Microstructure and Performance of Ni-Based Alloy Cladding Deposited by Semiconductor Laser on Q235 Steel

Literature Overview

This study by Li Zhengqiu and colleagues from Lanzhou University of Technology investigates the effect of laser power on the microstructure, hardness, and wear resistance of Ni60 alloy powder single-track cladding layers deposited on Q235 carbon steel substrates using a large-spot semiconductor laser system. Published in Applied Laser (Vol. 36, Issue 5, 2016, pp. 547-552), the work was supported by the National Torch Program (2015GH531401) and the Wenzhou Laser and Optoelectronics Industry Cluster Science and Technology Special Project (j20130014). The research addresses a critical question in surface engineering: how does the input energy parameter of laser power govern the dilution rate, microstructural evolution, and tribological performance of hardfacing deposits?

Core Technical Content and Process Parameters

The experimental design maintains all other process parameters constant while varying the laser power across five levels: 900 W, 1100 W, 1300 W, 1500 W, and 1700 W. The substrate material is Q235 structural steel, and the cladding powder is Ni60 alloy, a widely used nickel-based hardfacing material containing significant amounts of Cr, Mo, and B. The key measured parameters include penetration depth, weld width, dilution rate, dendrite size, Vickers hardness, and wear resistance.

The monotonic relationship between laser power and dilution rate is physically intuitive: higher power increases the heat input into the substrate, melting more base metal and incorporating more iron into the molten pool. This dilution fundamentally alters the alloy composition of the cladding layer, reducing the effective concentration of Ni, Cr, and Mo that contribute to hardness through solid solution strengthening and carbide precipitation.

Microstructural Analysis and Metallurgical Interpretation

The microstructure of Ni60 cladding layers typically consists of a dendritic austenitic matrix with interdendritic precipitation of hard carbides, primarily M7C3 and M23C6 type chromium carbides, along with borides. As laser power increases, the cooling rate decreases due to greater thermal mass and deeper melt penetration, resulting in coarser dendrite spacing. This coarsening reduces the density of hard carbide precipitation sites and diminishes the overall hardness of the deposit.

From a welding metallurgy perspective, the dilution rate is the single most critical factor governing cladding performance. For Ni60 alloy systems, the optimal dilution rate should generally be maintained below 30% to preserve the intended carbide-forming chemistry. When dilution exceeds this threshold, the increased iron content promotes the formation of softer, more ductile phases at the expense of the hard carbide network. The transition from fine to coarse dendritic structures also has implications for thermal fatigue resistance, as coarser structures are more susceptible to crack initiation under cyclic thermal loading.

The hardness decrease with increasing laser power follows the classical inverse relationship between cooling rate and dendrite arm spacing. At 900 W, the rapid solidification associated with the semiconductor laser's high energy density produces fine microstructures with superior hardness values, potentially exceeding 800 HV. However, excessively low power may compromise metallurgical bonding and produce incomplete melting of the powder particles.

Engineering Practice Implications and Process Optimization

The study demonstrates that the optimal single-track laser power for Ni60 cladding on Q235 is in the range of 900-1100 W, where the dilution rate is minimized and the microstructure remains fine enough to achieve high hardness and wear resistance. The authors then proceed to multi-pass cladding using the optimized parameters, achieving a macroscopically smooth and uniform cladding surface. This two-step approach—single-track parameter optimization followed by multi-pass deposition—is a standard methodology in production laser cladding operations.

For engineering applications in pipe and fitting surface hardfacing, this research provides valuable guidance. In the context of pipeline components subjected to erosional wear, such as valve seats, pump impellers, and pump casing surfaces, the selection of appropriate laser power is essential to achieving the target surface hardness while maintaining adequate metallurgical bond strength. The semiconductor laser's large-spot configuration offers the advantage of processing larger areas with acceptable quality, which is particularly relevant for industrial-scale surface engineering of large pipe fittings.

The study also highlights an important trade-off: while lower power yields superior surface properties, the productivity of the process decreases. In production environments, this trade-off must be balanced against economic considerations. Multi-pass cladding at optimized parameters provides a practical solution, as each subsequent pass benefits from the heat input of the preceding pass, potentially allowing slightly higher power without excessive dilution.

Study Insights and Reflections

This work exemplifies the systematic approach required in laser cladding process development. The isolation of a single variable (laser power) while holding all others constant is methodologically sound and produces clear, actionable results. The findings reinforce the principle that dilution rate is the master variable in laser cladding of dissimilar alloy systems, and that laser power is the most direct means of controlling it.

For practitioners in the pipe and fitting industry, the key takeaway is that laser cladding parameters must be optimized specifically for each substrate-cladding combination rather than relying on generic parameter sets. The Q235 substrate, being a low-carbon steel with relatively low melting point and thermal conductivity characteristics, is representative of many industrial substrates used in pump and valve manufacturing. The Ni60 alloy, with its excellent resistance to abrasive and erosive wear, is among the most widely specified cladding materials in the oil and gas, mining, and power generation sectors.

The research also implicitly addresses the challenge of maintaining consistent cladding quality in multi-pass operations. As each pass is deposited, the thermal history of the substrate changes, and the effective dilution rate may differ between the first pass and subsequent passes. This necessitates careful process control and potentially adaptive parameter adjustment, which connects to the broader theme of process automation discussed in other studies on cladding technology.

Reference Value and Outlook

The study provides a solid foundation for understanding the fundamental relationships in semiconductor laser cladding of Ni-based alloys on carbon steel substrates. Its practical value lies in the specific parameter recommendations and the clear demonstration of the dilution-hardness-wear resistance relationship. Future work in this area should consider the effects of other process parameters such as scanning speed, powder feed rate, and spot diameter, as well as the influence of substrate preheating on dilution control. The integration of real-time monitoring and feedback control systems for dilution rate measurement would further enhance the reproducibility and quality assurance of laser cladding operations in industrial settings.