GTA Oscillating Remelting of FeCrBSiWNb HVOF Coating

Literature Overview and Research Background

This paper by Zong Xuemei and colleagues from the Xuzhou Construction Machinery Research Institute and the State Key Laboratory of Intelligent Manufacturing for High-End Construction Machinery, published in Surface Technology (2017, Vol. 46, No. 7, pp. 195-200), addresses a practical and persistent problem in the surface engineering of construction machinery components. The FeCrBSiWNb coating system, applied via high-speed arc spraying (HSAS), is widely used for wear and corrosion protection on critical structural parts such as hydraulic cylinders, boom joints, and bucket teeth in earthmoving equipment. However, the as-sprayed coating typically suffers from porosity, residual stress, and poor metallurgical bonding with the substrate. The conventional remelting approach using stationary GTA (GTAW) produces narrow single-pass welds with uneven dilution, and multi-pass overlap regions are prone to porosity and cracking due to the high residual stress concentration. The authors propose an oscillating GTA remelting process designed through geometric analysis and numerical calculation to widen the single-pass remelted zone, reduce internal residual stress, and improve the metallurgical integrity of the coating.

Core Technical Approach and Process Parameters

The fundamental design philosophy behind the oscillating remelting process is to use a programmed two-directional oscillation of the tungsten electrode to broaden the effective heat-affected width of a single pass, thereby achieving more uniform melting of the sprayed coating and reducing the number of passes required. This is conceptually similar to the oscillating welding techniques used in automatic pipe welding, where electrode oscillation is employed to fill wide root gaps. The authors performed geometric modeling of the remelting pool shape as a function of oscillation amplitude and frequency, then optimized the process parameters to achieve complete remelting without excessive substrate dilution.

The X-direction speed governs the lateral oscillation amplitude, while the Y-direction speed controls the longitudinal travel. The ratio between these two speeds determines the effective width of the remelted track. With the optimized parameters, the single-pass remelted width increased significantly compared to the non-oscillating process, effectively eliminating the narrow-track problem and the associated inter-pass stress concentrations.

Microstructural Characterization and Phase Analysis

The remelted layer microstructure was characterized using XRD, optical microscopy, and scanning electron microscopy (SEM). The matrix phases identified in the remelted coating include martensite, retained austenite, and Fe-Cr solid solution. The hard phases consist of borides (likely Fe₂B and FeB) and Laves phase (Fe₂W or (Fe,W)₂B). This multiphase microstructure is critical for the coating's wear resistance, as the combination of a tough martensitic matrix with hard boride and Laves phase particles provides a balanced toughness-hardness relationship.

The hardness distribution across the cross-section reveals a clear gradient: the substrate at 242 HV0.3, the HAZ at 563 HV0.3, and the remelted coating at 820 HV0.3. The HAZ hardness increase indicates martensitic transformation in the substrate due to the rapid cooling rates experienced during remelting. The coating hardness of 820 HV0.3 is consistent with the presence of hard boride and Laves phase precipitates dispersed in a martensitic matrix. The authors note that the remelted layer exhibits multiple strengthening mechanisms including solid solution strengthening, precipitation strengthening, and dispersion strengthening from the hard carbide/boride particles.

A particularly important finding is that no internal cracks were observed in the remelted layer. This is attributed to the wider single-pass width, which reduces the peak residual stress gradient and allows for more uniform stress distribution across the remelted zone. The oscillation effectively distributes the heat input laterally, avoiding the localized thermal cycling that promotes cracking in narrow non-oscillating tracks.

Engineering Practice Implications and Critical Reflection

From a practical standpoint, this oscillating remelting approach has significant implications for the repair and maintenance of construction machinery components. Traditional remelting of HVOF coatings often requires multiple passes with careful overlap control, increasing cycle time and introducing quality variability. The oscillating process reduces the number of passes and improves consistency, which is particularly valuable in field repair scenarios where precision welding equipment may be limited.

However, several engineering concerns merit further investigation. First, the process parameters were optimized for a specific coating thickness and substrate geometry. In practice, coating thickness can vary due to spray gun trajectory variations, and the substrate may have complex curvature (e.g., cylinder surfaces, tapered boom sections). The process window may need adjustment for different geometries. Second, the residual stress state of the remelted layer, while improved compared to non-oscillating remelting, still warrants quantitative assessment through X-ray diffraction or neutron diffraction, as the authors did not report residual stress measurements. Third, the wear performance of the remelted coating under actual service conditions (abrasive wear, impact wear, corrosion-wear) should be validated through accelerated wear testing, as hardness alone does not fully predict service life.

The Laves phase formation is of particular metallurgical interest. In Fe-Cr-W-B-Si-Nb systems, Laves phase (typically Mo₂C-type or Cr₂B-type) forms preferentially at the eutectic boundaries between the matrix and boride phases. The presence of this phase contributes to high hardness but can also promote intergranular brittleness if the Laves phase network becomes continuous. The absence of cracking in the remelted layer suggests that the Laves phase morphology remains particulate rather than forming a continuous network, which is favorable for toughness. The oscillating remelting process may promote this beneficial morphology by providing a more uniform cooling rate across the wider track.

Summary

This study presents a well-conceived process innovation that leverages geometric analysis to design an oscillating GTA remelting process for FeCrBSiWNb HVOF coatings. The resulting single-pass width increase effectively eliminates internal cracking, and the microstructural analysis confirms a beneficial multiphase composition with multiple strengthening mechanisms. For engineers working on surface engineering of heavy machinery components, this approach offers a practical pathway to improve coating quality and reduce repair cycle time, provided that the process parameters are appropriately adapted to the specific geometry and coating thickness of the application.