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International Journal of Metalcasting

, Volume 13, Issue 2, pp 345–353 | Cite as

Parameter Optimization of Gas Tungsten-Arc Repair Welding Technique in Mg–6Gd–3Y–0.5Zr Alloy

  • D. H. Meng
  • B. Zhou
  • D. WuEmail author
  • Y. Q. Ma
  • R. S. ChenEmail author
  • P. J. Li
Article
  • 26 Downloads

Abstract

The feasibility of gas tungsten-arc repair welding was studied in Mg–6Gd–3Y–0.5Zr alloy. The processing parameters were optimized by sequential experiments including comparative study to find proper filler material and the type of welding groove by butt welding, and the investigation of influence of preliminary heating, area of repair welding, post-weld heat treatment and thickness of welding plate on the mechanical properties of welded joints. In addition, the analysis of microstructure and tensile properties of Mg–6Gd–3Y–0.5Zr alloy was performed for comparison with welded joints. The results showed that the microstructure in as-cast condition was characterized by equiaxial α-Mg and eutectic phase, including Mg24(Gd,Y)5 and Mg5(Gd,Y) phase. The strength was improved greatly after T6 treatment, while the ductility was remarkably decreased. The filler material and the type of welding groove were chosen as Mg–6Gd–3Y–0.5Zr alloy and V-type, respectively. The optimal process parameters of repair welding consisted of preliminary heating, T6 treatment after welding, 4 mm-thickness plates and welding area with the diameter of Φ15 mm. The tensile strength of welded joints reached the maximum value up to 295 MPa, equivalent to 99.7% of base material.

Keywords

gas tungsten-arc (TIG) repair welding Mg–6Gd–3Y–0.5Zr processing parameters optimization mechanical properties 

Introduction

The application development of magnesium alloys in automotive and aerospace industries has been attributed to their low density, high specific strength, good energy efficiency and recyclability.1, 2, 3 Most of Mg alloy parts are produced by casting. However, their further applications are restricted by casting defects, such as porosity, inclusion and misrun. Some of these defects in complex castings are unavoidable even though they are processed by advanced casting techniques and in a modified cooling system.4, 5, 6 For castings with these inevitable defects, the economical gas tungsten-arc repair welding is an advantageous solution in terms of both time and cost saving.7

Recently, Mg–Gd–Y system alloys have received considerable attention for their higher strength and better creep resistance than commercial Mg alloys.8 Extensive work has been focused on the investigations to find the relationship between mechanical properties and microstructures,9, 10, 11 while study on their weldability and repair welding process has not been done. To promote their applications, a reliable process to repair the casting parts of Mg–Gd–Y alloy by gas tungsten-arc repair welding is necessary.

In present investigation, Mg–6Gd–3Y–0.5Zr alloy12, 13, 14 was selected to investigate and optimize some typical processing parameters of gas tungsten-arc repair welding, including the determination of filler material, welding groove, preliminary heating, area of repair welding, post-weld heat treatment and thickness of welding plates. It is believed that this research is of benefit to improve the reliability and consistency of Mg–6Gd–3Y–0.5Zr alloy castings due to effective removal of the defects by gas tungsten-arc repair welding.

Experimental Procedure

The base material used in this study was nominal Mg–6Gd–3Y–0.5Zr (wt%) alloy plates and fabricated by differential pressure casting. The actual chemical composition is shown in Table 1, which was analyzed by an inductively coupled plasma (ICP).
Table 1

Actual Chemical Composition of the Test Mg–6Gd–3Y–0.5Zr Alloys

Element

Gd

Y

Zr

Mg

Content/(wt%)

5.66

2.75

0.44

Balance

Gas tungsten-arc welding process was applied to repair the cast plates with voltage of 380 V and current of 150A under the protection of argon. The processing parameters of butt welding and repair welding are listed in Table 2. The butt welding was designed to optimize the filler material and the type of welding groove. The influence of other parameters on the tensile properties of welded joints was analyzed through repair welding. It should be noted that prior to welding, the machined plate surface was slightly ground using SiC paper and cleaned by methanol to remove contaminants.
Table 2

