N. P. Hung

Machinability of aluminum alloys reinforced with silicon carbide particulates

Abstract Cold isostatic pressed then hot extruded AlLi alloys reinforced with 10–20 wt% SiC particulates have been produced successfully. Such composites are well-known for their high specific strength, high wear resistance, but poor machinability. Analytical based published papers on the machinability of metal matrix composites are very limited. This study utilizes Taylors tool life equation to model a facing process such that the performance of a single-point cutting tool can be predicted and optimized for different cutting conditions. Different approaches based on linear accumulative damage results in two different models. The models were first verified with data from the turning and facing of steel, then applied to study the machinability of AlLi SiCp composites. Performances of tested tool materials (i.e. high speed steel, titanium nitride coated high speed steel, tungsten carbide, cubic boron nitride (CBN), and polycrystalline diamond) used in machining these composites were ranked. Sub-surface damage of faced samples was assessed by measuring the micro-hardness of the plastically deformed matrix, and microscopically examining cross sections of chemically etched samples. CBN and diamond tools fracture the SiC particulates along their crystallographic planes and induce little damage in the matrix, while other tools not only delaminate the particulates from the matrix, but also roughen the particulates, and significantly deform the matrix.

Machinability of cast and powder-formed aluminum alloys reinforced with SiC particles

Abstract Metal matrix composites (MMCs) have been increasedly used in the industries because of their improved properties over those of non-reinforced alloys. High hardness silicon carbide (SiC) particles are commonly used to reinforce the aluminum alloys, but the full application of such MMCs is however cost sensitive because of the high machining cost. This study investigates the machinability of cast and powder-formed aluminum alloys reinforced with SiC particles. Models for tool wear are validated, while the effect of tool materials, particle distribution, and sub-surface damage are studied and compared. Roughing with uncoated tungsten carbide inserts then finishing with polycrystalline diamond tools is the most economical route to machine SiC reinforced MMCs. The cast MMC exhibits higher machinability than that of the powder-formed MMC mainly because of the favourable shape and distribution of the particles, but weakly because of the fabricating processes. Regardless of the cutting tool materials used for machining, cracked SiC particles and debonded matrix-reinforcement interface were found underneath a machined surface. Such machine-induced defects could be a concern when using the MMCs in a critical application.

Cumulative tool wear in machining metal matrix composites Part II: Machinability

Abstract Models for cumulative tool wear are used to study the machinability of aluminium matrix composites reinforced with SiC or Al2O3 particles. Cutting conditions to simulate a finishing operation were employed. Effects of cutting tool materials, heat treatment, and hot isostatic pressing were investigated. Carbide tools can be utilized in a roughing operation, while cubic boron nitride and polycrystalline diamond tools could be used to finish-machine the composites. The latter tools have acceptable tool lives, and cause minimum damage to the sub-surface. The hard aluminium matrix as a result of heat treatment significantly shortens the tool life. Fractured and delaminated particles along or underneath a machined surface could be of concern when composites are used in critical applications.

Effect of cutting fluid on the machinability of metal matrix composites

Abstract This paper utilizes models for cumulative tool wear to study the effect of cutting fluid on the machinability of aluminum-based matrix composites reinforced with SiC or Al 2 O 3 particles. Pressurized cutting fluid neither improves nor worsens the tool life because of either effective flushing of the chips or lack of a lubricating film. The surface finish and the cutting forces are insensitive to cutting fluid and cutting speed when machining with a new diamond tool, but deteriorate with greater tool wear.

Chip formation in machining particle-reinforced metal matrix composites

Abstract This paper compares the chip forming mechanism in monolithic metals and in particle-reinforced metal matrix composites. A quick stop device was designed to collect the chip-root for microscopic examination. The eutectic silicon and reinforcing particles formed flow lines due to different velocity zones across a chip. The reinforcing particles interfered with plastic deformation of the matrix, retarded crack growth, and were not delaminated from the matrix. High local temperature, induced by high strain rate in machining, improved the material ductility and healed microcracking in a chip. Heat treating a composite to a T6-temper affects the shear deformation at the tool-chip interface, and shortens the tool life.

