Investigation and management of residual stresses in selective laser melting of maraging steel

Mugwagwa, Lameck (2019-04)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Selective Laser Melting (SLM) is a leading metal additive manufacturing process that has gained a lot of traction since the turn of the new millennium. Despite many benefits associated with SLM, a major setback that continues to impede its wider application and uptake is the inherent phenomenon of residual stresses. Although post-processing methods such as heat treatment can significantly reduce the magnitude of generated residual stresses, these methods cannot reverse the cracking, delamination and warping distortions that occur during the process. This dissertation focuses on the investigation of residual stresses and explores effective ways through which these stresses can be managed in-situ. An experimental study was conducted to establish the influence of input parameters on residual stresses and their accompanying effect on residual stresses. First, a study of the distribution of residual stresses was carried out on parts of different thickness. Secondly, scanning strategies and process parameters were studied through a structured experimental programme. Specimens were manufactured from maraging steel 300 powder on an M2 LaserCUSING as well as an EOSINT M280 machine. Residual stresses were measured using the neutron and X-ray diffraction methods whilst a coordinate measurement machine was used to measure distortions that arose from these stresses. The results show that residual stresses increase as part thickness increases, and that these stresses are not uniform, even at the same depth of measurement. From the scanning strategy perspective, reducing the scan vector length lowered residual stress magnitudes, but increased porosity significantly. Whilst rescanning lowered tensile stresses and increased the magnitude of compressive stresses, it is also clear that maintaining the same laser parameters as the initial beam pass leads to overheating and a marginal rise in porosity. An improved scanning pattern, called the successive chessboard strategy, yielded up to 40 % reduction of residual stresses against the default island scanning strategy. The correlations between input parameters and process outcomes show that increasing laser power and scanning speed increases residual stresses and distortions for the range of parameters tested. On the other hand, increasing the layer thickness from 30 to 45 μm generally reduces residual stresses and distortions but promotes porosity. However, a satisfactory process parameter combination was found at 180 W and 600 mm/s for the 45 μm layer thickness. At this point, residual stresses and distortions were reduced by 31 % and 46 % respectively, relative to the 30 μm layer at the same laser power and scanning speed. As original contribution, a method for evaluating and selecting residual stress management techniques was developed. Furthermore, new scanning sequences were developed, with the successive chessboard contributing to reduction of residual stresses and distortions. A process window was also devised for SLM of maraging steel 300. The process window demonstrates the porosity and residual stress state of final parts at different combinations of laser power and scanning speed. Finally, correlations were formulated between input parameters and the responses. This was extended to analysing the interdependencies between process outcomes, for example, residual stresses vs distortions and porosity vs distortions.

AFRIKAANSE OPSOMMING: Selektiewe laserstraalsmelting (SLM) is ‘n prominente byvoegingsverdaardigingsproses wat veel momentum begin gekry het sedert die nuwe millennium. Ten spyte van die voordele geassosieër met SLM, is ‘n groot hindernis wat wyer topassing terug hou die inherente fenomeen van oorblywende spanning. Alhoewel na-bearbeidingsmetodes soos hittebehandeling gegenereerde oorblywende spanning noemenswaardig kan verminder, kan die metodes nie krake, delaminasie, en skeeftrekking wat tydens die proses voorkom omkeer nie. Hierdie proefskrif fokus op die ondersoek van oorblywende spanning en verken effektiewe maniere hoe hierdie spanning in-situ bestuur kan word. ‘n eksperimentele studie was uitgevoer om die invloed van inset parameters op oorblywende spanning en hul gepaardgaande effekte van oorblywende spanning vas te stel. Eerstens, ‘n studie van die verdeling van oorblywende spanning was uitgevoer op parte van verskillende dikte. Tweedens, skandeer strategieë en proses parameters was bestudeer deur ‘n gestruktureerde ekperimentele program. Monsters was vervaardig van martensietverouderde staal 300 poeier op ‘n M2 LaserCUSING sowel as ‘n EOSINT M280 masjien. Oorblywende spanning was gemeet met neutron en x-straal diffraksie metodes terwyl ‘n koördinaatmetingsmasjien gebruik was om die afwykings te meet wat onststaan het as gevolg van hierdie spanning. Die resultate toon dat oorblywende spanning toeneem soos part dikte toeneem en dat hierdie spanning nie uniform is nie, selfs by dieselfde metingsdiepte. Van ‘n skanderingsstrategie perspektief, vermindering van die skandeervektor lengte het die oorblywende spanninggrootte verlaag, maar die porositeit beduidend laat toeneem. Terwyl herskandering trekspanning verlaag en die grootte van drukspanning laat toeneem het, is dit ook duidelik dat deur dieselfde laserparameters as die oorsponklike straal pad te handhaaf tot oorverhitting en ‘n marginale toename in porisiteit lei. ‘n Verbeterde skanderingspatroon, genoem die skaakbordstrategie, het tot ‘n 40 % verlaging van orblywende spanning in vergelyking met die standaard eiland skandeer strategie gelei. Die korrelasies tussen inset parameters en proses uitkoms toon dat verhoging van lasersterkte en skandeerspoed oorblywende spanning en afwykings verhoog vir die getoetsde gebied van parameters. Aan die ander kant, verhoging van die laagdikte van 30 na 45 μm verlaag normaalweg die oorblywende spanning en afwykings maar bevorder porositeit. Nietemin, ‘n aanvarbare prosesparameterkombinasie was gevind by 180 W en 600 mm/s vir ‘n laagdikte van 45 μm. By dié punt was die oorblywende spanning en afwykings respektiewelik verminder met 31 % en 46 % relatief tot die van ‘n 30 μm laagdikte by dieselfde lasersterkte en skandeerspoed. As oorspronklike bydrae, ‘n metode vir die evaluering en seleksie van oorblywendespanningbestuurstegnieke was ontwikkel. Verder, nuwe skandeervolgordes was ontwikkel, met die opvolgende skaakbord wat bygedra het tot die verlaging van oorblywende spanning en afwykings. ‘n Prosesvenster was ook versin vir SLM van martensietverouderde staal 300. Die prosesvenster demonstreer die porositeit en oorblywende spanning toestand van finale parte by verskillende kombinasies van lasersterkte en skandeerspoed. Laastens, korrelasies was geformuleer tussen inset parameters en die reaksies. Dit was uitgebrei na die analise van onderlinge afhanklikhede tussen proses uitkomste, byvoorbeeld, oorblywende spanning teenoor afwykings en porositeit teenoor afwykings.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105857
This item appears in the following collections: