Cladding-Spraying Composite Process for Strengthening Wear Parts

Literature Overview and Background

This study investigates a hybrid approach that combines weld cladding with thermal spraying to enhance the performance of wear parts. The motivation behind this composite process is straightforward: individual cladding or spraying methods each have inherent limitations in terms of hardness, toughness, and layer thickness. By combining the metallurgical bonding strength of cladding with the surface hardening capability of spraying, the composite process aims to achieve a synergistic improvement in wear resistance, fatigue life, and overall durability of critical components such as pump impellers, valve seats, cylinder liners, and conveyor rollers.

The study examines the sequential application of a weld overlay layer followed by a thermal spray coating, and vice versa, to determine the optimal processing sequence for different material systems and service conditions. The research also explores how the interaction between the two layers affects the overall mechanical behavior of the component under cyclic loading and abrasive wear conditions.

Core Technical Points and Process Analysis

The composite process involves two primary stages: the base cladding stage and the surface spraying stage. In the cladding stage, a weld overlay layer is deposited using methods such as submerged arc welding (SAW), gas metal arc welding (GMAW), or electroslag welding (ESW), depending on the component geometry and required overlay thickness. In the spraying stage, a hard coating is applied using flame spraying, plasma spraying, or high-velocity oxy-fuel (HVOF) spraying to achieve the desired surface hardness and wear resistance.

The processing sequence is critical. When cladding is applied first and spraying second, the spraying process provides a hard, dense surface layer on top of a tough, well-bonded cladding layer. This sequence is preferred when the component requires both high surface hardness and good resistance to impact loading. When spraying is applied first and cladding second, the cladding process can partially remelt the sprayed layer, which may reduce its hardness but improve the interfacial bonding. This sequence is less common but may be used in specific applications where a graded hardness profile is desired.

Microstructural and Performance Characteristics

The interface between the cladding layer and the sprayed coating is a critical region that determines the overall performance of the composite system. Metallographic analysis reveals that the sprayed layer typically consists of flattened lamellae with retained porosity and oxide inclusions, while the cladding layer exhibits a more homogeneous, fully dense microstructure. The bonding at the interface depends on the surface preparation between the two stages. Shot blasting or grit blasting of the cladding surface prior to spraying is essential to remove oxides and create a mechanically interlocking surface.

The wear resistance of the composite system is evaluated using pin-on-disc or block-on-ring wear tests. The results consistently show that the composite process outperforms either method alone. A typical finding is that the cladding-only layer achieves a wear rate of approximately 10–20 mg/N·m, while the sprayed-only layer achieves 2–5 mg/N·m. The composite system achieves a wear rate of 1–3 mg/N·m, representing a 50–70 percent improvement over the cladding-only approach. The hardness profile across the composite cross-section shows a gradient from the base material through the cladding layer to the ultra-hard sprayed surface, which is beneficial for stress distribution under impact loading.

Defect Analysis and Countermeasures

The composite process introduces additional potential defects compared to single-method approaches. Delamination between the cladding and sprayed layers is the most critical failure mode, often caused by insufficient surface preparation, contamination between stages, or thermal mismatch during the spraying process. The countermeasure involves rigorous surface cleaning, including acid pickling and thorough drying, followed by immediate spraying to minimize oxidation. Cracking in the cladding layer can propagate to the sprayed interface, reducing the effective life of the composite system. This is mitigated by controlling the residual stress in the cladding layer through appropriate heat treatment and by ensuring that the spraying process does not introduce excessive thermal loads.

Porosity in the sprayed layer is another concern, particularly with flame spraying methods. HVOF spraying produces denser coatings with porosity below 1 percent, while flame spraying can yield porosity levels of 5–10 percent. For critical applications, HVOF or plasma spraying is preferred to ensure coating integrity. The study also notes that the substrate temperature during spraying must be controlled to prevent thermal damage to the underlying cladding layer; a maximum substrate temperature of 200 °C during spraying is generally recommended.

Engineering Practice and Case Study

A practical case examined in the study involves the strengthening of pump impellers used in mineral processing applications. The impellers were first clad with a stainless steel overlay layer (3 mm thick, using GMAW with ER309L wire) to provide corrosion resistance, followed by HVOF spraying of a WC-Co composite coating (0.3 mm thick) to enhance abrasion resistance. The resulting composite impeller achieved a service life of 18 months compared to 6 months for the uncoated impeller and 10 months for the clad-only impeller. The bond strength between the cladding and sprayed layers was measured at 45 MPa, well above the minimum requirement of 30 MPa specified by ASTM C236.

Study Insights and Conclusions

This study demonstrates that the cladding-spraying composite process is a powerful approach for enhancing the performance of wear parts, offering a synergistic combination of metallurgical bonding, toughness, and surface hardness that neither method can achieve alone. The key to successful implementation lies in careful process sequencing, rigorous surface preparation between stages, and thorough quality control at each step. Engineers should conduct process qualification tests to optimize the parameters for their specific material system and service conditions, paying particular attention to interface integrity and thermal management. The composite approach represents a practical and cost-effective solution for extending the service life of critical components in harsh industrial environments.