Cumulative tool wear in machining metal matrix composites Part I: Modelling

Abstract Analytical models are derived for cumulative wear of facing and turning tools. An aluminium-based metal matrix composite reinforced with silicon carbide particles was used to verify the validity of the models. Experimental data show that flank wear of a cutting tool does not depend on the order of different cutting speeds since abrasive wear dominates the wear mechanism. Since the data for turning and facing are correlated, it is more economical to rank machinability using data for facing, and to convert them for turning if required.

SURFACE ROUGHNESS OF SPUTTERED SILICON. II. MODEL VERIFICATION

Experimental verification of the mathematical surface roughness model for sputtered silicon was performed. The beam shape and its significant level of intensity were determined first by measuring the topography of craters sputtered by focused ion beam (FIB). Then the beam function was generated for various combinations of beam parameters. The material function was developed both by theoretical and experimental analysis. These two functions were then used in the model to calculate the theoretical surface roughness. Microsurface analysis was formed by FIB sputtering of a (100) silicon wafer. The surface roughness at the bottom of the sputtered features was then measured using an atomic force microscope. The theoretical surface roughness was found to be within ±1 and ±5 nm of the measured surface roughness with the measurement uncertainty (standard deviation) of about ±0.36 and ±0.85 nm for R a and R t, respectively.

DUCTILE-REGIME MACHINING OF PARTICLE-REINFORCED METAL MATRIX COMPOSITES

ABSTRACT This paper presents research results on ultraprecision machining of metal matrix composite (MMC) composed of aluminum matrix and either SiC or A12 03 particles. Ductile-regime machining of both SiC and aluminum was evaluated to improve the surface integrity of the composite. Both polycrystal-line diamond (PCD) and single crystalline diamond (SCD) tools were used to ultraprecision machine the composites at a depth of cut ranging from 0 to 1μm using a taper cut. The feedrate was normalized to the tool nose radius. A model is proposed to calculate the critical depth of cut for MMCs reinforced with either A1203 or SiC. The critical depths of cut were found to be 1 p.m and 0.2 u.m for MMCs reinforced with A12 0 or SiC3, respectively. Both depth of cut and crystallographic direction of the ceramic particles are the sufficient conditions for ductile-regime machining. Although both tools produce similar surface finish, a SCD tool removed the MMC as chips while a PCD tool simply smeared the surface. A dif...

SURFACE ROUGHNESS OF SPUTTERED SILICON. I. SURFACE MODELING

A mathematical model for the calculation of surface roughness was developed for focused ion beam (FIB) sputtering. The surface roughness function is a combination of the beam function and the material function. The beam function includes ion type, ion acceleration energy, and beam parameters. Furthermore, the beam parameter incorporates ion flux, the ion beam intensity distribution profile, tailing and neighboring of the successive beams, dwell time, etc. The intensity distribution inside the ion beam is considered to be Gaussian. The cumulative intensity over the total milling area is calculated by the algebraic summation of individual beam intensity delivered to every pixel successively. The material function includes the inherent material properties related to the ion beam micromachining. If one knows the beam function and material function, surface roughness at the bottom of the sputtered features can be calculated using this model.

Precision grinding and facing of copper-beryllium alloys

Abstract This paper investigates the machinability of Cu-Be alloys by ultraprecision grinding and facing. The material temper, tool geometry, and machining parameters are varied to assess their effects on surface finish. The study shows that microgrinding of Cu-Be with a diamond wheel generates a rougher surface finish as compared to that produced by microfacing with a single-point diamond tool. Similar chip formation mechanisms are observed when the depths of cut vary from few millimeters to submicron levels. A mathematical model is derived to compare the theoretical and experimental surface finish. Good agreement between predicted and measured data is obtained, providing grain boundaries are visible on a machined surface when being observed under a microscope. Feedrate and tool radius are the most influential parameters on surface finish. Flatness of 20 nm on the 9.5 mm diameter rod and roughness of 2 nm Ra and 8 nm Rt are achieved. Although the material’s micromachinability is the same for both the aged and unaged alloys, size and distribution of beryllides must be controlled for better tool life and surface finish.