The relentless demand for durability in industrial components often places materials like grey iron casting at the forefront of engineering challenges. As a widely used material for complex geometries such as engine cylinder heads, brake discs, and machinery housings, grey iron casting offers excellent castability, machinability, and damping capacity. However, its service life is frequently compromised by issues like severe wear, thermal fatigue cracking, and corrosion, leading to premature failure and significant resource waste. The inherent microstructure of grey iron casting, characterized by graphite flakes within a ferritic or pearlitic matrix, creates natural stress concentrators and weak interfaces. When components like cylinder heads operate under cyclic thermal loads, micro-cracks can initiate and propagate, ultimately causing catastrophic failure. Traditionally, replacing these large, intricate castings is costly and time-consuming. Therefore, developing effective repair and surface enhancement technologies is paramount for sustainable manufacturing practices.
Our research focuses on harnessing the potential of laser cladding, a directed energy deposition technique, to restore and fortify damaged grey iron casting components. The process involves melting a precisely delivered stream of metallic powder synchronously with a high-power laser beam onto the substrate surface, creating a dense, metallurgically bonded coating. For grey iron casting repair, this technique must overcome specific hurdles, including high crack sensitivity due to carbon dilution and the formation of brittle phases, as well as ensuring minimal distortion and optimal coating quality. While single-track cladding provides fundamental insights, practical repair of larger damaged areas necessitates multi-track overlapping. The quality of these multi-track coatings is critically dependent on the overlap strategy, which directly influences geometric characteristics, dilution with the substrate, and the resulting mechanical properties. In this comprehensive study, we detail our investigation into optimizing the multi-lap cladding process on grey iron casting substrates, employing iron-based alloy powder to achieve coatings with superior forming quality, tailored microstructure, and significantly enhanced hardness.
The core of our experimental work was conducted using a high-power direct diode laser system with a coaxial four-port powder feeding nozzle. The substrate material was grey iron casting grade HT250, a common grade for automotive and industrial parts. Prior to cladding, the substrate surfaces were ground clean and degreased. The clad material was a custom iron-based alloy powder designed for hardfacing, with a composition optimized for compatibility with the grey iron casting substrate and for producing a wear-resistant microstructure. The powder was thoroughly dried to ensure consistent flowability. Based on prior parametric optimization for single tracks, a constant set of laser power, scanning speed, and powder feed rate was selected. To study the overlap effect, we produced a series of four-track clad layers with varying overlap ratios, defined as the percentage of the subsequent track that re-melts the previous one. The primary metrics for evaluating the forming quality were the dilution ratio and the surface flatness, both derived from cross-sectional geometric analysis.
The dilution ratio ($DR$) is a fundamental parameter in laser cladding, indicating the degree to which the substrate material intermixes with the clad material. Excessive dilution can incorporate undesirable elements from the grey iron casting substrate (like carbon and silicon) into the coating, potentially leading to brittleness and crack formation. Insufficient dilution may result in poor metallurgical bonding. The optimal $DR$ for repair applications is typically below 10%, with a target around 5%. It is calculated from the cross-sectional area of the clad ($A_c$) and the area of substrate dilution ($A_d$) as follows:
$$DR = \frac{A_d}{A_c + A_d} \times 100\%$$
Surface flatness ($SR$) is another crucial indicator, especially for coatings requiring minimal post-machining. A flatter surface implies better material utilization and easier finishing. We define it as the ratio of the clad area to the product of its total width ($W$) and maximum height ($H$):
$$SR = \frac{A_c}{W \times H}$$
A higher $SR$ value indicates a coating that is wider and flatter relative to its height, which is generally desirable. The cross-sectional geometries for different overlap ratios were meticulously measured using stereo microscopy and image analysis software. The collected data is summarized in the table below, revealing clear trends in how overlap affects coating quality on grey iron casting.
| Overlap Ratio (%) | Clad Area, $A_c$ (mm²) | Dilution Area, $A_d$ (mm²) | Dilution Ratio, $DR$ (%) | Total Width, $W$ (mm) | Max Height, $H$ (mm) | Surface Flatness, $SR$ |
|---|---|---|---|---|---|---|
| 25 | 21.1 | 1.69 | 7.42 | 15.17 | 1.90 | 0.73 |
| 35 | 24.08 | 1.31 | 5.16 | 14.27 | 1.92 | 0.88 |
| 45 | 19.62 | 1.57 | 7.41 | 12.23 | 1.99 | 0.81 |
| 55 | 18.69 | 1.60 | 7.89 | 10.17 | 2.35 | 0.78 |
Analysis of the data and macro-morphologies led to several key findings. At a low overlap ratio of 25%, the individual tracks were distinct, leading to a wavy surface profile and a relatively high dilution rate. As the overlap ratio increased to 35%, a remarkable improvement was observed. The $DR$ reached its minimum value of 5.16%, which is very close to the ideal target for repairing grey iron casting. Simultaneously, the $SR$ peaked at 0.88, indicating the flattest and most efficient coating geometry. This optimal condition arises because the increased overlap provides sufficient energy to remelt the previous track’s crest, smoothing the surface, while the powder stream’s shielding effect and the thermal mass of the prior track limit excessive heat conduction into the grey iron casting substrate, thereby minimizing dilution.
