Papers by Keyword: Work-Hardening

Abstract: Cross-contamination occurring after the blending of metal powders in multi-material laser powder bed fusion (PBF-LB/M) is a frequent manufacturing issue and poses a major obstacle to the further development of this emerging additive manufacturing process. To evaluate the influence of such contamination on a technologically important material system for multi-material PBF-LB/M, this study investigates the impact of CuCr1Zr foreign particle contamination within AlSi10Mg powder on the resulting metallurgical characteristics and mechanical performance of the fabricated parts. Several contamination levels of CuCr1Zr were considered, namely 0.5 wt.%, 3.0 wt.% and 5.0 wt.%, with the results benchmarked against samples produced from uncontaminated powder. Tensile testing demonstrated that the cross-contamination contribute to material embrittlement. This investigation focuses on the tensile work-hardening behaviour of the investigated materials showcasing that the specimens exhibit only the first two work-hardening stages, while for the higher contamination level studied, the material becomes brittle and fractures even in the first work hardening stage.

Abstract: We recently proposed a new mechanism to simultaneously improve the strength and ductility of multiphase alloys, named “Anisotropic mechanical property-induced ductilization (AMID)”. In this paper, the variations in tensile deformation behavior of Mg/LPSO extruded alloys depending on the volume fraction of the LPSO phase were examined, to deepen the understanding on AMID. As expected, the work-hardening rate of the Mg/LPSO two-phase extruded alloy increased as increasing in the volume fraction of LPSO phase. This demonstrates the validity of the AMID mechanism. However, the increase in the volume fraction of the LPSO phase decreased the elongation, even though the work-hardening rate was increased in them. The present study revealed that an appropriate amount of Mg grains is necessary to obtain the effect of AMID in improving the uniform elongation of the alloy, by the suppressing the development of microcracks formed in the LPSO phase grains into macroscopic fracture.

Abstract: Czochralski silicon single crystals were deformed in tensile tests along the direction at between 1173 K and 1373. Yield point phenomenon were observed in the specimens deformed at below 1273 K while continues yield was observed in the specimens deformed at above 1323 K. It is due to the effect of dislocation starvation in the used crystals. Work-hardening rates in stage II were consistent with those reported in fcc crystals such as copper. The onset of stage II was found to be active before the Schmid factor of the second slip system becomes larger than that of the primary slip system. Electron backscattered diffraction images indicated clear kink bands near grips and in the parallel portion. The kink bands were formed at the middle of stage I, which suggest that the formation of kink bands is a trigger of stage II.

Abstract: The conventional characterization of work-hardening is to approximate the stress-strain diagram using the empirical curve-fitting of Hollomon or Voce. The new method uses the Taylor slip analyses to derive a functional form which is optimally fitted to the data. This constitutive relations analysis (CRA) duplicates the data using at least two fit loci. The fit parameters relate to the slip motion within the microstructure and hence its interpretation reveals the possible dynamic shape-change reactions. The fit-process defines a new yield stress which separates the yielding from the deformation mechanisms at large strains that breaks up into two regions separated by intersection parameters. The applications of CRA to nanovoid formation and growth leading to ductile failure, plane stress yield locus prediction using tensile tests and decoding the stress-strain diagram for age-hardened aluminum alloys have been successful. Using super-pure aluminum, this study confirms that CRA is based on crystal plasticity principles and that CRA can predict the correlation of the obstacle strength factor, α, with work-hardening, hence permitting conversion of flow stress at given strains to obstacle density. The derived results show that the inherent annihilation process and the changing strength factor are coordinated to result in a self-consistent constitutive relation.

