hat started as a curious new metal-casting technology inside a Tesla plant is on the verge of being a hot industry trend. Megacasting, which uses larger-than-normal factory presses to create enormous individual auto pieces, is now spreading to Volvo and may soon show up at Mercedes-Benz and the Chinese EV-maker Nio. The new approach allows a vehicle assembler to produce a single large aluminum piece, for example a vehicle’s entire rear structure. In the past, dozens or even more than 100 castings would have been required — all needing to be welded together. Volvo has estimated the new technology promises a 75 percent time savings compared with how large aluminum body parts are traditionally put together.