CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of Arc-Welded Light Multi-Principal-Element Alloy Overlay Layers
Literature Overview
This 2021 paper from Anhui University of Science and Technology and Nanjing University of Aeronautics and Astronautics investigates the microstructure and mechanical properties of arc-welded overlay layers composed of light multi-principal-element alloys (MPEAs) on steel substrates. The authors, Huang Shaofu and Guo Yu, were supported by the Jiangsu Provincial Key Laboratory of Precision and Microfine Manufacturing Technology. The work represents an emerging frontier in cladding technology: the application of multi-principal-element alloys — which have gained significant attention in materials science for their exceptional mechanical properties — to surface engineering through conventional arc welding processes.
Technical Background and Material System
Multi-principal-element alloys, also known as high-entropy alloys (HEAs), contain five or more principal elements in near-equal atomic percentages. When light elements such as Al, Ti, V, Cr, and Si are incorporated into the alloy system, the resulting MPEAs can achieve high specific strength — strength normalized by density — which is particularly valuable for aerospace and lightweight structural applications. The overlay alloy system investigated in this study was AlTiVCrSi, deposited on a Q345 low-alloy steel substrate using gas metal arc welding (GMAW).
| Parameter | Specification |
|---|---|
| Substrate | Q345 low-alloy steel |
| Overlay alloy | AlTiVCrSi (near-equal atomic %) |
| Wire diameter | 1.2 mm |
| Shielding gas | 80% Ar + 20% CO₂ |
| Welding current | 120–160 A |
| Arc voltage | 20–24 V |
| Travel speed | 150–250 mm/min |
| Heat input | 8–14 kJ/mm |
| Number of passes | 3–5 |
| Interpass temperature | ≤250 °C |
Microstructural Characteristics
The metallographic analysis of the overlay layer revealed a complex microstructure consisting of multiple phases:
- Primary phase: Body-centered cubic (BCC) solid solution with a lattice parameter of approximately 3.18 Å, composed of Al, Ti, V, Cr, and Fe in near-equal proportions.
- Secondary phase: B2 ordered intermetallic compounds (TiCr, TiV) distributed along grain boundaries and within the BCC matrix.
- Tertiary phase: Small amounts of Cr-rich carbides (M₇C₃ type) formed at the overlay-steel interface due to carbon diffusion from the substrate.
- Grain size: 20–40 μm in the overlay, with columnar grains near the interface transitioning to equiaxed grains at the surface.
The hardness of the overlay layer was measured at 580–650 HV, significantly higher than the BCC solid solution alone (approximately 400 HV), indicating that the B2 intermetallic phase contributed substantially to the hardness. The overlay density was measured at 6.2 g/cm³, which is approximately 20% lower than the steel substrate (7.85 g/cm³), confirming the lightweight character of the MPEA overlay.
Mechanical Properties
| Property | Overlay Layer | HAZ | Base Material |
|---|---|---|---|
| Hardness (HV) | 580–650 | 350–420 | 200–240 |
| Density (g/cm³) | 6.2 | 7.5 | 7.85 |
| Specific hardness (HV·cm³/g) | 95–105 | 47–56 | 26–31 |
| Micro-hardness gradient | Smooth over 2–3 mm | — | — |
| Peel strength (MPa) | 380–420 | — | — |
The specific hardness — the ratio of hardness to density — is the key metric for lightweight applications, and the MPEA overlay achieved a specific hardness nearly 4 times that of the base steel. This represents a significant performance advantage for weight-sensitive components.
Defect Analysis and Countermeasures
The arc welding process introduced several potential defects that were identified and addressed:
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking in overlay | High cooling rate, residual stress | Increase heat input, control interpass temperature |
| Porosity | Gas entrapment in MPEA wire | Improve wire deoxidation, use pure Ar shielding |
| Delamination at interface | Dilution and composition mismatch | Use a transition layer (dilution buffer) |
| Incomplete fusion | Low wetting of MPEA on steel | Increase current, pre-clean substrate |
The most critical finding was that the overlay-steel interface required careful management. The dilution rate in the first pass was approximately 35–45%, which caused significant compositional deviation from the intended MPEA design. The authors recommended a three-pass strategy: a transition pass with a diluted MPEA composition, followed by two passes of full-composition MPEA. This approach reduced the interface dilution to below 20% in the final overlay.
Engineering Practice and Future Directions
The application of MPEA overlays to steel substrates is still in its early stages, but the potential applications are compelling:
- Aerospace components: Lightweight, high-strength overlay layers on steel brackets, fasteners, and structural fittings.
- Automotive applications: Wear-resistant, lightweight coatings on engine components and drivetrain parts.
- Marine applications: Corrosion-resistant MPEA overlays on steel hulls and propeller shafts, leveraging the light weight to improve vessel performance.
The main challenges for industrial adoption are the cost of MPEA welding wire (significantly higher than conventional alloys) and the limited process knowledge base. The welding parameters identified in this study provide a starting point, but further optimization is needed for different substrate geometries and production scales.
Key Reflections
This paper represents a forward-looking approach to cladding technology, leveraging the latest advances in multi-principal-element alloy design and applying them through conventional arc welding processes. The key insight is that the exceptional properties of MPEAs — high specific strength, excellent corrosion resistance, and radiation tolerance — can be transferred to existing steel components through surface engineering, without the need to replace entire components with expensive MPEA forgings. This "surface MPEA" approach offers a practical pathway to integrating advanced materials into legacy infrastructure. However, I note that the long-term stability of the B2 intermetallic phase under thermal cycling and mechanical fatigue remains an open question, and more extensive durability testing is needed before widespread industrial adoption.
Study Insights
The work demonstrates that conventional arc welding processes — GMAW in this case — are capable of depositing complex multi-principal-element alloys with controlled microstructure and properties. This is significant because it means that existing welding infrastructure can be leveraged for MPEA overlay applications, reducing the barrier to adoption. The emphasis on specific hardness as a performance metric is appropriate for lightweight applications, and the results are promising enough to warrant further investigation into production-scale processes and long-term service behavior.