The use of unconventional machining (UCM) practices is increasing for product manufacture particularly when machining difficult to cut materials and when high precision is required. There is plenty of research conducted on improving sustainability of traditional machining. However, sustainability studies on unconventional/non-traditional machining practices are few. This review aims to determine the current state of the art in sustainability assessment of unconventional machining practices and identify gaps in research. An extensive review was carried out and analysed using a qualitative data analysing software. The analysis shows that only 25 publications directly and indirectly discuss the matter of sustainability of UCM. Out of this almost 70% of publications were recorded after year 2006 showing a clear evidence of uncovered research gap in the field with a growing interest. Despite this trend, evidence on studies which are explicitly dedicated to analyse the sustainability of UCM are rare

Plastic materials have been widely used to replace metals in functional parts due to their lower cost and comparable technical properties. However, the increasing use of virgin plastic material in consumer and industrial applications has placed a significant burden on waste management due to the volume of waste created and the potential negative effects of its end-of-life processing. There is a need to adopt circular economy strategies such as plastic recycling within industrial applications in order to reduce this significant waste management pressure. The present study used recycled polylactic acid (PLA) material as a feedstock for the 3D printing of a centrifugal semi-open pump impeller. The technical performance of 3D printed recycled PLA material and virgin PLA material was compared in this study. The environmental impacts for technically feasible impellers were assessed through the environmental life cycle assessment, while costs were evaluated by life cycle costing. The results were incorporated into a techno-eco-efficiency framework to compare the technical properties, environmental impacts, and costs. The social impacts of additive manufacturing and recycled feedstock material were also explored. The technical assessment results indicated that tensile strength, fatigue strength, density, and hardness decreased with recycled material content compared to virgin material. Microscopy of the fracture surfaces revealed the presence of slightly higher porosity and defects in recycled specimens, which could result in slightly lower technical properties. However, the recycled material was accepted for further ecological analysis as it offered higher pumping performance when compared to the original component and could reduce the burden on virgin material-based production and waste material disposal. Importantly, the results showed that 3D printed recycled PLA impellers are more eco-efficient when compared to 3D printed virgin PLA impellers.

The economic, environmental, and social impacts caused by the extensive resource consumption and harmful emissions from the metal manufacturing industry should be lowered through innovative sustainable manufacturing strategies. This study aims to investigate the techno-eco-efficiency performance of metal 3D-printed parts in comparison with CNC-machined parts to determine the technical, economic, and environmental performance as a decision support tool for selecting the most techno-eco-efficient manufacturing method. In this study, a novel metal extrusion 3D printing technology has been used to create a centrifugal semi-open pump impeller in 316L stainless steel material. The technical feasibility of the impellers has been determined by evaluating the geometry, build material, mechanics, morphology, and functional performance of the impellers. The eco-efficiency performance of technically feasible impellers was evaluated through environmental life cycle assessment, life cycle costing, and portfolio analysis. This eco-efficiency analysis helped ascertain the cost-competitiveness and environmentally friendliness of the 3D-printed impellers by comparing it with the conventional impellers. The findings reveal that the AM impeller is eco-efficient mainly due to lower normalised environmental impacts (54.6%) compared to the SM impeller. The functional parts made by metal extrusion 3D printing are technically feasible, cost-effective, and environmentally friendly compared to the SM counterparts.