Processing Parameters of Butt Welding and Repair Welding

Welding manner

Number

Plate thickness

Welding groove

Filler material

Welding area

Preliminary heating

Post-weld heat treatment

Butt welding

BSP01

4 mm

V-type

Mg–6Gd–3Y–0.5Zr

No

T6

BSP02

4 mm

V-type

Pure Mg

No

T6

BSP03

4 mm

V-type

Mg–9Gd–3Y–0.5Zr

No

T6

BSP04

8 mm

X-type

Mg–6Gd–3Y–0.5Zr

No

T6

Repair welding

BSB01

4 mm

V-type

Mg–6Gd–3Y–0.5Zr

Φ15 mm

No

T6

BSB02

4 mm

V-type

Mg–6Gd–3Y–0.5Zr

Φ5 mm

Yes

T6

BSB03

4 mm

V-type

Mg–6Gd–3Y–0.5Zr

Φ25 mm

Yes

T6

BSB04

4 mm

V-type

Mg–6Gd–3Y–0.5Zr

Φ15 mm

Yes

BSB05

4 mm

V-type

Mg–6Gd–3Y–0.5Zr

Φ15 mm

Yes

T6

BSB06

8 mm

V-type

Mg–6Gd–3Y–0.5Zr

Φ15 mm

Yes

T6

Tensile tests were carried out by a universal tensile testing machine. Samples containing the weld zone in gauge section for tensile tests have a rectangular cross section (3 mm thick and 10 mm wide) and a gauge length of 36 mm. The tests were performed in compliance with GB/T228A223.15 The final data of tensile tests were based on the average result of three samples to ensure the reproducibility of result. Vickers hardness testing was taken using 4.9 N load and holding time of 10 s. The hardness was determined as the average value of five indentations in a row parallel to welding line. Microstructural examination was performed by an optical microscope (OM) and a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX). Samples for OM and SEM investigations were ground and polished to produce a mirror-like finish. The as-cast samples were etched in a 4 vol.% nitric acid alcohol solution, while the T6-treated samples were etched in a solution of 6 g picric acid, 40 ml acetic acid, 40 ml water and 100 ml ethanol. The mean grain size was determined using a linear intercept method, using the equation d = 1.74 L, where L is the linear intercept.16

Experimental Results and Discussion

Microstructure and Mechanical Properties of Mg–6Gd–3Y–0.5Zr Alloy

Figure 1 shows the microstructure of Mg–6Gd–3Y–0.5Zr alloy observed by optical microscopy. As shown in Figure 1a, the microstructure of as-cast Mg–6Gd–3Y–0.5Zr alloy mainly consisted of equiaxial α-Mg and discontinuous network of eutectic phase situated at grain boundaries. After T6 treatment, the eutectic phase was mostly dissolved into the matrix, as shown in Figure 1b. The grain size of Mg–6Gd–3Y–0.5Zr alloy was measured to be about 96 μm in T6 condition.
Figure 1

Microstructure of Mg–6Gd–3Y–0.5Zr alloy in (a) as-cast and (b) cast T6 condition.

Figure 2 presents the typical SEM images and EDX results of Mg–6Gd–3Y–0.5Zr alloy. In accordance with the optical results, the eutectic phase can be observed in as-cast condition in Figure 2a. According to the zoom-in area in Figure 2b, two typical types of second phase were observed, including skeleton-shape phase and cuboid phase. The EDX analysis illustrated that the chemical compositions of skeleton-shape phase and cuboid phase were 88.22 Mg–4.10Y–7.68Gd (at.%) and 70.43 Mg–12.83Y–16.74Gd (at.%), respectively. Consistent with previous researches,17 skeleton-shape phase was determined to be Mg24(Gd,Y)5 and cuboid phase was determined to be Mg5(Gd,Y). After T6 treatment, Mg24(Gd,Y)5 phase was almost completely dissolved into the matrix (Figure 2c). However, some Mg5(Gd,Y) phase still existed. It should be noted that some Mg5(Gd,Y) phase will not be eliminated even after solution treatment at 565 °C for 30 h (approaching to the solidus temperature).18
Figure 2

(a) SEM image and (b) EDX results of Mg–6Gd–3Y–0.5Zr alloy in as-cast condition. (c) SEM image of Mg–6Gd–3Y–0.5Zr alloy in-cast T6 condition.

The tensile properties of Mg–6Gd–3Y–0.5Zr alloy both in as-cast and cast T6 condition are shown in Figure 3. Mg–6Gd–3Y–0.5Zr alloy belongs to precipitation hardening alloy which has a remarkable aging hardening effect. The precipitation sequence was reported as follows: α-Mg supersaturated solid solution (S.S.S.S.) → β″ (D019) → β′(cbco) → β1(fcc) → β(fcc), among which β′ phase was the main strengthening phase.19,20 The ultimate tensile strength (UTS) and yield strength (YS) were considerably improved after T6 treatment (UTS: 193 MPa to 296 MPa, YS: 121 MPa to 200 MPa). However, the elongation (EL) decreased from 4.5 to 3.7%.
Figure 3

Tensile properties of Mg–6Gd–3Y–0.5Zr alloy in as-cast and cast T6 condition.