Abstract: Grain refinement is attracting attention as a strengthening method which does not depend on the alloying elements added to steels. Many reports have described the manufacturing methods and mechanical properties of ultra-fine grained steels. In ultra-fine grained steels, it is well known that yielding stress drastically increases in accordance with the Hall-Petch relationship, while uniform elongation significantly decreases. These tendencies imply that grain size affects not only yielding but also work-hardening behavior. However, the influence of grain size on work-hardening behavior has not been clearly understood. Therefore, in this study, we investigated the work-hardening behavior during tensile deformation of 12Cr stainless steel with various grain sizes. Grain refining was conducted by cold-rolling of annealed and quenched steel specimens, followed by recrystallization annealing. The grain size of the specimens decreased as the cold-rolling reduction rate increased. The minimum grain size obtained by this method was approximately 5 μm. With decreasing grain size, 0.2% proof stress increased and the strain which reached the plastic instability condition decreased. In the session, we report the dislocation accumulation behavior estimated by grain hardness and XRD and the dynamic recovery behavior assessed by the Kocks-Mecking model.

Abstract: An experiment of face milling of Invar36 was conducted by using coated carbide insert, the microhardness was tested and the metallographic structure was observed to figure out the principles of work-hardening. The results showed that the depth of work-hardening ranges from 80μm to 160μm among the parameters selected in the experiments. The degree and the depth of work-hardening were significantly affected by the axial depth of cut and feed per tooth. The degree and the depth of work-hardening showed a tendency to increase with the increase of the axial depth of cut and feed per tooth. Compared with the axial depth of cut and feed per tooth, cutting speed had less influence on the degree and depth of work-hardening. The degree and depth of work- hardening decreased slowly with the increase of cutting speed. Metallographic observation showed that work-hardening layer consisted of the thermal force influenced layer and the force influenced layer, while the amorphous metallographic structure was observed in the thermal force influenced layer, and lattice distortion was observed in the force influenced layer.

Abstract: This study discussed about the influence of the cutting speed and radial depth of cut on surface integrity (microhardness and work-hardening) when performing hard milling of AISI D2 workpiece. By using PVD-TiAlN, nine experimental trials were performed at various cutting speeds of 80, 100 and 120 m/min and various radial depth of cut of 3, 4 and 5 mm as feed and depth of cut remain constant at 0.05 mm/tooth and 0.05 mm. From the result, due to high cutting temperature generated, the cutting speed adversely affects the microhardness value of the subsurface layer. At higher cutting speed of 120 m/min higher hardness values were obtained when compared with lower cutting speed of 80 m/min. As for the effect of the radial depth of cut it is clear that it influences the microhardness beneath the surface. Higher microhardness recorded from the radial depth of cut increment associated with high cutting temperature generated during machining.

Abstract: Work-hardening of machined surface plays an important role in the evaluation of surface quality and performance of wear resistance in the process of machining components. In this study work-hardening of machined surface during milling 7050-T7451 aluminum alloy is investigated using micro-hardness experiments under different cutting conditions. Moreover, the wear resistance of machined surface including wear quantity and friction coefficient are obtained and studied by means of high speed ring-block friction-wear tester. The work-hardening and wear resistance are particularly sensitive to cutting speed. Friction coefficient has marked drop trends and the tendency of wear quantity is ascend in first and descend at last as work-hardening increases. The comparison of wear resistance under different cutting conditions shows that the wear resistance of machined surface can be directly affected by work-hardening and machined surface obtained by high speed milling with higher micro-hardness have more superior in wear resistance performance.

Abstract: The orthogonal experiments of end milling were designed. Workpiece surfaces hardening were studied in high-speed milling. The results indicate that the degree of surface work-hardening in high-speed milling 8407 mold-steel is lower than that in conventional speed milling;By choosing the appropriate cutting parameters, to improve the surface quality of the workpieces and the processing efficiency is beneficial;Through changing the high-speed milling parameters to control the work hardening, the effect is not so obvious.

Abstract: With cold rolling base plate of low carbon steel by CSP process, the cold deformation experiments were carried out by the two-roller reverse-mill in the laboratory. The work-hardening was studied for different deformation plates through the room temperature tensile and microhardness measured, and the microstructure was also studied after deformation. It was found that the steel yield and tensile strength increased and work-hardening marked with the deformation augment, the test steel microstructure is ferrite with mingle small amount pearlite, its grain is refined and elongated with deformation increasing.