Papers by Author: Thai Nguyen

Abstract: This paper investigates the effect of the grinding-induced cyclic heating on the hardened layer properties generated in plunge cylindrical grinding. It was found that by increasing the number of grinding cycles, the hardened layer becomes thicker. The cyclic grinding stressing together with the heating initiates a plastic deformation zone of a highly oriented microstructure. The surface residual stresses of the layer are compressive. Under a lower infeed rate, the tempering of the hardened layer material will occur more severely.

Abstract: Grinding-hardening is a thermo-mechanical process capable of simultaneous material removal and surface enhancement. However, there are difficulties in dealing with cylindrical workpieces, because of the complex 3D-helical movement of the grinding-induced heat source. This paper investigates the effects of some key grinding parameters on the profile and microstructure variations of the hardened layer generated by a traverse cylindrical grinding, both experimentally and numerically. It was found that the ratio of the longitudinal wheel feed to the rotational speed of the workpiece R is most influential on the hardened layer profile. A larger R results in a thicker hardened layer but a more pronounced discontinuity of two adjacent hardening zones. Although a smaller R yields a relatively thinner hardened layer, the non-uniformity could be minimised by the overlapping of the grinding heating. The microstructure of the hardened layer can be altered by the tempering in subsequent grinding heating. A thermal analysis with the aid of the finite element method in conjunction with an experimental microstructural examination showed that the temperature distribution in the tempered region would lead to a variation of the precipitated carbide, and that a higher tempering temperature could result in a higher degree of the carbide precipitation and a more significant reduction of hardness.

Abstract: A finite element heat transfer model incorporating a moving heat source has been developed to predict the temperature field in traverse cylindrical grinding. The model was then applied to analyse the grinding-hardening of quenchable steel 1045. It was found that in the region where the grinding wheel had an entire contact with the workpiece, material would experience a heating-cooling cycle, enabling the generation of a uniform hardened layer. In the transient regions at the two ends of the workpiece where the wheel-workpiece contacts were partial, the material was not hardened but experienced an annealing process. The results were in good agreement with the experimental observations.

Abstract: This paper reviews two cooling methods for surface grinding. These include i) replacing the toxic coolant by a mixture of cold air and vegetable oil mist, and ii) minimising the use of coolant using a segmented grinding wheel system. The discussion concludes that the cold air-oil mist provides an environmentally conscious mean for surface grinding. However, due to the limit of its cooling capacity, it is more applicable for fine grinding. Using the segmented wheel system can minimise coolant consumption and can improve grindability.

Abstract: A three-dimensional finite element heat transfer model incorporating a moving heat source was developed to investigate the heat transfer mechanism in grinding-hardening of a cylindrical component. The model was applied to analyze the grinding-hardening of quenchable steel 1045 by two grinding methods, traverse and plunge grinding. It was found that the heat generated can promote the martensitic phase transformation in the ground workpiece. As a result, a hardened layer with a uniform thickness can be produced by traverse grinding. However, the layer thickness generated by plunge grinding varies circumferentially. The results are in good agreement with the experimental observations.

Abstract: This paper presents a study of using an abrasive slurry jet for the machining of micro channels on brittle glasses. The machined surface morphology and channel dimensions are used to assess the technology. Surface morphology was found featuring with two types of wave patterns; one was along the channels with large wave lengths as a result of the jet deflection during the motion of nozzle, and the other was due to viscous flow that resulted in smooth surface eroded predominantly by ductile mode. The investigation showed that using higher jet pressure and higher particle concentration enables to create channels with higher depth, although these widened the channels and degraded the surface quality in some cases by inducing a larger number of pit fragments on the surface. With proper control of the operating parameters, this technology can be used for machining micro channels on brittle materials with high quality of surface finish.

Abstract: This paper investigates the temperature field in plunge cylindrical grinding with the aid of the finite element analysis. The analysis included the effect of the variation of the depth of cut and that of the repeated heating and cooling caused by workpiece rotation. It was found that the predictions agree well with the experimentally measured results. The analysis showed that the heating in the consequent rotations further raises the temperature in the workpiece. The reheating in the hardened layer can lead to material’s tempering.

Abstract: This paper presents a temperature-dependent finite element heat transfer model, incorporating a triangular moving heat source and various cooling conditions, to predict the three-dimensional temperature field in plunge surface grinding. The model was applied to analyse the grinding-hardening of quenchable steel 1045 using dry air and liquid nitrogen as the cooling media. The temperature field variation under such grinding conditions was also measured experimentally. It was found that the temperature history predicted by the model agrees well with the measured results. The model provides a fundamental study as a first step in optimisation and control of the hardened layer thickness and its compositions in grinding-hardening technology.

Abstract: This investigation aims to develop a quantitative model to estimate the material removal of polycrystalline diamond composites by dynamic friction polishing. The model accounts for not only the polishing parameters that govern the material removal mechanisms, but also the constitutive properties of the diamond composites subjected to polishing. The model prediction was justified by relevant experimental measurements.

Abstract: The erosion process in micro-machining of brittle glasses using a low pressure slurry jet is discussed. The process capability of the technique is assessed by examining the machined surface integrity in relation to fluid flow dynamics in micro-hole generations. The holes produced are characterised by a “W” shape in the cross section, while the surface morphology is distinguished by three zones associated with the fluid flow behaviour, i.e. a direct impact zone, a wavy zone and an accumulation zone. The surfaces appear to be smooth and without cracks, indicating a predominance of the ductile mode erosion process. With the increase of pressure, the erosion rates can be enhanced as a result of the expending of the accumulation zone while the outer diameter of the holes remains unchanged. This study shows that this technique can be used for micro-machining with high surface quality, and provides an essential understanding for further research in the avenue.