Electrical discharge machining (EDM) is a modern technology that is widely used in the production of difficult to cut conductive materials. The basic problem of EDM is the stochastic nature of electrical discharges. The optimal selection of machining parameters to achieve micron surface roughness and the recast layer with the maximal possible value of the material removal rate (MRR) is quite challenging. In this paper, we performed an analytical and experimental investigation of the influence of the EDM parameters: Surface integrity and MRR. Response surface methodology (RSM) was used to build empirical models on the influence of the discharge current I, pulse time ton, and the time interval toff, on the surface roughness (Sa), the thickness of the white layer (WL), and the MRR, during the machining of tool steel 55NiCrMoV7. The surface and subsurface integrity were evaluated using an optical microscope and a scanning profilometer. Analysis of variance (ANOVA) was used to establish the statistical significance parameters. The calculated contribution indicated that the discharge current had the most influence (over the 50%) on the Sa, WL, and MRR, followed by the discharge time. The multi-response optimization was carried out using the desirability function for the three cases of EDM: Finishing, semi-finishing, and roughing. The confirmation test showed that maximal errors between the predicted and the obtained values did not exceed 6%.

Multi-Response Optimization of Electrical Discharge Machining Using the Desirability Function

Additive manufacturing, including 3D printing (3DP), is one of the critical pillars of Industry 4.0 and the next construction revolution. Several countries, including China, have utilized 3DP on larger scales or real projects. However, reviews of the lessons learned from previous large-sized practices of 3DP utilization are scarce. This paper presents a few practical applications of implementing 3DP over the past decade and suggests a direction for future research work. Recent publications on 3DP practices are systematically reviewed using an interpretivist philosophical lens, and more specifically, the nozzle characteristics are focused upon. The Scopus and China National Knowledge Infrastructure (CNKI) journal databases are utilized, resulting in the examination of 54 English and 62 Chinese papers. The selected practices from Mainland China, Hong Kong, Taiwan and Macao are considered for this review. A content critical review approach is adopted, and the identified papers are critically reviewed. These papers reported key challenges and advantages from their reported practices, such as limitations in aggregate sizes, nozzle sizes, standards, post-occupancy satisfaction, final product quality, productivity challenges and other associated risks. The paper reports upon prominent limitations and signposts directions for future investigations.

Additive Manufacturing Applications for Industry 4.0: A Systematic Critical Review

The tensile flow properties of austenitic (S316-L) and martensitic (S410-L) stainless steel wall structures deposited by powder-fed laser additive manufacturing (LAM) process are evaluated. The properties obtained by the LAM process are compared to commercial rolled sheets of these metals. Strain-rate sensitivity, work hardening, and fracture behavior are assessed by conducting uniaxial tensile testing at different strain rates (0.001, 0.01, 0.1, and 1.0 sec−1). Moreover, a correlation between the final microstructure and mechanical properties is established for the LAM products through detailed analyses of grain structures and hardness indentation measurements. The results indicate a strong dependency for the strain rate in martensitic alloys compared to austenitic alloys produced by the LAM process. Interestingly, the tensile strength of commercial rolled martensitic stainless steel sheet doubles (∼100% increase) by increasing the strain rate, while preserving its elongation to failure. Comparing the two manufacturing methods, a lower strain-rate sensitivity factor is recorded for the additive manufacturing material (m of ∼0.0336) compared to the commercial sheet (m of ∼0.0775). This lower sensitivity is attributed to coarser grain structure and greater microstructural heterogeneity of the LAM product, which stems from directional solidification and cooling phenomenon during the layer-upon-layer deposition process. In contrast, the work hardening exponent (n value) varies little (0.1834–0.2854) for the different materials and manufacturing methods. Fractographic studies reveal that the fracture mode varies from ductile rupture towards ductile-brittle with the formation of greater martensitic phases, which is in combination with the failure component changing from shear to tensile at high strain rates.

Microstructure, strain-rate sensitivity, work hardening, and fracture behavior of laser additive manufactured austenitic and martensitic stainless steel structures

Problems with the laser additive manufacturing of metal parts related to its low efficiency are known to hamper its development and application. The method of selective laser melting of metallic powders can be improved by the installation of an additional laser beam modulator. This allows one to control the power density distribution optically in the laser beam, which can influence the character of heat and mass transfer in a molten pool during processing. The modulator contributes alternative modes of laser beam: Gaussian, flat top (top hat), and donut (bagel). The study of its influence includes a mathematical description and theoretical characterization of the modes, high-speed video monitoring and optical diagnostics, characterization of processing and the physical phenomena of selective laser melting, geometric characterization of single tracks, optical microscopy, and a discussion of the obtained dependences of the main selective laser melting (SLM) parameters and the field of its optimization. The single tracks were produced using the advanced technique of porosity lowering. The parameters of the obtained samples are presented in the form of 3D graphs. The further outlook and advanced applications are discussed.

Alexander Laskin

Papers

On productivity of laser additive manufacturing