Microstructure and Hardness of Butt-Welded Joints Using Mg–9Gd–3Y–0.5Zr Alloy as Filler Material

Figure 4 shows the microstructure evolution from weld zone to base material of butt-welded joints using Mg–9Gd–3Y–0.5Zr (wt%) alloy as filler material. The grain size in weld zone was measured to be about 15 μm, which was much smaller than the grains in heat affected zone and base material (about 130 μm). In the process of welding, the alloy in weld zone was remelted and resolidified. Both small amount of the remelted alloy compared with castings and attachment to unmelted heat affected zone and base material (acting as good heat transfer medium with low temperature) contribute to higher cooling rate of the remelted alloy than the melt of differential pressure casting,21 leading to fine grains in weld zone.
Figure 4

Microstructure evolution from weld zone to base material of butt-welded joints using Mg–9Gd–3Y–0.5Zr alloy as filler material.

Selected points for the hardness test and corresponding results of butt-welded joints using Mg–9Gd–3Y–0.5Zr alloy as filler material are presented in Figure 5. It exhibited that the base material possessed the highest hardness and the heat affected zone possessed the lowest hardness. The hardness variation of butt-welded joints was mainly related to gas pore (gas entrapment) introduced by welding process and grain size distribution. Gas pore is the discontinuity among microstructure, reducing the effective force bearing area, strength and hardness.22 The microstructure of weld zone, heat affected zone and base material is shown in Figure 6. Specifically, few of gas pore existed in base material, leading to the highest hardness. Although gas pore both existed in heat affected zone and weld zone, fine grain size in weld zone accounted for the higher hardness than that in heat affected zone.
Figure 5

(a) Selected points for the hardness test of butt-welded joints using Mg–9Gd–3Y–0.5Zr alloy as filler material and (b) corresponding results.

Figure 6

Microstructure of (a) weld zone (b) heat affected zone and (c) base material.

Filler Material and Welding Groove Optimization

The filler material and the type of welding groove were optimized through the analysis of butt-welded joints. Since it was well established that the filler material should be of similar chemical composition with base material, Mg–6Gd–3Y–0.5Zr alloy, Mg–9Gd–3Y–0.5Zr alloy and pure Mg (> 99.95%) were chosen as the potential filler material for repair welding of Mg–6Gd–3Y–0.5Zr alloy components. Pure Mg was not a good filler material for components welding of Mg–6Gd–3Y–0.5Zr alloy, since some conspicuous weld imperfections were observed by visual inspection of butt-welded joints in the macrostructure of welding line as shown in Figure 7. Figure 8 shows the tensile properties of butt-welded joints using Mg–6Gd–3Y–0.5Zr alloy and Mg–9Gd–3Y–0.5Zr alloy as filler material. Butt-welded joints using Mg–6Gd–3Y–0.5Zr alloy as filler material exhibited a better mechanical behavior than that using Mg–9Gd–3Y–0.5Zr alloy. The fracture surface of butt-welded joints with Mg–6Gd–3Y–0.5Zr alloy and Mg–9Gd–3Y–0.5Zr alloy filler material is shown in Figure 9. Some large visual fracture planes in Figure 9b (illustrated in red rectangle) were associated with anomalous grain growth and large cleavage planes, leading to the lower tensile properties of butt-welded joints with filler material of Mg–9Gd–3Y–0.5Zr alloy. Accordingly, the best filler material was determined as Mg–6Gd–3Y–0.5Zr alloy, i.e., the same as base material.
Figure 7

Welding line processed by butt welding using pure magnesium as filler material.

Figure 8

Tensile properties of butt-welded joints using Mg–6Gd–3Y–0.5Zr alloy and Mg–9Gd–3Y–0.5Zr alloy as filler material.

Figure 9

Fracture surface of butt-welded joints with different filler materials: (a) BSP01 Mg–6Gd–3Y–0.5Zr alloy (b) BSP03Mg–9Gd–3Y–0.5Zr alloy.

The mechanical properties of butt-welded joints processed by X-type and V-type welding groove are illustrated in Figure 10. There was no considerable difference in strength between butt-welded joints with X-type and V-type welding groove. However, better ductile properties were found in butt-welded joints with V-type welding groove. Consequently, no advantage was introduced by complex X-type welding groove. Actually, X-type welding groove was implemented by V-type welding on both sides of welding plate, which means two welds were needed. Much more defects may be introduced into the welded joints processed by X-type groove, leading to its lower ductility. The welding groove was determined as V-type.
Figure 10

Tensile properties of butt-welded joints with different welding grooves.

