Upgrading additively manufactured parts using laser shock peening
Additive manufacturing (AM) has brought enormous benefits to mankind and has already shown its usefulness in a variety of cutting edge fields.
Metallic additive manufacturing, which fosters the development of smart technologies and production systems, is one of the essential pillars of Industry 4.0.
Additionally, AM is a vital facilitator of the circular economy, aiming to reduce use of resources while increasing commercial prospects. As a result, less material will be used, and physical inventory will be minimised.
Selective laser melting (SLM) is one of today’s most sophisticated AM processes. SLM uses a high-energy laser to fuse metallic powder and achieve layer-by-layer fabrication of complicated geometries. The complexity of part design can be beyond the average person’s comprehension, and producing such designs via traditional manufacturing or machining processes can be extremely difficult, if not impossible. Nevertheless, SLM’s advantages, such as less material usage, higher process precision and control, denser material creation and shorter manufacturing cycles, make it a viable and trustworthy AM technique. However, despite many benefits, AM also has limitations such as poor surface quality, reduced material strength, voids inherent in the AM process and excessive tensile residual stresses present in as-built parts. All these highly detrimental effects significantly influence the fatigue life of the AM component.
These effects necessitate the need for post-processing of AM parts, and laser shock peening (LSP) has emerged as a possible method to improve their fatigue life, surface roughness and surface integrity if performed within a controlled experimental environment. LSP increases the quality of AM parts and allows them to be used in even the most demanding applications.
How does LSP work?
LSP consists of four stages: (i) delivery of a laser beam with defined energy; (ii) plasma generation on the material’s surface under the confining medium (the confining medium is critical in preventing plasma from expanding away from the surface and generating strong, high-pressure shock waves); (iii) propagation of high-pressure waves through the materials; and (iv) plastic deformation and generation of compressive residual stresses inside the material. Figure 1 depicts a schematic of the LSP process.
Figure 1: Illustration of the laser shock peening mechanism
Improving AM parts using LSP
By applying LSP, it is possible to change the state of residual stresses in materials. LSP can therefore be used to transform the harmful tensile residual stresses found in AM parts into beneficial compressive residual stresses that work to improve part performance. Figure 2a illustrates the typical residual stress depth profile before and after applying LSP on AM-produced 316L stainless steel. While other surface enhancement technologies impart the compressive residual stresses up to a few hundred micrometres, LSP can impart residual stress several millimetres below the part surface. Such change in the residual stress state leads to a significant increase in the component’s fatigue life. Figure 2b presents the fatigue life improvement of Ti6Al4V where, for certain cases, it is possible to make improvements of even 130 times. In this graph, once the 10 million cycles have been reached, the testing is stopped.
Figure 2: a) Residual stress depth profile for AM SS 316M before and after LSP; b) Fatigue life results for AM Ti6Al4V before and after LSP
Why upgrade AM using LSP?
These two results are already a good sign that LSP is an excellent solution for improving AM parts for applications that require high-quality parts, either because of technical requirements or safety reasons. Furthermore, by applying LSP as a post-process of AM, the performance of the part can be dramatically improved in terms of fatigue life. Such performance improvement can allow AM to be implemented in highly sensitive applications and a more comprehensive range of industries, such as aerospace or biomedical. Moreover, LSP can be effectively used to shorten the manufacturing time in certain cases by allowing thicker printing layers. Also, proper application of LSP can decrease the cost of the feedstock by eliminating imperfections created in parts printed out of older or lower-quality powder. All the reasons mentioned make LSP an upcoming technology for additionally upgrading the quality of AM-produced parts, as well as for improving the commercial aspects of AM.
Delivering it to industry
HiLASE is equipped with a state-of-the-art LSP cell, which is effectively utilised for post-processing AM parts. Several different materials, including smart materials (NiTi), stainless steel, aluminium and its alloys, titanium alloys and others, have been successfully treated with LSP at HiLASE. The results have shown impressive improvements in the treated parts in terms of residual stresses and fatigue life.
Together with our partners, HiLASE can deliver the equipment required to start using LSP as a post-processing technique for AM. Furthermore, HiLASE is now offering consulting services to help integrate LSP technology into industrial production, as well as develop LSP processing for AM parts for particular applications.
On 9 March at 15:00 CET we will be hosting a webinar: ‘LSP and Metal Additive Manufacturing’, to share this topic further with industry. Our expert will give a more detailed explanation on LSP capabilities to improve performance of AM-produced parts and consequently how it can help AM technology be widely accepted across numerous industries. Register at: bit.ly/LSP_AM
Sanin Zulic is a junior researcher at HiLASE
Dr Sunil Pathak is a postdoctoral researcher at HiLASE