Numerical and Experimental Study on Constrained Groove Pressing

Constrained groove pressing (CGP) is a new severe plastic deformation method suitable for producing ultra-fine grained sheet metals. Based on Taguchi optimization method, the influence of processing parameters such as groove width, groove angle, friction coefficient and deformation rate on deformation homogeneity of constrained groove pressing (CGP) was studied numerically utilizing DEFORM-3D. A multi-pass CGP was carried out on 1060 commercially pure aluminium, copper and Ni sheets. Through a series of experimental research, the evolution of microstructure, tensile properties, forming load, and die parameters during the process was investigated

Groove pressing and its research progress

In order to systematically study the application of groove pressing in preparing bulk mass ultra-fine grain sheet metal, the fundamental principles, plastic mechanics analysis and classification of groove pressing are introduced, and on the basis of which, the influence rules and mechanism of processing factors such as pass number, deformation temperature and die structure on groove pressing are reviewed, and some key problems such as thermal stability of processed materials, process improvements and deformation path, deformation homogeneity analysis and process optimization are summarized. The research trends of groove pressing in the fields of application to sheet metals difficult to deform at room temperature, improvement of ductility, toughness and formability of processed sheets as well as its plastic deformation mechanism are prospected: in the future, groove pressing will be used for sheet metals which have limited formability but wide industrial applications such as magnesium alloys and titanium alloys, and much attention should be paid to investigation on performance adjustment mechanism and methods for processed sheets and exploration of effective technological measures for preventing cracks; meanwhile, the evolution mechanism of microstructure and properties of groove pressed materials must be systematically and thoroughly revealed

The Fracture Behavior and Thermal Stability of Commercially Pure Nickel Sheets Processed by Constrained Groove Pressing

Constrained groove pressing (CGP) is one of the most promising severe plastic deformation (SPD) techniques suitable for producing ultra-fine grained (UFG) sheet or plane metallic materials with unique physical, chemical, and mechanical properties. However, the heavily deformed sheets are usually with high strength but low ductility due to work hardening and micro-cracking, and the UFG microstructure and high strength are unstable especially when exposed to high temperatures. Thus, in this work, CGP was conducted on commercially pure nickel sheets and, firstly, the fracture mechanism of the processed sheets was examined. Then, the annealing behavior of CGP nickel sheets was investigated in detail to explore their thermal stability and improve their ductility. The results showed that significant grain refinement and strength improvement of the nickel sheets were achieved with great loss of ductility. The fracture surface morphology of the sheet sample exhibited typical characteristics of fatigue fracture, and inhomogeneous strain distribution and work hardening determined the micro-crack initiation position and propagation direction. The CGP sheets by one and two passes showed high thermal stability up to 650 and 600 °C, respectively, owing to different stored internal stresses and accumulated energy. In both cases, obvious recovery of elongation to failure from 12.7% and 10.6% to 29.3% and 27.3% were achieved by CGP with post-deformation annealing treatment, respectively, with acceptable drop of strength

Experimental investigation of pure aluminum sheets processed by constrained groove pressing

121-127Recently, a new severe plastic deformation method named constrained groove pressing (CGP) has been invented for producing ultra-fine grained sheet metals. Here, a multi-pass CGP is carried out on 1060 commercially pure aluminum sheets. Through a series of experimental research, the evolutions of microstructure, tensile properties, forming load and surface residual stress during the process are investigated. The grain size is greatly refined from 29 μm of annealed samples to about 18 μm after pass four. Polygonized and dislocation-free substructures of submicron level with well-defined boundaries is obtained. The ultimate tensile strength and yield strength increase continuously until pass three and then decrease by further deformation. Residual tensile stress is observed on the surface of all pressed samples due to the unique stress-strain state, and it increases dramatically during the first pass. The results show that lubrication reduces the average residual tensile stress and its distribution homogeneity along the longitudinal direction

A study on the tribological behavior of AZ31 magnesium alloy sheets processed by temperature-assisted ultrasonic shot peening

Mg alloy, as a promising lightweight structural material, finds applications in various fields. However, poor surface properties like wear resistance greatly limit its applications. As a surface modification technology, ultrasonic shot peening (USP) has gained significant popularity. In this study, temperature-assisted USP (TA-USP) was firstly applied to AZ31 Mg alloy. The influence of peening duration and temperature on microstructural evolution and mechanical properties was investigated in details. The ball-on-disk dry sliding wear test was performed to examine the tribological behavior. The results show that a gradient microstructure and surface nanocrystallization are introduced by TA-USP. The gradient becomes significantly pronounced as the peening duration extends. The surface hardness increases from the as-annealed 60.9 ± 1.8 HV to 159.0 ± 6.1 HV after processed at room temperature with 800 s, and a depth of about 600 μm for the affected region is achieved. When the peening temperature is raised, the gradient weakens, even disappears at 300 °C, and the hardness decreases. Notably, the gradient microstructure and enhanced hardness exhibit significant thermal stability below 200 °C. The wear curves and morphology analysis suggest that TA-USP obviously improves wear resistance. The surface nanocrystalline results in an easy formation of MgO patches and reduces the coefficient of friction. Moreover, the presence of gradient microstructure also retards the delamination phenomenon. Increasing peening duration or temperature to a moderate level will lead to an enhanced wear resistance, owing to its dependence on both hardness and toughness. A simplified wear transition map is roughly established to predict optimum wear resistance

Influence of erbium addition on the defects of selective laser-melted 7075 aluminium alloy

Selective laser-melted (SLMed) 7075 aluminium alloy is prone to cracking. Erbium (Er) could modify the aluminium alloys to prevent cracking in traditional manufacturing processes. In the present study, Er-modified 7075 aluminium alloy powders are prepared to investigate the influence of Er element on crack reduction in SLM. The evolution of Er is characterised by a scanning electron microscope (SEM), a X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM). The results show that the crack density is reduced with Er addition, but the porosity increases, a new discovery. Based on the SEM and TEM results, Er addition exists in the form of unmelted particles and Al3Er of sub-micrometer scale. The Er particles in the molten pool deteriorate the fluidity and result in higher porosity. The coherency between aluminium matrix and Al3Er favours the grain refinement, therefore the crack reduction