Khamis Essa

Process optimisation of selective laser melting using energy density model for nickel based superalloys

The main challenge associated with the application of selective laser melting (SLM) to Ni based superalloys is the performance of process optimisation to maximise the mechanical properties. The energy density parameter has typically been used as a semiquantitative approach to identify the energy threshold beyond which the material achieves virtually full consolidation. Nonetheless, some Ni superalloys are susceptible to crack formation during SLM, which cannot be avoided via process optimisation. In the present report, a comparative study is presented showing the utility of the energy density parameter in process optimisation for γ′ and γ′/γ″ strengthened Ni based superalloys. For both classes, it was found that the build density increases [i.e. void area (%) decreases] with the increase in the energy density. Nonetheless, no direct correlation can be found between the energy density parameter and the cracking density.

Optimisation of selective laser melting for a high temperature Ni-superalloy

Purpose – The purpose of this paper is to optimise the selective laser melting (SLM) process parameters for CMSX486 to produce a “void free” (fully consolidated) material, whilst reducing the cracking density to a minimum providing the best possible fabricated material for further post-processing. SLM of high temperature nickel base superalloys has had limited success due to the susceptibly of the material to solidification and reheat cracking. Design/methodology/approach – Samples of CMSX486 were fabricated by SLM. Statistical design of experiments (DOE) using the response surface method was used to generate an experimental design and investigate the influence of the key process parameters (laser power, scan speed, scan spacing and island size). A stereological technique was used to quantify the internal defects within the material, providing two measured responses: cracking density and void per cent. Findings – The analysis of variance (ANOVA) was used to determine the most significant process parameter...

Net-Shape Manufacturing using Hybrid Selective Laser Melting/Hot Isostatic Pressing

Purpose The purpose of this study is to develop a manufacturing technology using hybrid selective laser melting/hot isostatic pressing (SLM/HIP) process to produce full density net-shape components more rapidly and at lower cost than processing by SLM alone. Design/methodology/approach Ti-6Al-4V powder was encapsulated in situ by the production of as-SLMed shell prior to the HIP process. After HIPping, the SLM shell is an integral part of the final component. Finite element (FE) modelling based on pure plasticity theory of porous metal coupled with an iterative procedure has been adopted to simulate HIPping of the encapsulated Ti-6Al-4V powder and SLMed shell. Two demonstrator parts have been modelled, designed, produced and experimentally validated. Geometrical analysis and microstructural characterisation have been carried out to demonstrate the efficiency of the process. Findings The FE model is in agreement with the measured data obtained and confirms that the design of the shell affects the resulting deformed parts. In addition, the scanning electron microscope (SEM) and Electron backscatter diffraction EBSD (EBSD) of the interior and exterior parts reveal a considerably different grain structure and crystallographic orientation with a good bonding between the SLMed shell and HIPped powder. Originality/value An approach to improve SLM productivity by combining it with HIP is developed to further innovate the advanced manufacturing field. The possibility of the hybrid SLS/HIP supported by FEA simulation as a net shape manufacturing process for fabrication of high performance parts has been demonstrated.

Effect of casting practice on the reliability of Al cast alloys

Abstract: The properties of aluminium castings are strongly affected by their inclusion content, particularly entrained surface alumina films. These form due to the surface turbulence associated with mould filling, which causes the oxidised surface of a liquid metal to fold-over onto itself and be submerged into the bulk liquid with a thin layer of air entrapped within it. This is known as entrainment action. These flaws have been reported to increase the variability of the fracture strengths of Al alloy castings. This means that shape castings in light alloys can have inconsistent properties, which makes designing structures employing shape castings more difficult. Entrained surface layers can cause premature failure, but also have been associated with other defects, such as hydrogen porosity, shrinkage porosity, intermetallic compounds and hot tearing. Recent research has suggested that the air inside the defect would react with the surrounding melt leading to its consumption, which may enhance the mechanical properties of the casting. In this work, liquid aluminium was poured into three identical ceramic moulds which were immediately placed in a furnace to preserve the molten metal at 800 °C, for different periods of time prior to freezing. The Weibull moduli of the plate castings were determined under tensile conditions, and their fracture surfaces examined using SEM. Investigation of the fracture surfaces of the specimens detected many alumina layers at different locations. Many of which were found inside pores, reflecting the role of entrained defects in the formation of porosity. The results also suggested that opposite phenomena may take place during the holding treatment. The consumption of air inside the entrained defects due to reaction with the surrounding molten metal may lead to improvements in mechanical properties, but this may be accompanied by hydrogen passing into the defects, which has a deleterious effect on properties.

Prediction of springback in multi-point forming

Abstract Flexible forming techniques, such as multi-point forming (MPF), are employed in manufacturing to reduce the time and cost of production. MPF uses a set of height-adjustable pins to construct free-form three-dimensional surfaces. Springback is a common phenomenon in forming including MPF which, if not properly catered for, will lead to parts that are out of specification. This paper introduces a detailed numerical approach for predicting springback in MPF. FE models were developed to simulate MPF of doubly curved panels in Aluminium alloy 5251-O. The Response Surface Method and the analysis of variance technique were employed to identify the most significant process parameters and to determine their optimal setting. The influence of these parameters on thickness variations across the formed panel and the subsequent effect of those variations on the amount of springback were investigated. It was found that the radius of curvature had the most significant effect on springback and thickness variation. Minimum springback can be achieved by introducing high strains through sheet stretching.

Laser additive manufacturing of 3D meshes for optical applications

Selective laser melting (SLM) is a widely used additive manufacturing process that can be used for printing of intricate three dimensional (3D) metallic structures. Here we demonstrate the fabrication of titanium alloy Ti–6Al–4V alloy based 3D meshes with nodally-connected diamond like unit cells, with lattice spacing varying from 400 to 1000 microns. A Concept Laser M2 system equipped with laser that has a wavelength of 1075 nm, a constant beam spot size of 50μm and maximum power of 400W was used to manufacture the 3D meshes. These meshes act as optical shutters / directional transmitters and display interesting optical properties. A detailed optical characterisation was carried out and it was found that these structures can be optimised to act as scalable rotational shutters with high efficiencies and as angle selective transmission screens for protection against unwanted and dangerous radiations. The efficiency of fabricated lattice structures can be increased by enlarging the meshing size.