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Build failures due to residual stress
How residual stress can cause major build failures, and what you can do to prevent it Anyone who starts out on their own evaluation of the opportunities presented by metal Additive Manufacturing will soon stumble across the phrase ‘build failure’. Whilst there can be many causes for a failed build, which of course comes at a significant cost in terms of both material wastage and machine time, residual stress is often at the top of the list. As Olaf Diegel and Terry Wohlers explain, residual stress can be both anticipated and managed though a combination of basic design rules, process settings and post-build heat treatment.
Residual stress is one of the most common causes of catastrophic build failures in metal Additive Manufacturing. A recent LinkedIn post by Alexander Liu, a wellknown metal AM trouble-shooter from Temasek Polytechnic in Singapore, demonstrated this problem with the failed build of a topology-optimised bracket. The parts kept detaching from the build plate or ripping away from the support material. It was caused by considerable residual stress within the parts.
Residual stresses remain in a solid material after the original causes of the stresses have been removed. In itself, residual stress is not always a bad thing, and can sometimes be desirable. Shot peening, for example, imparts beneficial compressive residual stresses into metal parts. Residual stress is also used in toughened glass to allow for the
large, thin, scratch-resistant glass used in smartphones. Unintentional residual stress in a designed structure, however, can cause it to fail prematurely and becomes problematic. Many factors can cause residual stress in a part, but the chief one is controllable through design. Residual stress is a result of temperature
What is residual stress? Similarly to welding, metal Additive Manufacturing induces a substantial amount of stress on parts. This is one of the principal reasons why support material is often needed. This residual stress, as well as stress concentrations, must be relieved through heattreatment after the build and before parts are removed from the build plate.
Vol. 4 No. 4 Š 2018 Inovar Communications Ltd
Fig. 1 Examples of parts detaching from the build due to residual stress (Courtesy Alexander Liu)
Metal Additive Manufacturing | Winter 2018
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