An experimental and analytical investigation of flow instability and shear localization in the orthogonal machining of grade 2 commercially pure titanium was made. A criterion for thermo-plastic instability was developed from torsion test results and applied to the analysis of the chip formation process. It was shown that flow instability followed by flow localization occurs when machining titanium at all cutting speeds and that a transition in the chip type from uniform to segmented does not occur. Orthogonal machining experiments were conducted in the speed range from 8.75 × 10 −5 to 3.20 m/s for various depths of cut and the shear strain in the chip was calculated. It was shown that shear localization occurred in the chip formation process when the uniform shear strain involved in producing a chip segment reached a critical value and that this critical shear strain correlates fairly well with the instability shear strain predicted by the thermo-plastic instability criterion.

Analysis of the phenomenon of adiabatic flow instability during the torsional straining of CP titanium has been made and the instability shear strain predicted. The constitutive equation for isothermal deformation of CP titanium was integrated over the total shear strain received by the material. Thermal softening caused by adiabatic heating was calculated and used to derive the adiabatic shear stress-shear strain curves. The instability shear strain was determined as the value that corresponds to zero slope of these curves. The effect of strain rate, temperature, and specific heat on the instability shear strain was also discussed. Comparison of the experimental results with predictions showed that flow instability in the torsion test occurred at strains approximately equal to the theoretical thermo-plastic instability strains for a temperature of 150°C but at lower strains at higher temperatures. The difference between the theoretical and experimental instability strains increases with an increase in temperature. A discussion of this difference is also presented.

Flow stress data for CP titanium were obtained using the torsion test in the temperature range from ambient to 750°C and at shear strain-rates in the range from 0.192 to 122.0 s −1 . The temperature rise because of deformation heating was calculated numerically. It was found that the temperature rise is insignificant for strain rates below 0.192 s −1 where the deformation conditions are essentially isothermal while for strain rates above 27.0 s −1 deformation conditions are adiabatic. Large temperature gradients exist in the longitudinal direction of the specimen with the maximum temperature occurring at the middle of the gage length. The temperature gradients in the radial direction are much smaller and can be neglected for most practical applications. Isothermal shear stress-shear strain curves for CP titanium were obtained from the experimental torsion test data by accounting for the temperature rise produced by deformation. It was found that the data exhibits a good fit to a Johnson-Cook relationship with exponential thermal softening. The work emphasizes the importance of the need to consider the role of deformation heating when interpreting data on the effect of strain rate and temperature on the mechanical properties of materials.

A novel electrolytic etching technique is used to determine the residual stress distribution in the machining of annealed 18 percent nickel maraging steel. Ring shaped specimens were machined under unlubricated orthogonal conditions with carbide cutting tools having wear lands of 0.125, 0.25, and 0.5 mm length at cutting speeds ranging between 0.05 and 1.60 ms −1 . The results of the investigation show that the residual stresses are tensile at the machined surface and decrease with an increase in depth beneath the machined surface. The maximum (near surface) residual stress and depth of the severely stressed region increase with an increase in cutting speed and tool wear land length. The results are interpreted in terms of the variations in the amount of surface region deformation produced by changes in cutting conditions.

The effects of cutting speed, chip-tool contact length, and tool surface finish on the surface integrity of solution-treated and aged 2024-aluminum alloy, that is machined under lubricated orthogonal conditions are investigated. The surface region of machined workpieces is examined using a variety of diagnostic techniques. In addition, tool forces are measured and the cutting geometry is determined. The results show that a variety of geometrical surface features are generated that contribute to the natural surface roughness, and that a plastically deformed surface region is generated that contains variations in hardness, microstructure, and residual plastic strain. The results are interpreted in terms of the type of chip produced, the mechanics and thermodynamics of the cutting process and the lubricating action of cutting fluids. When the results of the investigation are compared with similar results derived under nonlubricated conditions, it is found that at low cutting speeds application of a lubricant produces a reduction in tool forces, power consumption, surface roughness, surface defects, and subsurface deformation, whereas at high cutting speeds application of a lubricant has little influence on the cutting process.

The effect of cutting speed and tool wear land length on the surface integrity of quenched and tempered AISI 4340 steel machined under dry, unlubricated orthogonal conditions is determined. The surface region of machined test pieces is examined using optical microscopy, scanning electron microscopy, X-ray microprobe analysis, microhardness measurements, and profilometry. In addition, tool forces are measured and tool temperatures calculated. The results of the investigation show that during machining a damaged surface region is produced which is quite different from the bulk of the material. It is found when cutting at low speeds with sharp cutting tools that the damage is restricted to a variety of geometrical defects associated with the surface. It is found when cutting at high speeds or with tools having large artificially controlled wear lands that considerable subsurface damage involving changes in metallurgical structure and hardness is produced. The results are interpreted in terms of the type of chip produced during machining, the temperatures generated during machining, and the interaction between the tool nose region and workpiece. It is suggested that observations based on scanning electron microscopy are more indicative of the true surface condition than surface roughness measurements.

The effect of cutting speed and wear land length on the phenomenon of microchip formation during machining of quenched and tempered AISI 4340 steel under dry orthogonal conditions is determined. Machined test pieces are examined using scanning electron and optical microscopy. Surface roughness is determined using a profilometer. A possible mechanism of microchip formation based on the interaction between the surfaces of the tool and freshly machined work piece is discussed. It is suggested that the grooves left by the generation of microchips may act as sources for the initiation of failures by creep, fatigue and stress corrosion cracking. It is also suggested that the results obtained using scanning electron microscopy may be more indicative of the true surface condition than surface roughness measurements.

An idealized model of the orthogonal metal cutting process is used to determine the stresses, strain rates, and temperatures at the tool rake face for the machining of several materials under dry, unlubricated conditions, where a continuous chip is produced with an absence of a built-up edge. An attempt is made to correlate stress with strain rate and temperature using a temperature compensated strain rate parameter, and velocity modified temperature.

An analysis is presented which shows how the theory of anisotropy based on a von Mises criterion of yielding first proposed by Hill may be extended to the plastic torsional straining of a hollow cylinder. Expressions are given for the anisotropic parameters, and the yield stresses along the anisotropic axes in terms of certain quantities, namely; the change in axial and tangential strain with shear strain, the principal yield shear stress, and the through thickness yield stress of the hollow cylinder. Experimental measurement of these quantities is made for the plastic torsional straining of hollow cylinders of A1-1100. Experimental data are analyzed using the previously derived expressions, and the anisotropic parameters, and yield stresses along the anisotropic axes determined. Finally, the results of the investigation are discussed, and conclusions drawn.

A review is presented concerning the mechanics of the metal-cutting process where a continuous chip is produced with the absence of a built-up edge on the tool face. It is shown that previous theories which predicted a linear relationship of the form φ = A − B (β − α) between the angle parameters cannot be used to interpret the experimental data. The apparent linear relationships observed in some previous experimental work are thought to be due partly to the method of presentation of the results. It is suggested that a more significant combination of the angle parameters is the difference between the shear angle φ and the rake angle α, that is, (φ − α). It is pointed out that two parameters which can be used to describe the frictional conditions on the tool face are the mean friction stress and the mean normal stress. Since these can vary independently, it is thought unrealistic to group them together as a single variable. It is therefore concluded that the mean angle of friction is insufficient in itself to describe the frictional conditions on the tool face.