Repair Welding Parameters Optimization

The repair welding parameters variations had a great influence on the tensile behavior of welded joints. Figure 11 presents the tensile properties of welded joints with varied processing parameters, including preliminary heating, area of repair welding, post-weld heat treatment and welding plate thickness. The welded joints of BSB02 (with area of repair welding in diameter of Φ5 mm) and BSB03 (with area of repair welding in diameter of Φ25 mm) possessed similar mechanical properties, which were worse than BSB05. The optimal area of repair welding was testified to be in diameter of Φ15 mm (BSB05). A great effect of post-weld heat treatment on the tensile properties of welded joints was observed. Both the UTS and YS of BSB04 (without post-weld heat treatment) were conspicuously lower than other welded joints, yet with the best elongation (EL). Consequently, post-weld heat treatment was needed to obtain welded joints with high strength. The welding plate thickness influenced the welding electricity and welding time. It was found to affect the tensile properties of welded joints greatly in present research. The YS of BSB05 (4 mm thickness) was similar with BSB06 (8 mm thickness), while the UTS and EL of BSB05 were much higher than that of BSB06, with an increase of 10.1% and 79.1%, respectively. The thickness of welding plate was chosen as 4 mm.
Figure 11

Tensile properties of welded joints with varied processing parameters.

The macroscopic fracture surfaces of welded joints without preliminary heating (BSB01) and with preliminary heating (BSB05) are shown in Figure 12. A great amount of inclusion was found in the fracture surface of BSB01, while the amount of inclusion was much lesser in the fracture surface of BSB05. Inclusion is known as detrimental defect which accounts for lower UTS of BSB01 than BSB05. The lesser amount of inclusion during welding was attributed to the reduction of vapor after preliminary heating, though there still existed some inclusion in the fracture surface in BSB05. Therefore, it was necessary to heat welding plate prior to repair welding.
Figure 12

Fracture surfaces of welded joints with/without preliminary heating: (a) without preliminary heating and (b) with preliminary heating.

Finally, the best comprehensive tensile properties of welded joints (BSB05) were achieved by preliminary heating and post-weld heat treatment with welding area in diameter of Φ15 mm and welding plate thickness of 4 mm. Figure 13 compares the tensile properties of Mg–6Gd–3Y–0.5Zr alloy (in-cast T6 condition) with that of BSB05. There was no strength drop after repair welding when the parameters were optimized. However, a ductility drop with EL decreasing from 3.7 to 1.2% was observed in BSB05, which was introduced by the inclusion in weld zone and heat affected zone.
Figure 13

Tensile properties of Mg–6Gd–3Y–0.5Zr alloy and BSB05.

Conclusion

A series of experiments were performed to optimize repair welding process for Mg–6Gd–3Y–0.5Zr alloy, including the determination of filler material, welding groove, preliminary heating, welding area, post-weld heat treatment and welding plate thickness. The conclusions can be drawn as follows:
  1. 1.

    The microstructure of Mg–6Gd–3Y–0.5Zr alloy in as-cast condition mainly composed of equiaxial α-Mg and eutectic phase, including Mg24(Gd,Y)5 and Mg5(Gd,Y) phase. After T6 treatment, Mg24(Gd,Y)5 phase was almost completely dissolved into the matrix, while Mg5(Gd,Y) phase still existed. The ultimate tensile strength and yield strength increased up to 296 MPa and 200 MPa, respectively, while the ductility decreased from 4.5 to 3.7%.

     
  2. 2.

    For Mg–6Gd–3Y–0.5Zr alloy, the base material was believed to be the best filler material in comparison with pure Mg and Mg–9Gd–3Y–0.5Zr alloy. Complex X-type welding groove introduced no improvement in mechanical properties to welded joints, and the welding groove was determined as V-type.

     
  3. 3.

    Gas tungsten-arc (TIG) repair welding was testified feasible in Mg–6Gd–3Y–0.5Zr alloy plates. No remarkable reduction in strength of welded joints was achieved in optimal process. The tensile strength of welded joints reached the maximum value up to 295 MPa, equivalent to 99.7% of base material (in-cast T6 condition), yet the inclusion in weld zone and heat affected zone would lead to a ductility drop.

     

Notes

Acknowledgements

This work was funded by the National Science and Technology Major Project of China through Project No. 2017ZX04014001, the National Key Research and Development Program of China through Project No. 2016YFB0301104, the National Natural Science Foundation of China (NSFC) through Projects Nos. 51301173, 51531002, 51601193 and 51701218, and the National Basic Research Program of China (973 Program) through Project No. 2013CB632202.

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Copyright information

© American Foundry Society 2018

Authors and Affiliations

  1. 1.Department of Mechanical EngineeringTsinghua UniversityBeijingChina
  2. 2.The Group of Magnesium Alloys and Their Applications, Institute of Metal ResearchChinese Academy of SciencesShenyangChina
  3. 3.School of Materials Science and EngineeringUniversity of Science and Technology of ChinaHefeiChina
  4. 4.Beijing Institute of Astronautical Systems EngineeringBeijingChina

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