Microstructure and Mechanical Properties of TIG and A-TIG Welded AZ61/ZK60 Magnesium Alloy Joints

Literature Overview

This study, published in the Transactions of the Nonferrous Metals Society of China in 2019, investigates the microstructure and mechanical properties of gas tungsten arc welded (TIG) and arc-transferred TIG (A-TIG) welded joints of AZ61 and ZK60 magnesium alloys. The research was conducted by Bo QIN and colleagues from Xiangtan University and the Hunan Provincial Key Laboratory of Vehicle Power and Transmission System, in collaboration with researchers from the National Engineering Research Center for Magnesium Alloys at Chongqing University. The work was supported by the National Natural Science Foundation of China (Project No. 51771160) and the Hunan Provincial Natural Science Foundation (Project No. 2018JJ4048). Magnesium alloys are increasingly used in automotive, aerospace, and biomedical applications due to their excellent specific strength and stiffness, but their weldability remains a significant challenge.

Core Technical Content

AZ61 and ZK60 Alloy Characteristics

AZ61 is an Al-Zn-Mg alloy that is widely used due to its good combination of strength, corrosion resistance, and castability. ZK60 is a Zn-K-Mg alloy that offers higher strength than AZ61 due to the strengthening effect of zinc and potassium. The dissimilar welding of AZ61 and ZK60 presents unique challenges due to the differences in their chemical composition, thermal properties, and solidification behavior.

TIG vs. A-TIG Welding Processes

A-TIG welding uses a high-frequency arc transfer to initiate and maintain the welding arc, eliminating the need for contact between the tungsten electrode and the workpiece. This results in improved arc stability, reduced spatter, and the ability to achieve higher travel speeds with deeper penetration. For magnesium alloy welding, A-TIG offers particular advantages in terms of process consistency and reduced contamination.

Microstructural Analysis

Weld Metal Microstructure

The weld metal in AZ61/ZK60 dissimilar joints exhibits a complex microstructure influenced by the dilution ratio, cooling rate, and solidification behavior:

The formation of intermetallic phases such as Mg17Al12, MgZn2, and Mg2Zn3 at the fusion boundary and in the HAZ can significantly affect the mechanical properties of the joint. These phases are typically hard and brittle, and their distribution and morphology are critical factors in determining joint strength and ductility.

Mechanical Properties of Welded Joints

The A-TIG welded joints exhibit superior mechanical properties compared to conventional TIG welds, primarily due to the deeper and more uniform penetration, which reduces the dilution ratio and minimizes the formation of brittle intermetallic phases. The higher travel speed in A-TIG welding also reduces the heat input, limiting grain growth and phase coarsening in the HAZ.

Welding Defects and Countermeasures

Engineering Practice and Quality Control

Shielding Gas Requirements

Magnesium is highly reactive at elevated temperatures, and the shielding gas must provide excellent protection against atmospheric contamination. The following gas parameters are recommended:

The use of backing gas is essential for magnesium alloy welding to prevent oxidation on the root side of the weld. Without adequate backing protection, the root pass will exhibit significant oxidation, leading to porosity and reduced joint strength.

Quality Inspection Criteria

Key Questions and Reflections

The dissimilar welding of AZ61 and ZK60 magnesium alloys presents a practical challenge in lightweight structural applications where different sections of a component may require different material properties. The key question is whether the mechanical properties of the dissimilar joint can be made sufficiently uniform to meet the design requirements for critical structural applications.

The A-TIG welding process offers a promising solution to the challenges of magnesium alloy welding, particularly in terms of process consistency, reduced contamination, and improved mechanical properties. However, the equipment requirements for A-TIG welding are more complex and expensive than conventional TIG welding, which may limit its adoption in some manufacturing environments.

Study Insights and Implications

This research provides valuable insights into the welding of dissimilar magnesium alloys, which is an important consideration for lightweight structural design. The A-TIG welding process demonstrates clear advantages over conventional TIG welding for magnesium alloy applications, particularly in terms of weld quality and mechanical properties. For engineers involved in the fabrication of magnesium alloy components, the key takeaways are: (1) A-TIG welding should be preferred for critical applications where joint strength is paramount; (2) the shielding gas system must be carefully designed to provide adequate protection against oxidation; (3) the dilution ratio in dissimilar joints must be controlled to minimize the formation of brittle intermetallic phases; and (4) thorough quality inspection is essential to ensure the integrity of magnesium alloy welded joints. Future research should focus on developing automated A-TIG welding procedures for complex geometries, investigating the long-term mechanical properties of magnesium alloy joints under cyclic loading, and developing standardized welding procedures for dissimilar magnesium alloy combinations.