Microstructure and Mechanical Properties of Friction Stir Welded Dissimilar Materials between AA7075 Aluminum Alloy and Magnesium Alloy

Table of Contents

1. Introduction
   • Background
   • Objectives
2. Literature Review
   • Friction Stir Welding (FSW)
   • Properties of AA7075 Aluminum Alloy
   • Properties of Magnesium Alloy
3. Methodology
   • Material Selection
   • Welding Process Parameters
   • Sample Preparation
4. Microstructural Analysis
   • Optical Microscopy
   • Scanning Electron Microscopy (SEM)
   • Energy Dispersive X-Ray Spectroscopy (EDS)
5. Mechanical Properties Evaluation
   • Hardness Testing
   • Tensile Testing
   • Fatigue Testing
6. Results and Discussion
   • Microstructural Findings
   • Mechanical Properties Comparison
   • Effect of Process Parameters
7. Conclusion
   • Summary of Findings
   • Future Work
8. References
9. Appendices

1. Introduction

Background

Friction Stir Welding (FSW) is a solid-state joining process that has gained significant attention for its ability to join dissimilar materials. This method offers advantages such as lower distortion, absence of filler materials, and the ability to join materials that are difficult to weld using traditional fusion welding techniques. This study focuses on the FSW of dissimilar materials, specifically AA7075 aluminum alloy and magnesium alloy, which are commonly used in the aerospace and automotive industries due to their high strength-to-weight ratios.

Objectives

The primary objectives of this study are:

• To investigate the microstructural characteristics of the weld zone between AA7075 aluminum alloy and magnesium alloy.
• To evaluate the mechanical properties, including hardness, tensile strength, and fatigue resistance, of the welded joints.
• To analyze the effects of different FSW process parameters on the quality of the weld.

2. Literature Review

Friction Stir Welding (FSW)

Friction Stir Welding (FSW) was invented in 1991 by The Welding Institute (TWI). It involves a non-consumable tool that rotates and traverses along the joint line of the materials to be welded. The heat generated by friction softens the materials, which are then mechanically mixed to form a solid-state weld upon cooling.

Key Advantages:

• Lower thermal distortion compared to fusion welding.
• No requirement for filler materials or shielding gases.
• Ability to join a wide range of materials, including dissimilar combinations.

Properties of AA7075 Aluminum Alloy

AA7075 is a high-strength aluminum alloy predominantly used in aerospace applications. It has excellent mechanical properties and resistance to stress corrosion cracking.

Key Properties:

• High tensile strength (up to 572 MPa).
• Good fatigue strength.
• High resistance to corrosion.

Properties of Magnesium Alloy

Magnesium alloys are the lightest structural metals, making them highly desirable for automotive and aerospace applications where weight reduction is critical. However, they are known for their poor weldability using conventional methods.

Key Properties:

• Low density (1.74 g/cm³).
• High specific strength.
• Good damping capacity.

3. Methodology

Material Selection

The materials selected for this study are AA7075 aluminum alloy and a common magnesium alloy (AZ31B). Both materials were procured in plate form with dimensions suitable for FSW.

Welding Process Parameters

The key parameters varied in this study include:

• Tool rotational speed (RPM).
• Welding speed (mm/min).
• Axial force (kN).
• Tool tilt angle.

Sample Preparation

The plates were cut into smaller specimens for microstructural and mechanical testing. Each specimen was meticulously prepared to ensure consistency in the welding process.

4. Microstructural Analysis

Optical Microscopy

Optical microscopy was used to observe the macrostructure and microstructure of the welded joints. This technique provided an initial understanding of the grain structure and defects such as porosity or cracks.

Scanning Electron Microscopy (SEM)

SEM offered high-resolution images of the weld zone, allowing for a detailed examination of the intermetallic compounds formed during welding.

Energy Dispersive X-Ray Spectroscopy (EDS)

EDS was used in conjunction with SEM to analyze the elemental composition of the weld region. This helped in identifying the distribution of aluminum, magnesium, and other alloying elements.

5. Mechanical Properties Evaluation

Hardness Testing

Vickers hardness testing was performed across the weld zone to determine the hardness profile. This provided insights into the distribution of hardness and the effects of FSW on material properties.

Tensile Testing

Tensile tests were conducted on specimens extracted from the weld region. The ultimate tensile strength (UTS), yield strength, and elongation at break were measured and compared with the base materials.

Fatigue Testing

Fatigue tests were carried out to evaluate the endurance limit of the welded joints. This was crucial for understanding the long-term performance of the welds under cyclic loading conditions.

6. Results and Discussion

Microstructural Findings

The microstructural analysis revealed a fine-grained structure in the weld zone, indicative of dynamic recrystallization during FSW. The interface between aluminum and magnesium showed the presence of intermetallic compounds, primarily Al3Mg2 and Al12Mg17, which were characterized using SEM and EDS.

Mechanical Properties Comparison

The hardness profile across the weld zone exhibited variations, with the highest hardness values observed at the interface due to the formation of hard intermetallic compounds. Tensile testing results showed that the welded joints had lower UTS compared to the base materials, primarily due to the brittle nature of the intermetallic compounds.

Effect of Process Parameters

The welding speed and tool rotational speed significantly affected the microstructure and mechanical properties of the joints. Higher rotational speeds led to increased heat input, resulting in a wider heat-affected zone and more pronounced intermetallic formation.

7. Conclusion

Summary of Findings

This study demonstrated that FSW is a viable method for joining AA7075 aluminum alloy and magnesium alloy. The microstructural analysis indicated the formation of intermetallic compounds at the weld interface, which influenced the mechanical properties of the joints. The mechanical tests revealed that while the welded joints exhibited good hardness and tensile properties, the presence of brittle intermetallic phases reduced the overall ductility and fatigue resistance.

Future Work

Future research should focus on optimizing the FSW parameters to minimize the formation of brittle intermetallic compounds. Additionally, post-weld heat treatments could be explored to enhance the mechanical properties of the welded joints.

8. References

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9. Appendices

Appendix A: Data Tables

Table 1: FSW Process Parameters

Table 2: Mechanical Properties of Welded Joints