Grain Refinement and Surface Modification Technique by Equal Channel Angular Pressing and Laser Shock Peening on Magnesium Alloy

Abstract

Design and development of any part in mechanical design consists of three elemental parameters. Such as, selection of material, geometrical constrains (dimensioning) and loading or boundary conditions. Boundary conditions are the functional requirements of design which need to be satisfied from the geometry of the component by allowing optimal material to execute the function. Hence, selection of materials is the primary building block of any component in mechanical system. Selection of material is very crucial and based on type of loading, environment conditions and reliability to withstand for long duration. Magnesium and its alloys have drawn great interest from the past decade due to its superior strength to weight ratio, bio-compatibility, effective manufacturing process and other positive attributes, but there are some limitation, such as effective strength, low fatigue life, low wear resistance and high corrosion rate. These properties can be altered by grain boundary characteristics, hence reformation of grain boundary to change the grain behaviour is of significant interest. In most of the methods, one principle technique (controlling cooling rate while solidification, alloying, severe plastic deformation) is used to alter the grain size of a material, which affects the grains in whole material or at near surface. Hence, there is a research gap while combining two different techniques to achieve combination of grains for better application. Severe plastic deformation (SPD) is a top down approach to form fine grains from coarse grain, and equal channel angular pressing (ECAP) is one of the simple procedures in SPD to achieve fine grains effectively. Samples during the ECAP process experience severe shear strain followed by deformation of grains, which rupture the coarse grains into new grains with redistributed grain boundaries. Formation of fine grains and grain boundary redistribution by ECAP enhances the strength and other mechanical properties in accordance with Hall-Petch relation. Conversion of coarse grains into fine grains occurs throughout the sample and resultant grain size depends on number of passes and route of the pass. But the original shape of the sample doesn’t get altered after processing. Laser shock peening (LSP) is a surface treatment process, to induce compressive residual stresses at the surface. This technique involves creating permanent deformation at the surface, which causes grain refinementat near surface. Grain refinement of bulk sample is obtained by ECAP process, whereas grain refinement at the surface of already deformed ECAP processed sample, is obtained by laser shock peening process. Present work describes the combined effect of ECAP and LSP on AM80 magnesium alloy. As-received (cast) AM80 (Wt. 8% of Al, Wt. 1 % of Mn, balance Mg) material is homogenized and processed by ECAP upto 4 passes under route BC. The samples were tested for mechanical properties, which showed enhancement of strength and ductility in ECAP processed samples. Microscopic investigation revealed the formation of fine grains, due to applied shear strain. By increasing the number of ECAP passes, more fine grains are reported. 2–pass ECAP processed sample shows heterogeneous grains, where the large grains were surrounded by small grains, and possess maximum tensile strength of 310 MPa compared to 1, 3 and 4-pass samples. Therefore, 2-pass ECAP processed sample is considered for further processing by LSP. LSP is carried out with a power density of 8 GWcm-2 and repeatedly to achieve different percentage of coverages, LSP processed samples are analysed for mechanical properties and microstructural characterization. Microscopic examination revealed the formation of fine grains in the range of few nanometers after peening near the surface. Scanning electron microscope revealed the formation of flower petal like structures, and transmission electron microscope revealed elongated grains in the form of bands, and these bands overlapped as the percentage of coverage increases. There was a slight increase in tensile strength in LSP processed samples, due to strain hardening at surface. Dimples of various sizes were observed on fracture surface of ECAP+LSP processed region. Mg17Al12, Mg2Al3, MnAl6 with Mg phases were identified by X-ray diffraction. Wear studies of LSP processed region showed an increase in wear resistance, and microscopic image of wear surface reveals the wear mechanisms due to oxidation and ploughing of hard particles. Roughness measurement was carried out on ECAP+LSP processed samples and there was significant influence of peening in increasing roughness of the surface. Nano indentation experiments help to understand the hardness behaviour of processed material at nano scale. An increase in surface hardness is observed with LSP processed samples compared to as-cast and ECAP processed samples. Further, there was anincrease in toughness and yield strength in peened region. D-space measurements were done by X-ray diffraction to measure the lattice space before and after peening, and relative strains were converted into stresses and residual stresses were identified. Tensile residual stress profile is identified in as-cast sample due to solidification of molten metal, and homogenized sample showed decrease in tensile residual stress value due to kinetic grain growth. ECAP processed sample shows compressive residual stresses due to strains induced in between the lattice. But ECAP+LSP processed sample shows higher compressive stress at near surface (peened region). Fatigue experiments played crucial role to characterize the material in cyclic loads for reliability. Experiments were conducted at maximum stress of 120 MPa, with a stress ratio of 0.125. ECAP+LSP processed sample with 100 % coverage took 85268 cycles of load compared to homogenized sample (1 cycle of load). Investigation of fractured surface of fatigue samples showed crack initiation and propagation region followed by rupture. ECAP+LSP processed sample with 100 % of coverage shows, significant gap between crack initiation and rupture region. Hence delay in crack initiation and propagation was observed.