With rising interest in metal additive manufacturing and, specifically, in powder bed fusion processes, it is essential to understand relevant process parameters and the behavior of powders used in such processes. At the time of writing, the flow behavior of powders is in the spotlight whereas little research into the spreadability of a powder is being conducted. While the two characteristics are related, powders that are being spread are subject to different force loads which are not present during the simple flow of a powder. This work develops a testing system capable of qualitatively and quantitatively assessing spreadability in metal powders. Seven gas atomized powders of varying size distributions and four different chemistries supplied by Uddeholms AB were used to examine the efficacy and accuracy of the system. Image analysis of the spread layers was found to be effective in measuring the areas of the powder layers. It was also possible to assess the quality of powder coverage in the layer in terms of defects, which were sensitive to process parameters such as layer thickness and rake speed. From measurements of the mass of powder in each layer, a layer density was calculated and shows greater sensitivity than powder coverage to changes in layer thickness parameters. The spreadability data collected were compared to relevant existing flowability metrics, including some derived from powder rheometry. Two metrics were created to assess how well the rheometry data can predict spreading behavior. Firstly, the change in area coverage as a function of rake speed correlated to an increase in basic flowability energy, both of which became less sensitive to rake speed at higher speeds. Finally, an equation was formulated to assess the gap that forms between the true height of a spread layer and the nominal layer thickness. This gap showed great sensitivity to the cohesion values attained from shear cell tests: highly cohesive powders produced larger spread layer gaps. This work is expected to contribute to moving toward a standardized method to attain a powder characteristic for spreadability.With rising interest in metal additive manufacturing and, specifically, in powder bed fusion processes, it is essential to understand relevant process parameters and the behavior of powders used in such processes. At the time of writing, the flow behavior of powders is in the spotlight whereas little research into the spreadability of a powder is being conducted. While the two characteristics are related, powders that are being spread are subject to different force loads which are not present during the simple flow of a powder. This work develops a testing system capable of qualitatively and quantitatively assessing spreadability in metal powders. Seven gas atomized powders of varying size distributions and four different chemistries supplied by Uddeholms AB were used to examine the efficacy and accuracy of the system. Image analysis of the spread layers was found to be effective in measuring the areas of the powder layers. It was also possible to assess the quality of powder coverage in the layer in terms of defects, which were sensitive to process parameters such as layer thickness and rake speed. From measurements of the mass of powder in each layer, a layer density was calculated and shows greater sensitivity than powder coverage to changes in layer thickness parameters. The spreadability data collected were compared to relevant existing flowability metrics, including some derived from powder rheometry. Two metrics were created to assess how well the rheometry data can predict spreading behavior. Firstly, the change in area coverage as a function of rake speed correlated to an increase in basic flowability energy, both of which became less sensitive to rake speed at higher speeds. Finally, an equation was formulated to assess the gap that forms between the true height of a spread layer and the nominal layer thickness. This gap showed great sensitivity to the cohesion values attained from shear cell tests: highly cohesive powders produced larger spread layer gaps. This work is expected to contribute to moving toward a standardized method to attain a powder characteristic for spreadability. / Med ett ökande intresse för additiv tillverkning av metaller i allmänhet, och pulverbäddsprocesser i synnerhet, är det viktigt att förstå relevanta processparametrar och beteendet av pulver som används i sådana processer. I skrivande stund är flytbarheten i fokus, medan väldigt lite forskning görs på spridbarheten. Dessa två egenskaper är relaterade, men pulver som sprids utsätts för andra krafter vilka inte återfinns i simpla flytbarhetstester. I detta arbete utvecklas ett testsystem som är kapabelt till att undersöka spridbarheten kvalitativt och kvantitativt. Sju gasatomiserade pulver som tillhandahålles av Uddeholms AB, med varierande storleksfördelningar och fyra olika sammansättningar användes för att undersöka effektiviteten och noggrannheten av systemet. Bildanalys av de utspridda pulverlagren visade sig vara effektivt för att mäta arean av lagren. Det var också möjligt att undersöka kvalitén på pulvertäckningen med avseende på defekter, som visade sig vara känsliga för processparametrar så som lagertjocklek och rake-hastigheten. Från mätningarna av pulvermassan från varje lager kunde en lagerdensitet räknas ut, och denna visar större känslighet med avseende på processparametrar än pulvertäckningen. Spridningsdatan jämfördes med relevanta flytbarhetsmätningar, inklusive reometrimätningar av pulver. Två mätetal användes för att undersöka hur väl reometri kan användas för att förutse spridbarheten. Först användes ändringen i täckning som en funktion av rake-hastigheten korrelerat till ökningen av grundläggande flytbarhetsenergi, där båda parametrarna blev mindre känsliga vid högre rake-hastigheter. Sedan formulerades en ekvation för att redogöra för glappet mellan den verkliga höjden av ett pulverlager och den nominella lagertjockleken. Detta glapp visade stor känslighet för koherensen som mättes med hjälp av skjuvcellstest: koherenta pulver gav större glapp i spridlagren. Detta arbete förväntas bidra till utvecklingen av en standardiserad metod att undersöka spridbarhet hos pulver.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:kth-301259 |
| Date | January 2021 |
| Creators | Rosala, Fabrice |
| Publisher | KTH, Materialvetenskap |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | TRITA-ITM-EX ; 2021:537 |
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