electron beam.

Currently, there is no domestic study that published regarding electron beam repair welding using filler metal wires. The results of this study confirmed the applicability of electron beam repair welding technology to lightweight mechanical parts that use aluminum alloys.

Acknowledgment

This work was supported by the Construction for SMR Auxiliary device parts Manufacturing Pilot-Center Program funded by the Korea Institute of Energy Technology Evaluation and Planning (No: 20241E000 00020) and the E-Mobility Laser Utilization Technology Manufacturing Equipment Advancement Program funded by Korea Institute for Advancement of Technology (No: 00430048).

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Article information Continued

This Journal follow the http://creativecommons.org/licenses/by-nc/4.0/ Creative Commons Attribution Non-Commercial No Derivatives License

Material		Al	Mg	Si	Fe	Cu	Mn	Cr	Zn	Ti
base metal	Al5083	Bal.	4.30	0.12	0.18	0.03	0.60	0.06	0.04	0.06
filler wire	Al5356	Bal.	4.60	0.05	0.10	0.00	0.12	0.11	0.00	0.06

Specimen	Position		Al	Mg	Si	Fe	Mn	Cr	Ti
A	WM	2 mm	94.87	4.27	0.03	0.11	0.54	0.08	0.10
A	WM	5 mm	94.74	4.38	0.04	0.08	0.60	0.11	0.05
B	RWM	2 mm	95.80	3.39	0.10	0.15	0.42	0.11	0.04
B	WM	5 mm	94.63	4.31	0.11	0.20	0.53	0.17	0.04
C	RWM	2 mm	95.69	3.64	0.05	0.09	0.42	0.07	0.04
C	WM	5 mm	95.00	4.20	0.06	0.06	0.65	0.00	0.03
D	RWM	2 mm	95.67	3.49	0.00	0.07	0.48	0.16	0.12
D	WM	5 mm	94.94	4.19	0.03	0.22	0.43	0.13	0.07

Specimen		Voltage (kV)	Current (mA)	Weld speed (mm/min)	Repair weld speed (mm/min)	Wire supply speed (mm/min)	Defocus beam (mm)	Underfill (mm)
A	Bead welding	120	18	1200	-	-	0.5	-
B	Repair welding	120	18	-	600	2400	1.0	0.60
C	Repair welding	120	18	-	600	2400	2.0	0.65
D	Repair welding	120	18	-	600	2400	3.0	0.65