Milling of Overlay Alloy with Carbide Coated Cutting Tools

Literature Overview

This 2024 paper published in Machinery Design and Manufacturing (Vol. 2024, No. 10, pp. 230-234) by Yin Liang, Zhao Hongrui, and Li Haibao from Baotou Vocational Technical College, China North Industries Group Corporation, and Inner Mongolia Institute of Metal Materials, investigates the milling of a rare earth Cr-Ni-W overlay alloy using APMT1604PDER-type carbide coated cutting tools. The research was funded by Baotou Vocational Technical College (KJ202002). The overlay alloy was produced using inverse DC welding machines with specially formulated rare earth flux-cored wear-resistant overlay welding electrodes. The study addresses a practical manufacturing challenge: the machining of hard overlay welds that are commonly applied to equipment surfaces in mining, power generation, and material handling industries where severe abrasive wear is a primary failure mode.

Experimental Methodology

The experimental approach involved several stages: overlay welding of the test plate, mechanical property and microstructure characterization of the overlay layer, machining trials at various cutting parameters, and subsequent tool wear analysis using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).

Tool Microstructure and Coating Analysis

The APMT1604PDER tool substrate consists of irregular carbide particles (WC and Co-based carbides) with grain sizes ranging from 500 nm to 2 μm, which is characteristic of fine-grained cemented carbide grades designed for hard material machining. The AlTiN coating provides enhanced hardness, thermal stability, and wear resistance compared to conventional TiN or TiAlN coatings. The AlTiN coating is particularly suitable for machining hard materials at moderate cutting speeds due to its high thermal stability and low coefficient of friction against ferrous materials.

Wear Mechanism Analysis

The study identified three distinct stages of tool wear:

1. Initial stage: Abrasive wear predominates, caused by hard carbide particles in the overlay alloy mechanically abrading the tool surface. This is the expected primary wear mechanism when machining materials containing hard phases such as Cr₇C₃, Fe₃W₆C, or rare earth-containing carbides.
2. Intermediate stage: Adhesive wear becomes significant, with material transfer from the workpiece to the tool surface. This is attributed to the high temperature at the tool-chip interface, which softens the overlay material and promotes diffusion and mechanical bonding with the tool coating.
3. Late stage: Diffusion wear dominates, where elements from the workpiece (Cr, W, Co) diffuse into the tool substrate at high temperatures, degrading the coating-substrate interface and accelerating tool failure.

Discussion of Cutting Parameters

The recommended parameters are notably conservative, reflecting the extreme hardness and abrasive nature of the overlay alloy. The cutting speed of 0.085 r/min is extremely low, suggesting that the study may involve large-diameter cutter heads where peripheral speed is limited by rotational speed constraints. The feed per tooth of 0.05 mm and depth of cut of 0.2 mm indicate light machining conditions designed to minimize thermal input and mechanical stress on the cutting tool.

The selection of a 4-tooth cutter head provides a balance between material removal rate and cutting force per tooth. With multiple cutting edges, the total cutting force is distributed, reducing the load on each edge and extending tool life. However, the low feed and depth of cut limit the material removal rate, which is a practical consideration for production environments where throughput is critical.

Integration with Engineering Practice

In practice, overlay welds on large equipment surfaces such as crusher hammers, excavator buckets, and mill liners are often left as-welded without machining. However, when dimensional accuracy is required—for example, on mating surfaces or where a smooth finish is needed for fluid flow—machining becomes necessary. The findings of this study have direct implications for such applications:

• Tool selection: AlTiN-coated fine-grained cemented carbide tools are recommended for overlay alloy machining, but tool life will be limited and frequent tool changes should be planned.
• Process planning: Light cutting parameters should be used initially, with gradual parameter optimization based on observed tool wear patterns.
• Coolant strategy: Although not explicitly discussed in the paper, the high thermal loads indicated by the diffusion wear mechanism suggest that adequate coolant supply is essential to minimize tool temperature and extend tool life.
• Alternative approaches: For production applications, it may be more economical to apply the overlay weld after machining the base material to the required dimensions, avoiding the need to machine the overlay entirely.

Key Questions and Reflections

The study does not address the effect of overlay weld microstructure variation on machinability. Overlay welds often exhibit significant microstructural variation between layers, with different phase compositions, hardness profiles, and carbide distributions. This variation can lead to inconsistent cutting forces and unpredictable tool wear. Future studies should investigate how the number of overlay layers, welding sequence, and interpass temperature affect the machinability of the overlay surface.

Additionally, the extremely low recommended cutting speed raises questions about the practicality of machining overlay welds in production settings. If the overlay alloy cannot be machined at reasonable speeds, it may be more efficient to design the component geometry such that machining of the overlay is unnecessary. This is a design-for-manufacturability consideration that should be addressed early in the engineering process.

Summary

This study provides practical guidance for machining rare earth Cr-Ni-W overlay alloys using AlTiN-coated carbide tools, identifying the optimal cutting parameters and the progressive wear mechanisms that govern tool failure. The key finding is that light cutting conditions are essential, with the wear mechanism transitioning from abrasive to adhesive to diffusion as tool life progresses. For engineering practice, the study reinforces the principle that overlay welding and machining should be carefully coordinated in the manufacturing sequence to minimize the need for machining hard overlay surfaces, while also providing the technical foundation for cases where overlay machining is unavoidable.