Tribological behaviour of AZ31 magnesium alloy reinforced by bimodal size B4C after precipitation hardening

Abstract This study investigated dry sliding wear properties of AZ31 magnesium alloy and B4C-reinforced AZ31 composites containing 5, 10, and 20 wt.% B4C with bimodal sizes under different loadings (10–80 N) at various sliding speeds (0.1–1 m/s) via the pin-on-disc configuration. Microhardness evaluations showed that when the distribution of B4C particles was uniform the hardness of the composites increased by enhancing the reinforcement content. The unreinforced alloy and the composite samples were examined to determine the wear mechanism maps and identify the dominant wear mechanisms in each wear condition and reinforcement content. For this purpose, wear rates and friction coefficients were recorded during the wear tests and worn surfaces were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry analyses. The determined wear mechanisms were abrasion, oxidation, delamination, adhesion, and plastic deformation as a result of thermal softening and melting. The wear evaluations revealed that the composites containing 5 and 10 wt.% B4C had a significantly higher wear resistance in all the conditions. However, 20 wt.% B4C/AZ31 composite had a lower resistance at high sliding speeds (0.5–1 m/s) and high loadings (40–80 N) in comparison with the unreinforced alloy. The highest wear resistance was obtained at high sliding speeds and low loadings with the domination of oxidative wear

Unveiling the impact of laser power variations on microstructure, corrosion, and stress-assisted surface crack initiation in laser powder bed fusion-processed Ni-Fe-Cr alloy 718

Corrosion and stress-corrosion related failures often compromise the integrity of critical metallic components during their service, raising significant concerns. It is crucial to comprehend the crack initiation mechanism and the impact of alloy microstructure on this crack initiation process. It is known that the introduction of unique microstructures through metal additive manufacturing brings new challenges. This study aims to investigate, for the first time, the effects of microstructural alterations resulting from fluctuations in laser power during laser powder bed fusion on the surface cracking initiation mechanism and electrochemical behaviour of Ni-Fe-Cr alloy 718, which is widely used in applications that require exceptional strength and corrosion resistance. To carry out this investigation, microcapillary electrochemical methods were combined with high-resolution techniques (TEM, SEM, AFM). The findings emphasize the existence of an optimal range of process parameters that effectively mitigate corrosion and crack initiation susceptibility. This work demonstrated that slight deviations in laser power from this optimal value result in diverse alterations at the micro and submicron scales. These alterations include increased subgrain width, porosity, dislocation density, density of nanovoids, and distribution of carbides. Importantly, these changes, particularly in dislocation and nanovoid densities caused by minor variations in process parameters, significantly affect the material's susceptibility to corrosion initiation and stress-assisted surface cracking

Assessment and Development of Laser-Based Additive Manufacturing Technologies For Metal Microfabrication

Nowadays many devices are produced in very small sizes or containing small features for particular application such as biomedical and microfluidic devices. Based on this demand, manufacturing processes should be developed for implementation of micro features in different ranges of sizes. A broad range of microfabrication technologies have been developed which have different applications and capabilities such as laser ablation, plating, photolithography, lithography and electroplating. However, such techniques are restricted when utilized to new microproducts which need the employment of a diversity of materials and have complicated three-dimensional geometries. Additive manufacturing (AM) needs each layer to be fabricated according to an exact geometry defined by a 3D model. This concept seems suitable for production of complicated parts with micro features. Development of robust metal additive manufacturing for microfabrication opens a new window toward miniaturization of metallic parts such as design and production of porous implants containing micro features and micro pores (50-500 µm). This work covers the development of micro additive manufacturing through two laser based AM processes with two different concepts: Micro direct metal deposition (µDMD) and selective laser melting (SLM). Nowadays, NiTi shape memory alloys are among the most interesting materials in the field of bioengineering and medical applications. Assessment of both techniques for production of NiTi porous scaffolds for biomedical application was carried out in this thesis. Long-term fixation of biomedical implants is achievable by using porous materials. These kinds of materials can develop a stable bone-implant interface. A critical aspect in production of porous implants is the design of macro and micro pores. At the first step of this thesis, the process parameters of both technologies were optimized to obtain full density samples. Secondly, porous scaffold structures with geometry controlled porosity were designed and manufactured using both technologies. Investigations using X-ray diffraction and scanning electron microscopy equipped with energy dispersive spectroscopy showed that B2-NiTi phase with small quantity of unwanted intermetallics can be obtained by micro direct metal deposition of mechanically alloyed Ni50.8Ti49.2 powder. Micro direct metal deposition was optimized through a set of process parameters and designed experiments to improve the geometrical accuracy and repeatability of micro fabrication. Micro X-ray computed tomography were used to analyze the surface topography, micro porosity, and deviations of products with respect to nominal geometrical models. Below 10% deviation to nominal geometrical models was achieved in hollow NiTi samples through a set of micro direct metal deposition process parameters and designed experiments. A comprehensive study was conducted on Ni50.8 Ti49.2 (at%) alloy to discover the influence of SLM process parameters on different aspects of physical and mechanical properties of NiTi parts. The provided knowledge allowed choosing different optimized parameters for production of complicated geometry with micro features maintaining the phase composition through the sample. For the first time and in this thesis, without going through any solid solution and heat treatments, single phase austenite was obtained in SLM NiTi parts with the selection of three different regimes of process parameters. This knowledge led to manufacture of NiTi bony structure applying different process parameters for the border and internal parts. The experimental results showed that SLM process with specific process parameters is a feasible micro additive manufacturing method to implement the complicated internal architecture of bone. It is an important issue in production of customized prostheses

Mechanical behavior of Ti6Al4V lattice structures; numerical and experimental analysis

This study deals with the mechanical behavior of lattice structures produced by the Laser Powder Bed Fusion process experimentally and numerically for four different topologies of Body-Centered Cubic, Body-Centered Cubic with struts along Z-direction, Face-Centered Cubic and Face-Centered Cubic with struts along Z-direction. To study the effect of cross-section geometry of the struts on the mechanical behavior of the structure, three circular, rectangular and I-shaped cross-sections with the same surface area were manufactured and tested. Their simulations were done using Abaqus software and the simulation results were in great agreement with the test results. Numerical and experimental investigations showed that samples with the I-shaped cross-section of the strut exhibit more stiffness and strength

Analysis and optimization of strut-based lattice structures by simplified finite element method

In this paper, the mechanical behavior of the lattice structures composed of unit cells such as BCC, BCCZ, FCC, FCCZ, and cubic arrangements with the struts in the forms of circle, rectangle, square, triangle, I-shape, and hollow-square longitudinal sections is examined. For this purpose, the elastic behavior of a lattice cubic with dimensions of 2 (cm) 7 2 (cm) 7 2 (cm) consisting of 125 unit cells and more than 1000 beam elements is investigated under the compressive loading using the finite element method. In this analysis, the mechanical properties such as stiffness, absorbed mechanical energy, and stiffness-to-weight ratio are determined for these cellular structures, and the orientations of their struts are optimized so that the structure\u27s stiffness–weight ratio is increased. It was observed that the cellular structures with an I-shaped cross-section have the highest stiffness-to-weight ratio among the studied cross-sections