Temperature Field Simulation and Experimental Investigation of Laser Cladding Additive Manufacturing of Martensitic Ageing Steel

Literature Overview

This study investigates the temperature field characteristics and resulting microstructural evolution during laser cladding additive manufacturing (AM) of martensitic ageing steels, such as 18Ni(250) maraging steel and precipitation-hardened stainless steels. Martensitic ageing steels offer exceptional combinations of strength and toughness, making them ideal candidates for high-performance structural components fabricated by additive manufacturing. However, the complex thermal cycling inherent in layer-by-layer laser cladding AM introduces significant challenges in controlling the heat-affected zone (HAZ) properties, residual stress distribution, and phase transformation sequence.

Process Configuration and Parameters

The laser cladding AM process studied employs a 4 kW fiber laser with a coaxial powder delivery system. The base powder composition is optimized for 18Ni(250) maraging steel equivalent properties, with the following nominal composition: Fe balance, Ni 18 wt%, Co 8 wt%, Mo 5 wt%, Ti 0.8 wt%, Al 0.3 wt%. The process parameters are selected to achieve a dilution ratio below 5 percent and a single-layer deposition thickness of 0.3–0.5 mm.

Core Technical Findings

Temperature Field Characteristics

The finite element simulation reveals that the temperature field during laser cladding AM of martensitic ageing steel exhibits several distinctive features. The peak temperature in the melt pool center reaches approximately 1800–2200 °C, while the temperature gradient at the melt pool boundary is approximately 100–300 K/mm. The cooling rate at the melt pool boundary is estimated at 10³–10⁴ K/s, which is significantly higher than conventional casting cooling rates and approaches the cooling rates of gas atomization.

The layer-by-layer deposition creates a complex thermal history where each new layer is deposited onto a partially cooled substrate. The simulation shows that the temperature of the previously deposited layer at the time of the next layer deposition is approximately 300–600 °C, depending on the interlayer time. This interlayer temperature is critical because it determines whether the previously deposited layer undergoes additional phase transformation during subsequent heating cycles.

Phase Transformation Sequence

The rapid cooling rates inherent in laser cladding AM produce a fully martensitic microstructure in the as-deposited condition, which is the starting point for subsequent ageing heat treatment. The simulation confirms that the cooling rate exceeds the martensite start temperature (Ms) cooling rate threshold of approximately 100 K/s, ensuring complete austenite-to-martensite transformation. However, the high cooling rate also produces a fine lath martensite morphology with lath width of 100–200 nm, which is finer than the 300–500 nm laths produced by conventional water quenching.

The interlayer thermal cycling during multi-layer deposition creates a tempering effect on previously deposited layers. Layers deposited early in the build sequence experience up to 20–50 subsequent heating cycles at temperatures of 300–600 °C, which partially tempers the martensite and reduces hardness by 20–40 HV compared to the final layers. This creates a gradient in hardness through the build height, which must be addressed by post-build ageing heat treatment.

Residual Stress Distribution

The simulation predicts that residual stresses develop primarily in the longitudinal direction (along the build direction) and reach peak values of 600–750 MPa in the as-deposited condition. The stress distribution is non-uniform through the build height, with the highest stresses occurring in the middle layers where thermal constraint from both the baseplate and overlying layers is maximum. The top and bottom layers exhibit lower residual stresses due to reduced constraint.

Post-build ageing heat treatment at 480 °C for 8 hours reduces residual stresses by 40–55 percent through stress relaxation and diffusion mechanisms. However, the residual stresses after ageing still reach 300–400 MPa, which is significant but generally acceptable for structural applications.

Experimental Validation

The experimental results confirm the simulation predictions with reasonable accuracy. Hardness measurements show an as-deposited hardness of 420–460 HV in the final layers and 380–420 HV in the early layers, consistent with the predicted tempering effect. After ageing at 480 °C for 8 hours, the hardness increases to 520–560 HV uniformly across the build height, confirming that the ageing treatment effectively homogenizes the microstructure and properties.

Engineering Practice Implications

For engineers implementing laser cladding AM of martensitic ageing steels, this study provides critical guidance on process parameter selection and post-processing. The recommended approach includes: using a scan strategy that alternates between contour and hatch patterns to reduce residual stress concentration; maintaining interlayer time of 15–30 seconds to allow sufficient cooling between layers; applying preheat of 150–200 °C to reduce thermal gradient; and performing a full ageing heat treatment at 480 °C for 8 hours followed by air cooling. The dilution ratio must be monitored and controlled below 5 percent to ensure the as-deposited composition remains within the specified maraging steel range.

Study Insights and Reflections

The most significant finding of this research is the demonstration that laser cladding AM can produce martensitic ageing steel components with mechanical properties comparable to or exceeding those of conventionally cast and heat-treated counterparts. The as-built microstructure exhibits a finer lath martensite morphology that provides a favorable starting point for ageing precipitation, resulting in slightly higher post-ageing hardness and strength. However, the residual stress levels and hardness gradient through the build height represent practical challenges that must be managed through careful process design and post-processing. The study underscores the importance of understanding the thermal history of each layer in a multi-layer AM build, as this thermal history directly determines the final microstructure and properties. This research should be referenced by engineers developing qualification procedures for AM-produced martensitic ageing steel components in accordance with NB/T 47014 and emerging additive manufacturing standards such as ASTM F3184.