CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Co-based Alloy Cladding Layer after Laser Remelting and Aging Treatment
Literature Overview
The study under review investigates the influence of laser remelting followed by aging treatment on the microstructure and mechanical properties of cobalt-based alloy cladding layers. This topic sits at the intersection of advanced surface engineering and thermal processing, two disciplines that are critical for extending the service life of components operating in severe environments such as hot gas erosion, cavitation, and high-temperature oxidation. The research addresses a practical challenge: how to optimize the post-weld heat treatment of PTA or laser-cladded Co-based layers to achieve a superior balance between hardness, toughness, and thermal stability without compromising the bond strength at the interface.
Core Technical Points
Laser Remelting Parameters and Their Effects
Laser remelting serves as a critical post-processing step after the initial cladding pass. The primary objective is to refine the columnar dendritic microstructure formed during the initial deposition, eliminate residual stresses, and promote a more homogeneous distribution of carbide phases. The study examines laser power ranging from 1.5 to 3.0 kW, scanning speeds between 5 and 15 mm/s, and spot diameters of 2 to 4 mm. A key finding is that the optimal laser remelting window lies at approximately 2.2 kW power with a scanning speed of 8 mm/s, yielding a heat input of roughly 2.75 J/mm. At this condition, the remelted zone exhibits a fine equiaxed dendrite structure with an average grain size reduced by approximately 40% compared to the as-cladded state.
| Parameter | Range Investigated | Optimal Value | Effect |
|---|---|---|---|
| Laser power | 1.5–3.0 kW | 2.2 kW | Controls remelt depth and thermal gradient |
| Scanning speed | 5–15 mm/s | 8 mm/s | Balances cooling rate and dilution |
| Spot diameter | 2–4 mm | 3 mm | Affects energy density distribution |
| Remelt depth | 0.3–0.8 mm | 0.5 mm | Ensures full re-melting of as-cladded layer |
| Dilution ratio | 8–18% | 10–12% | Maintains Co-based alloy chemistry |
Aging Treatment and Phase Evolution
Following laser remelting, an aging treatment was conducted at temperatures between 700°C and 1000°C for durations of 1 to 8 hours. The Co-based alloy in question contains significant amounts of Cr, Mo, and W, which promote the precipitation of M6C and M23C6 carbides during aging. The study reveals that aging at 900°C for 4 hours produces the finest and most uniformly dispersed carbide network, resulting in a hardness increase from 420 HV to 510 HV. However, aging beyond 1000°C leads to excessive grain coarsening and carbide agglomeration, which degrades toughness and increases susceptibility to intergranular corrosion.
The precipitation hardening mechanism follows a classic TTT (Time-Temperature-Transformation) behavior. At lower aging temperatures (700–800°C), fine M6C carbides nucleate preferentially at dendrite boundaries, providing moderate hardening. At intermediate temperatures (850–950°C), both M6C and M23C6 phases coexist, and the combined precipitation strengthening yields the peak hardness. At temperatures exceeding 1000°C, the coarsening kinetics accelerate dramatically, and the Orowan strengthening mechanism becomes less effective as inter-particle spacing increases beyond the optimal range of 0.2–0.5 μm.
Bond Strength and Interface Integrity
A critical concern in laser remelting is the potential for cracking or delamination at the cladding-to-substrate interface. The study employed micro-shear testing in accordance with ASTM G119 to evaluate bond strength. The results show that laser remelting at optimal parameters (2.2 kW, 8 mm/s) increases the shear strength from 320 MPa to 385 MPa, attributable to the elimination of micro-porosity and the refinement of the fusion zone microstructure. However, excessive laser power (above 2.8 kW) caused localized overheating, leading to intergranular cracking along the prior austenite grain boundaries in the heat-affected zone of the substrate.
Microstructural Characterization
The microstructural analysis was conducted using optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The as-cladded layer exhibited a typical columnar dendritic structure with a primary dendrite arm spacing (PDAS) of approximately 8–12 μm. After laser remelting, the PDAS decreased to 4–6 μm, and the dendrite orientation shifted from columnar to equiaxed in the fully remelted region. The partially remelted zone at the interface retained a mixed microstructure, which provided a gradual transition in thermal expansion and mechanical properties between the cladding and the substrate.
The XRD analysis confirmed the presence of γ-Co solid solution as the matrix phase, with secondary phases of M6C (Co₃W₆C), M23C6 (Cr23C6), and M7C3 (Co7W6C) identified. After aging at 900°C for 4 hours, the volume fraction of precipitated carbides increased from approximately 8% to 15%, consistent with the observed hardness improvement.
Process Analysis and Engineering Implications
The combined laser remelting and aging treatment represents a two-step optimization strategy that addresses distinct metallurgical objectives. Laser remelting primarily targets microstructural refinement and defect elimination, while aging treatment focuses on precipitation hardening and thermal stability enhancement. The sequence is critical: performing aging before laser remelting would be counterproductive, as the thermal cycle of remelting would dissolve the precipitated phases and negate the hardening effect.
From a manufacturing standpoint, the process parameters identified in this study can be directly applied to repair and refurbishment operations on turbine components, pump impellers, and valve seats. The dilution ratio of 10–12% is achievable with standard PTA consumables such as Stellite 6 or Haynes 25, provided that the pre-weld preparation ensures adequate edge preparation and root cleaning. The recommended aging temperature of 900°C is well within the reheat cracking susceptibility range of low-alloy steel substrates, necessitating careful control of heating and cooling rates during the aging cycle. A heating rate of no more than 100°C/h and a cooling rate of no more than 50°C/h are recommended to minimize residual stresses in the substrate.
Key Questions and Reflections
The study raises several important questions for engineering practice. First, the long-term thermal stability of the aged microstructure at service temperatures above 600°C remains uncertain. While the 900°C/4h aging condition maximizes room-temperature hardness, it may not necessarily optimize the high-temperature creep resistance or oxidation behavior. Second, the effect of multiple laser remelting passes on the cumulative thermal exposure of the substrate warrants further investigation, particularly for thick cladding layers exceeding 2 mm. Third, the compatibility of the aged Co-based cladding with subsequent machining operations should be evaluated, as the increased hardness may affect tool wear rates and surface finish quality.
The dilution issue deserves particular attention in engineering applications. When cladding thick sections, the dilution ratio tends to increase in the early passes due to the relatively thin deposited layer and the high thermal mass of the substrate. This can lead to a chemistry that deviates from the nominal Co-based alloy composition, potentially resulting in a microstructure that is more susceptible to cracking during aging. A practical solution involves applying a transition layer of lower dilution alloy before the final Co-based cladding pass, ensuring that the final layer chemistry remains within the desired range.
Study Insights and Implications
This literature provides valuable guidance for engineers involved in the surface treatment of cobalt-based cladding layers. The key takeaway is that laser remelting and aging treatment are synergistic processes that, when properly sequenced and parameter-controlled, can significantly enhance the performance of Co-based cladding systems. The optimal processing window identified—laser remelting at 2.2 kW and 8 mm/s followed by aging at 900°C for 4 hours—offers a practical recipe that can be adapted to various component geometries and service conditions. Engineers should, however, remain vigilant about the interplay between thermal processing parameters and substrate metallurgical response, particularly when dealing with high-strength low-alloy steels or austenitic stainless substrates where reheat cracking and sensitization are known concerns. A thorough understanding of the thermal history and the resulting microstructural evolution is essential for ensuring the long-term reliability of cladded components in demanding industrial applications.