Further increasing the overlap ratio to 45% and 55% proved detrimental. The dilution rate began to climb again, and the surface flatness decreased. At 55% overlap, visible pore defects appeared in the overlap zones due to excessive remelting and potential gas entrapment. At 65% overlap (data not fully tabulated due to poor quality), the coating formation was severely compromised with large defects, rendering it unsuitable. Therefore, we conclusively identified a 35% overlap ratio as the optimal parameter for multi-track cladding on grey iron casting, achieving the best balance between minimal dilution and maximum surface flatness.
The microstructural evolution within the coating, particularly under the optimal 35% overlap condition, is fascinating and dictates the final properties. Metallographic examination of the etched cross-section reveals a gradient microstructure from the interface to the top surface, a hallmark of rapid solidification processes like laser cladding on grey iron casting. At the bottom of the coating, adjacent to the substrate, the thermal gradient ($G$) is extremely high, and the solidification rate ($R$) is relatively low. This results in a planar to cellular growth front, leading to coarse dendrites and cellular crystals epitaxially growing from the partially melted grains of the grey iron casting substrate. This zone ensures strong metallurgical bonding.
Moving upwards into the middle region of the coating, $G$ decreases while $R$ increases. The constitutional supercooling ahead of the solid-liquid interface rises, promoting a transition to columnar dendritic growth. These columnar grains often grow in a cross-oriented manner, following the local heat flow direction. In the overlap region between two tracks, a distinct bright band is visible. This area undergoes a complete remelting and re-solidification cycle during the deposition of the subsequent track. The rapid thermal cycle here typically results in a refined microstructure of sparsely distributed cellular crystals.
Finally, at the top surface of the coating, the thermal gradient is at its minimum, and the solidification rate is at its maximum. This creates a large zone of constitutional supercooling, triggering heterogeneous nucleation and the formation of fine equiaxed grains. This microstructural gradient—from coarse dendrites at the interface to fine equiaxed grains at the surface—is beneficial as it can help mitigate thermal stresses and may influence toughness. The heat-affected zone (HAZ) in the grey iron casting substrate beneath the interface showed evidence of a quenching effect, likely transforming the matrix to harder phases like martensite, which is common when laser processing grey iron casting.
The ultimate test of the repair’s effectiveness lies in its mechanical performance. Microhardness profiling across the coating section provides a direct correlation with the observed microstructure. Hardness measurements were taken from the grey iron casting substrate, through the HAZ and interfacial zone, and across the entire clad layer at the optimal 35% overlap condition. The grey iron casting substrate itself showed a typical hardness range. The HAZ exhibited a significant increase in hardness due to the formation of hard, quenched structures. The most dramatic jump in hardness occurred in the dilution zone immediately above the interface. Here, carbon from the dissolved graphite flakes in the grey iron casting substrate enters the melt pool, leading to the formation of high-carbon martensite and carbides upon rapid cooling, resulting in peak hardness values.
Within the main body of the clad layer, the hardness remains consistently high but shows some local variation. This is attributed to the combined effects of grain refinement (Hall-Petch strengthening) and solid solution hardening from the alloying elements (Cr, Mo, etc.) in the powder. The fine equiaxed zone and the refined overlap zones contribute to this sustained high hardness. The average microhardness of the multi-track coating at 35% overlap was calculated. The performance enhancement compared to the base grey iron casting substrate is profound, confirming that laser cladding can not only repair but also significantly strengthen the surface of grey iron casting components.
The relationship between overlap ratio and the average coating hardness can be further generalized. Let $H_{avg}$ represent the average microhardness and $\eta$ represent the overlap ratio. Our data suggests a non-linear relationship where an optimal $\eta$ exists for maximizing hardness, closely tied to minimizing dilution and defect formation:
$$H_{avg}(\eta) \approx H_0 – k_1 \cdot |\eta – \eta_{opt}| + k_2 \cdot DR(\eta)$$
where $H_0$ is a base hardness constant, $k_1$ and $k_2$ are positive coefficients, $\eta_{opt}$ is the optimal overlap ratio (~35%), and $DR(\eta)$ is the dilution rate as a function of overlap. This conceptual equation highlights that deviation from the optimal overlap or increased dilution negatively impacts the final hardness. The consistent high hardness across the coating, significantly exceeding that of the grey iron casting substrate, validates the process for creating wear-resistant surfaces.
In conclusion, our systematic investigation into multi-track laser cladding on grey iron casting has yielded a clear pathway for effective component repair and surface enhancement. By rigorously evaluating geometric quality indicators, we optimized the overlap ratio to 35%, achieving a near-ideal dilution rate of 5.16% and excellent surface flatness. This parameter set minimizes the risk of defect formation, such as cracks and pores, which are critical concerns when working with grey iron casting. The resulting coating exhibits a complex, graded microstructure that ensures strong bonding and high performance. Most importantly, the microhardness of the iron-based alloy coating is dramatically increased compared to the base grey iron casting, providing a robust, wear-resistant surface. This research provides a foundational reference and a viable technological framework for the laser-based remanufacturing of high-value grey iron casting parts, extending service life, conserving resources, and promoting sustainable industrial practices. The principles of overlap optimization, dilution control, and microstructure-property relationships established here are broadly applicable to the laser repair of various cast iron grades, contributing valuable knowledge to the field of additive manufacturing and surface engineering.