Excellent characteristics of Nitinol shape memory alloys (SMAs) makes them favourable for use in industrial applications. Precision machining of such advanced alloys becomes a key requirement for industrial applications. Conventional machining processes imposes many difficulties for nitinol SMAs. Electrical discharge machining (EDM) process is appropriate for fabricating intricate and complex profile geometries and also provides a better alternative for difficult-to-cut materials. Addition of nano-particles in an appropriate amount in the dielectric fluid improves the machining by producing good dimensional accuracy, higher productivity, and good surface finish for machining of newly developed advanced alloys. The current study investigated the performance of powder-mixed EDM of nitinol SMA with the considerations of design variables of current, pulse-on-time (T on ), nano-graphene powder concentration (PC), and pulse-off-time (T off ) on surface roughness, dimensional deviation (DD), and material removal rate (MRR). Taguchi's L9 (3 ˆ 4) design was employed to perform the experiments and Minitab 17 software was used for statistical analysis of design variables using ANOVA, residual plots, and main effect plots. ANOVA results depicted that PC, T on , and T off were identified to be the highest contributing parameters with 75.18%, 29.37%, and 45.72% to affect MRR, SR, and DD, respectively. Obtained results has depicted a preferred combined positive trend of increase in MRR with a simultaneous drop in SR and DD after the addition of nano-graphene PC. HST algorithm was used to optimize single and multiple responses. Validation trials were also conducted to reveal the ability and suitability of the HTS technique. Field emission scanning electron microscopy revealed the minor occurrence of resolidified debris particles, globules, micro-pores, and micro-cracks after the addition of nano-graphene PC at 2 g/L. 

Machining parameter optimization and experimental investigations of nano-graphene mixed electrical discharge machining of Nitinol Shape memory alloy

Additive manufactured magnesium (Mg) alloys have widespread applications in the medical industry as orthopedic implants and biomedical stents, and in the transportation/automotive industry due to their status as the lightest structural metallic alloys. The printability of Mg alloys is challenging due to the high oxidization rate, rapid evaporation, and susceptibility to gas trapping. Accordingly, there might be many defects in the as-built parts such as cracking and delamination, porosity and lack of fusion, residual/thermal stresses and distortion, inhomogeneous/columnar microstructure, anisotropy in mechanical/physical properties, inclusions, and non-equilibrium phases formed during solidification. The removal of these defects has a great practical significance, where the post-processing heat treatments without altering the geometry of the parts are often sought in this regard. Accordingly, the present overview article focuses on and critically discusses the recent progress/advances in the application of hot isostatic pressing (HIP), solution annealing (solutionizing), and aging/precipitation heat treatment for the modification/homogenization of the microstructure and improvement of mechanical/functional properties of Mg alloys for the first time. The main fabrication methods are selective laser melting (SLM) from the category of powder bed fusion (PBF) processes, as well as wire and arc additive manufacturing (WAAM) from the category of directed energy deposition (DED) processes. Moreover, the adaptation of the friction stir processing (FSP) technology into additive manufacturing for grain refinement via dynamic recrystallization (DRX) and defect/pore closure (due to the elevated-temperature thermomechanical processing effects), combined severe plastic deformation (SPD) and thermal post-processing, hybridization of additive manufacturing for Mg alloys, and future prospects have been summarized. 

Post-processing heat treatment of lightweight magnesium alloys fabricated by additive manufacturing: A review

