Papers by Keyword: Thermal Softening

Abstract: This paper is concerned with the validation of the dynamic hardening behaviors of metallic materials by comparing numerical and experimental results of the Taylor impact tests. Several uniaxial tensile tests are performed at different strain rates and temperatures by using three kinds of materials: 4130 steel (BCC); OFHC copper (FCC); and Ti6Al4V alloy (HCP). Uniaxial material tests are performed at a wide range of strain rates from 10⁻³ s⁻¹ to 10³ s⁻¹. Moreover, tensile tests are performed at temperature of 25 °C and 200 °C at strain rates of 10⁻³ s⁻¹, 10⁻¹ s⁻¹, and 10² s⁻¹, respectively. A modified Johnson–Cook type thermal softening model is utilized for the accurate application of the thermal softening effect at different strain rates. The hardening behaviors of the three materials are characterized by comparing the seven sequentially deformed shapes of the projectile from numerical and experimental results of Taylor impact tests.

Abstract: An experimental programme for determination of the Johnson Cook material and failure model constants for a typical armour steel material is reported. Tensile tests on specimens made from the armour material have been conducted at quasi-static and dynamic strain rates and at ambient and elevated temperatures. The analysis of the experimental data generates the model constants that are required as inputs during numerical simulation of dynamic events like armour impact using Johnson Cook constitutive relation and failure model implemented in most of the commercially available Finite Element codes.

Abstract: In this paper, the surface integrity is studied when machining the aeronautical titanium alloys. Surface roughness, lay, defects, microhardness and microstructure alterations are studied. The result of surface roughness judges that the CVD-coated carbide fails to produce better Ra value than the uncoated. Lay is characterized by cutting speed and feed speed directions. Feed mark, tearing surface, chip layer formation as built up layer (BUL), and deposited microchip are the defects. Microhardness is altered down to 350 microns beneath the machined surface. The first 50 microns is the soft sub-surface caused by thermal softening in ageing process. Microstructure alteration is observed in this sub-surface. Down to 200 microns is the hard sub-surface caused by the cyclic internal work hardening and then it is gradually decreasing to the bulk material hardness. It is concluded that dry machining titanium alloy is possible using uncoated carbide with cutting condition limited to finish or semi-finish for minimizing surface integrity alteration.

Abstract: In serrated chips of difficult-to-machining materials such as medium carbon steel and titanium alloy during metal cutting process at high strain rates, the fine grain structure of the narrow shear bands which results from thermal softening due to severe deformation have been observed. However, the theories which have been developed to analyze continuous chip formation and most FEM analyses based on the conventional models such as Johnson-Cook and Zerilli-Armstrong flow stress model fail to explain the adiabatic shear band formation and the serrated chip formation. This paper discusses the characteristic of the new flow stress model in the previous investigation [1,2] and FEM simulation results to predict the serrated chip formation results are shown.

Abstract: A model hot forging test was carried out using tools of matrix high-speed steel. Dynamic thermal loads were applied to tools subjected to ion nitriding on the surface layer of the tool, and the conditions of deterioration and thermal softening were examined by measuring Vickers hardness in the vicinity of the tool surface after the thermal load test. In addition, static tempering softening tests were carried out, and thermal softening equations are obtained using tempering parameter on the basis of the obtained master tempering curves. When the thermal load was applied under the same conditions to nitrided and heat-treated tools, the degree of surface deterioration and thermal softening was small for nitrided tools. Equations of softening for two cases, i.e., specimen subjected only to heat treatment and specimen subjected to both heat treatment and nitriding, were expressed using tempering parameter.