‘…discoveries have immediate impact on laser additive manufacturing of metals on both basic and applied research fronts.’
Researchers from the University of Virginia, US have discovered why structural defects occur during additive manufacturing of parts made from high-strength, light-weight titanium alloy, widely used in aerospace applications.
Publishing their findings in the journal Science the researchers, led by Tao Sun, Associate Professor of Materials Science and Engineering at the University of Virginia, focused on the two most important conditions of the additive manufacturing process, these being laser power and scan speed.
Capturing how these two conditions are set and interact in a power velocity process map, the researchers divided the map into a ‘good zone’ and three ‘bad zones.’ If the manufacturing process remained in the ‘good zone’ the build will ‘likely yield a high-quality part on a consistent basis.’
Two of the ‘bad zones’ could easily be identified, one being represented by a lack of fusion, seen by un-melted powder, and the second zone recognised by balling, when a single printed line rolls back on itself.
Focusing on the fourth ‘bad zone’ the team said this arises due to porosity. The tiny holes appear inside in the material making it hard to see and control. ‘You could print multiple test lines, and you still would not know by examining the part surface if porosity is occurring underneath, Professor Sun said.
What the team found was that the laser-metal interaction generates acoustic waves which can interact with a gas bubble in different ways, leading to the formation of pores. ‘Short-pulse laser waves were believed to be a source for generating acoustic waves in liquid, but we observed acoustic effects while using continuous-wave lasers. Apparently, there are still many intriguing problems that demand more research,’ Professor Sun said.
Currently additive manufacturing, widely used in aircraft manufacture, is limited to the fabrication of ductwork, interior components and other non-critical parts. Additive manufacturing of safety-regulated parts will help the industry achieve its aspirations for efficient and stable supply chain management, as well as fuel savings and emissions reductions that accompany a light aircraft.
The team say that their discoveries have immediate impact on laser additive manufacturing of metals on both basic and applied research fronts. In addition, the new observations afforded by synchrotron x-ray imaging, open up multidisciplinary research areas that will attract more scientists to carry out fundamental studies on laser additive manufacturing.