Pure aluminium poses inferior properties that limit its use in load-bearing applications. Reinforcing multiwall carbon nano-tube (solid lubricant) and graphene to aluminium matrix offers better (antifriction, hardness, and wear resistance) properties in composites for such applications. A stir casting processing route is employed to prepare the hybrid composite (aluminium-multiwall carbon nanotube-graphene Al-MWCNT-GR). The Taguchi L16 experimental matrix representing four variables (percent reinforcement of graphene, die temperature, melt temperature, and stir speed) operating at four levels were studied to analyze and obtain higher hardness and low wear rate in hybrid composites. Percent reinforcement of graphene showed maximum impact, and die temperature resulted with the least contribution towards both the responses. Criteria importance through intercriteria correlation (CRITIC) method is applied to determine the weight fractions (importance) for hardness and wear rate equal to 0.4752 and 0.5482, respectively. Grey relational analysis (GRA) and multi-objective optimization by the ratio analysis (MOORA) method converts multiple objective functions into a single objective function with weight fractions assigned to each output. Taguchi-CRITIC-MOORA outperformed the Taguchi-CRITIC-GRA method, which could result in 31.77% increase in hardness and a 36.33% decrease in wear rate compared to initial conditions. The optimal conditions ensure a dense microstructure with minimal pores, result in enhanced properties compared to that obtained for initial and average stir casting conditions. The worn-out surface results in a few thin and slender grooves between tracks with less crack propagation, ensuring self-lubrication in composites fabricated with the optimized condition. The better properties resulted in the hybrid composites correspond to optimized stir casting conditions and can be implemented in industries for large-scale applications.

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Stir Casting Process Analysis and Optimization for Better Properties in Al-MWCNT-GR-Based Hybrid Composites

Titanium is widely used in biomedical components. As a promising advanced manufacturing process, electropolishing (EP) has advantages in polishing the machined surfaces of material that is hard and difficult to cut. This paper presents the fabrication of a titanium microchannel using the EP process. The Taguchi method was adopted to determine the optimal process parameters by which to obtain high surface quality using an L9 orthogonal array. The Pareto analysis of variance was utilized to analyze the three machining process parameters: applied voltage, concentration of ethanol in an electrolyte solution, and machining gap. In vitro experiments were conducted to investigate the fouling effect of blood on the microchannel. The result shows that an applied voltage of 20 V, an ethanol concentration of 20 vol.%, and a machining gap of 10 mm are the optimum machining parameters by which to enhance the surface quality of a titanium microchannel. Under the optimized machining parameters, the surface quality improved from 1.46 to 0.22 μm. Moreover, the adhesion of blood on the surface during the fouling experiment was significantly decreased, thus confirming the effectiveness of the proposed method.

Electropolishing Parametric Optimization of Surface Quality for the Fabrication of a Titanium Microchannel Using the Taguchi Method

The microstructure evolution and precipitation behavior of TiB2/Inconel 718 composites manufactured by selective laser melting

In the current study, near net-shaped selective laser melting (SLM) technology was employed to build nickel-based superalloy Inconel 625 (IN625) parts with good quality. Taguchi method was employed to formulate a systematical study, analyze, and optimize the influencing factors, i.e., laser power (LP), scan speed (SS) and hatch distance (HD) on the resulting micro-hardness (MH) and surface roughness (SR) of the build samples. Scanning electron microscope (SEM) and X-ray diffraction analysis were carried out to characterize the powder morphology (spherical shaped particle possessing the size of 35 Â± 6 I¼m) and the surface of the build samples. Laser power was the most contributing factor on the analyzed parameters (MH and SR), followed by the scanning speed and hatch distance. Taguchi determined optimal condition (MH: LP = 270 W, SS = 800 mm/s, HD = 0.08 mm; SR: LP = 270 W, SS = 800 mm/s, HD = 0.08 mm) which resulted in higher microhardness of 416 HV and lower surface roughness of 2.82 I¼m. Higher MH was attributed to the minimal porosity, while the uniform smooth surface of the build samples resulted in low SR as evident from the SEM images and surface texture analysis. Super ranking concept (SRC) was used to optimize the MH and SR simultaneously, by determining a single optimal condition (LP = 300 W, SS = 600 mm/s, HD = 0.10 mm). The obtained optimal condition resulted in a MH of 382 HV, and a SR of 3.92 I¼m. The results of optimal conditions are validated subjected to SEM morphologies. Â© 2021 The Authors

Experimental investigation of selective laser melting parameters for higher surface quality and microhardness properties: Taguchi and super ranking concept approaches

Rohith Sheshadri

Papers

Experimental investigation of selective laser melting parameters for higher surface quality and microhardness properties: Taguchi and super ranking concept approaches