Microstructure Analysis of Al-Si Alloy Overlay Coating on AZ91D Magnesium Alloy

Literature Overview

This 2018 paper by Liu Chikai, Li Zhiyong, Pei Xiaolong, Yang Liuqing, Xue Tonghui, and Ren Jielang from the Welding Research Center of North University of China, published in Surface Technology (Vol. 47, No. 4, pp. 172-176), investigates the microstructure and formation mechanisms of Al-Si alloy overlay coatings deposited on AZ91D magnesium alloy substrates using DC pulse metal inert gas welding (DC-PMIG). The study was supported by the Shanxi Provincial Key R&D Plan Industrial Project (201603D121002-1), Shanxi Provincial Natural Science Foundation (2012011021-1), and Shanxi Provincial Overseas Returnee Research Fund (2013-07).

Technical Background

AZ91D Magnesium Alloy Characteristics

AZ91D is a widely used cast magnesium alloy (9% Al, 1% Zn, remainder Mg) valued for its excellent specific strength and weight savings potential. However, it suffers from several limitations:

DC-PMIG Welding Process Characteristics

DC-PMIG (Direct Current Pulse Metal Inert Gas) welding offers several advantages for overlay welding on magnesium alloys:

• Low heat input: Pulse welding allows precise control of heat input, minimizing thermal damage to the heat-sensitive magnesium substrate.
• Reduced porosity: The pulsing action promotes better gas protection and reduces porosity formation.
• Controlled dilution: Lower heat input reduces substrate dilution, enabling better control of overlay composition.
• Improved wetting: Pulse parameters can be optimized to improve molten metal spreading and wetting on the magnesium substrate.

Welding Parameters

Microstructural Analysis

Two-Zone Transition Region

The study identifies two distinct zones in the Mg/Al transition region:

Zone I (Mg-rich side)

Zone I is located adjacent to the magnesium substrate and contains the following phases in sequence:

1. Mg + Al₁₂Mg₁₇ (initial zone closest to substrate)
2. Al₁₂Mg₁₇ (continuous intermetallic layer)
3. Al₁₂Mg₁₇ + Al₃Mg₂ (mixed phase region)
4. Al₃Mg₂ (continuous intermetallic layer)

Irregular block-shaped Mg₂Si particles are dispersed throughout Zone I. The formation sequence in Zone I is:

• Mg and Al react to form Al₁₂Mg₁₇ first due to the high Mg activity at the interface.
• As the Al content increases moving away from the substrate, Al₃Mg₂ forms as the stable equilibrium phase.
• Mg₂Si precipitates preferentially in this zone due to the relatively high Mg content, which promotes Si activity and facilitates Mg₂Si nucleation.

Zone II (Al-rich side)

Zone II is located on the overlay coating side and consists of:

1. Columnar α-Al (primary phase)
2. Point-like Al₃Mg₂ precipitates on α-Al surfaces
3. Black point-like Mg₂Si at α-Al grain boundaries

The formation sequence in Zone II is:

• α-Al solidifies first as the primary phase during cooling.
• Al₃Mg₂ precipitates on α-Al surfaces as the local composition becomes supersaturated with respect to this intermetallic.
• Mg₂Si forms at α-Al grain boundaries as a secondary precipitate, appearing as fine black points.

Phase Formation Mechanism

Precipitation Sequence Difference

The key finding regarding Mg₂Si precipitation is that its morphology differs significantly between the two zones:

• Zone I: Mg₂Si precipitates first (before extensive Al solidification) and grows into irregular block shapes due to the relatively high Mg content and lower cooling rate in this region.
• Zone II: Mg₂Si precipitates later (after α-Al solidification) at grain boundaries, forming fine point-like particles due to the lower Mg content and higher cooling rate.

This difference in precipitation sequence and morphology is attributed to the varying Mg/Al ratios and cooling conditions in the two zones.

Engineering Implications

Coating Performance Characteristics

The Al-Si overlay coating on AZ91D magnesium alloy offers several potential performance benefits:

• Improved wear resistance: The Mg₂Si hard phase and α-Al matrix provide enhanced tribological properties compared to the AZ91D substrate.
• Enhanced corrosion resistance: The Al-rich coating provides a more stable surface, reducing the galvanic coupling effects that can accelerate magnesium corrosion.
• Reduced oxidation sensitivity: The Al₂O₃ layer that forms on the coating surface provides better oxidation resistance than the MgO/Mg(OH)₂ layers on bare magnesium.

Practical Considerations for Magnesium Alloy Overlay Welding

1. Hydrogen absorption: Magnesium alloys readily absorb hydrogen from the atmosphere, which can lead to porosity. The use of high-purity argon shielding and dry welding wire is essential.
2. Galvanic corrosion: The Al/Mg couple creates a galvanic potential difference. The coating should be designed to minimize this effect or provide complete isolation of the magnesium substrate.
3. Thermal management: Magnesium's low thermal conductivity (21 W/m·K) and high thermal expansion coefficient (26×10⁻⁶/K) require careful heat input control to prevent distortion and cracking.
4. Surface preparation: The AZ91D substrate surface should be cleaned and possibly preheated to promote wetting and reduce oxide interference.

FMEA for Overlay Coating Failure

Key Questions and Reflections

The study provides valuable insights into the Mg/Al interfacial reactions during overlay welding, but several questions remain open for further investigation:

Long-term interfacial stability: The intermetallic phases identified in the study (Al₁₂Mg₁₇, Al₃Mg₂) may continue to evolve during service, particularly under thermal cycling. Al₁₂Mg₁₇ is known to be metastable and may transform to Al₃Mg₂ at elevated temperatures. This transformation could affect the coating's mechanical properties and bonding strength over time.

Mechanical performance: While the microstructural analysis is thorough, the paper does not report mechanical testing results (hardness, tensile strength, fatigue performance) of the coating or the coating-substrate interface. These data would be essential for evaluating the practical engineering value of the overlay coating.

Cooling rate effects: The study mentions low heat input but does not quantify the cooling rates experienced by the different zones. Understanding the cooling rate distribution would help explain the observed microstructural differences and enable better process optimization.

Scalability: The study demonstrates successful coating deposition on a laboratory scale. Scaling to larger workpieces or complex geometries may introduce additional challenges related to heat dissipation, thermal distortion, and process consistency.

Study Insights and Implications

This research contributes significantly to the understanding of Mg/Al interfacial metallurgy during overlay welding, providing a detailed map of the phase formation sequence and the factors governing intermetallic morphology. The identification of two distinct transition zones with different precipitation behaviors offers a framework for predicting and controlling the microstructure of Mg/Al overlay coatings.

For engineers working with magnesium alloy components, this study demonstrates that overlay welding with Al-Si alloys is a viable technology for surface enhancement. The DC-PMIG process, with its controllable heat input, is particularly well-suited for the thermal sensitivity of magnesium alloys. The resulting coating, with its combination of α-Al matrix and Mg₂Si hard phases, offers potential improvements in wear and corrosion resistance.

However, practical implementation requires careful attention to several factors: ensuring complete protection against hydrogen absorption, managing the galvanic coupling between the Al coating and Mg substrate, and controlling the interfacial intermetallic thickness to balance bonding strength and brittleness. The narrow transition zone observed in the study is beneficial for maintaining coating integrity but may require careful process control to prevent excessive interdiffusion during service.