M.C. Kong

Response of titanium aluminide alloy to abrasive waterjet cutting: Geometrical accuracy and surface integrity issues versus process parameters

Abstract Due to their low thermal conductivity, high abrasion of cutting edges and a tendency to crack during machining, titanium aluminide (TiAl) alloys are notoriously difficult-to-cut materials when using conventional (e.g. turning, grinding, drilling, milling) cutting operations. Despite their low machinability TiAl alloys find niche application areas in the manufacture of complex shaped components for aerospace gas turbine engines. Abrasive waterjet (AWJ) machining is one of the most promising non-conventional machining processes for difficult-to-cut materials because of the reduced mechanical and thermal damage to workpiece surfaces produced using this technique. However, there is no information about the AWJ machining of TiAl alloys in the open literature. Based on a generic design of an aeroengine component, this paper investigates the response of a TiAl alloy to AWJ cutting process variables to enable generation of high-integrity surfaces. The effects of operating parameters (pump pressure, material removal rate, abrasive flow rate, and stand-off distance) on the output process quality measures have been investigated as follows: geometrical accuracy (kerf straightness); surface roughness homogeneity (measured along cutting direction on the cutting front surface to study the striation formation); workpiece surface integrity (grit embedment and possible recast micro-layers or material pull-out / micro-cracking after cutting). Advanced surface topography and metallurgical (e.g. SEM, EDX) analyses have been carried out to characterize the response of TiAl alloy to various AWJ conditions leading to the fulfilment of quality criteria of a group of targeted aeroengine components. It was found that the AWJ process has a very high capability (e.g. satisfactory geometrical accuracy, surface quality, minimum surface anomies) to cut TiAl alloy despite grit embedment. However, the authors have addressed this problem by plain waterjet cleaning know-how. In addition, a surprising phenomenon (bubble-like ‘TiO2-based spot’) caused by a strong exothermic reaction was found on the AWJ cut surfaces.

The Effect of Up-Cutting and Down-Cutting Directions on Materials Swelling in Ultra-Precision Raster Milling

The influence of materials swelling across the steps on surface roughness under up-cutting and down-cutting direction in ultra-precision raster milling was investigated. The normalized extent of swelling is characterized by a swelling significance index, defined based on the power spectral density of the roughness profile. Materials swelling was found to be significant in ultra-precision raster milling, especially when copper alloys machined in the up-cutting direction. The findings in the present study provide an important means for improving the surface finish of the raster-milled surfaces and optimizing the cutting conditions.

An Experimental Study of the Formation of Tool Marks Made by Facet Diamond Cutting Tools in Single-Point Diamond Turning

The formation of tool marks in single-point diamond turning is a fundamental study of the effect of materials swelling and recovery on surface roughness on a machined surface. A series of orthogonal face cutting tests has been conducted among plate aluminum alloy, oxygen-free high conductivity copper and electroless nickel phosphorus under the same cutting conditions by the use of facet tools with different front clearance angles. The results show that the regular width of the undulating pattern in tool marks could be explained by side swelling and the micro-waviness within a tool mark is caused by burnishing and recovery.