On Determining Lost Core Viability in High-Pressure Die Casting Using Computational Continuum Mechanics
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KTH Industrial Engineering and Management On determining lost core viability in high-pressure die casting using Computational Continuum Mechanics Sebastian Kohlst¨adt Doctoral thesis Stockholm 2019 KTH Royal Institute of Technology School of Industrial Engineering and Management Department of Material Science and Engineering Division of Processes SE-100 44 Stockholm Sweden Akademisk avhandling som med tillst˚andav Kungliga Tekniska H¨ogskolan i Stockholm, framl¨agges f¨or offentlig granskning f¨or avl¨aggande av teknologie doktorsexamen, fredagen den 31 maj 2019, kl 10.00 i H¨orsalen F3, Lindstedtsv¨agen 26, Kungliga Tekniska H¨ogskolan, Stockholm. ISBN 978-91-7873-206-7 a a Sebastian Kohlst¨adt: On determining lost core viability in high-pressure die casting using Computational Continuum Me- chanics KTH School of Industrial Engineering and Management Department of Material Science and Engineering Division of Processes SE-100 44 Stockholm Sweden ISBN 978-91-7873-206-7 Copyright c Sebastian Kohlst¨adt, 2019 Print: Universitetsservice US-AB, Stockholm 2019 a a To my beloved family Abstract The subject of this thesis work is to investigate whether Computational Con- tinuum Mechanics (CCM) can serve as a valuable tool for the casting engineer to determine a priori whether a housing concept with inlying geometries that so far only exists in Computer Aided Design (CAD) will have the desired coo- ling performance and will be manufacturable with an acceptable number of rejects. As of spring 2019, no application in serial production of lost cores, i.e. cores that are destroyed during deforming, in high-pressure die casting is known. The reason for this is believed to be the absence of an engineering tool that can tell upfront whether a concept of casting and process combined will be viable. This thesis aims to fill precisely that void by presenting, imple- menting and testing a CCM model inside the OpenFOAM toolbox in order to determine upfront whether a design of a housing will be manufacturable with lost cores. The two-phase flow of air and melt is modeled with the volume- of-fluid-concept. Turbulence modeling is done via the Reynolds-Averaged- Navier-Stokes (RANS) approach, mostly using the Menter SST k-!-model. An isotropic linear elastic model was assumed for the solid mechanics. Industrial operators and managers like short and easy to grasp conclu- sions. As it, however, turned out during the process of this research project, there is no clear and easy answer to the question whether salt cores in high- pressure die casting are a viable concept and will lead to sound castings. First of all, it was proven that housings made with lost cores can improve the heat transfer capabilities of castings. It was possible to produce castings with cores up to an impact velocity of 30 ms−1. The impact velocity was found to be the most decisive parameter. But the reader should bear in mind that this limit is only valid for the given setup. Each configuration has to be tested with the introduced model separately. The slamming events at first impact of the melt were found to be not failure-critical if crack-free cores are used. It was also found that the approach of evaluating only the peak force does not go far enough. Effects later in the process may have a more important impact due to larger force-time integrals. Also, different from the original assumptions, the heat transferred from the melt to the core may not be neglected even though filling times are below 0.1 s. Defining gene- ral numerical constraints for conditions under which salt cores are a viable technology is very difficult as geometry alterations play an important role too. This underscores the power and usefulness of the presented model even I further as the engineer is now capable of testing each setup individually. It soon became clear that a fully comprehensive model is still for future researchers to develop. It was found that it is not beneficial to attach the shot sleeve to the casting model with currently available open-source CFD techno- logy. The presented strategy in this thesis together with the developed CCM tools can therefore provide a powerful tool for the casting or CAD-engineer to decide case by case whether a concept for a casting will be producible or not. The tools range from a limited CFD approach for evaluating only the forces to a fully coupled FSI methodology describing the core deformation over time. All models have been tested and validated with high-pressure die casting experiments and are in line with previously published findings with deviations of 5-10 % at maximum. Key words: high-pressure die casting, lost salt cores, computational con- tinuum mechanics, two-phase compressible flow, OpenFOAM, CFD, fluid- structure interaction, volume-of-fluid method II Sammanfattning Fokuset i avhandlingsarbetet ¨ar att unders¨oka om ber¨akningskontinuumme- kanik (CCM) kan anv¨andas som ett v¨ardefullt verktyg f¨or gjutingenj¨oren f¨or att best¨amma om en h¨oljeskoncept med samh¨orande geometrier, som hittills endast existerar i computer-aided design (CAD), kommer att ha den ¨onskade kylningsprestandan och kommer att kunna tillverkas med ett acceptabelt l˚agt antal kassationer. Fram tills nu, v˚aren2019, s˚af¨orekommer ingen k¨and seriell till¨ampning av s˚akallade f¨orlorade k¨arnor"i pressgjutning vid h¨ogt tryck. Anledningen till detta antas vara bristen av ett tekniskt verktyg som kan prediktera om en ett t¨ankt gjutgods kan tillverkas genom att anv¨anda en viss gjutningsprocess. Denna avhandling syftar till att hantera denna problematik genom att pre- sentera, implementera och testa en CCM-modell i OpenFOAM-verktygsl˚adan f¨or att i f¨orv¨ag best¨amma om en h¨oljesdesign kommer att kunna tillverkas med f¨orlorade k¨arnor. Ett tv˚a-fasfl¨ode best˚aendeav luft och sm¨alta ¨ar mo- dellerat med "Volume of Fluid"(VOF) metoden. Turbulensmodellering g¨ors med anv¨andande av Reynolds-Averaged-Navier-Stokes (RANS) -metoden, mestadels med hj¨alp av Menter SST k-!-modellen. En isotrop linj¨ar elastisk modell antogs f¨or att beskriva mekaniken. Industrioperat¨orer och chefer ¨onskar slutsatser som ¨ar korta och l¨atta att f¨orst˚a.Som det visade sig under detta forskningsprojekt finns det ing- et tydligt och enkelt svar p˚afr˚aganhuruvida anv¨andning av saltk¨arnor vid h¨ogtryckspressgjutning ¨ar ett genomf¨orbart koncept som leder till gedigna gjutgods. F¨orst och fr¨amst bevisades att h¨oljen gjorda med f¨orlorade k¨arnor (lost cores) kan f¨orb¨attra v¨arme¨overf¨oringsf¨orm˚aganhos gjutgods. Det visa- de sig m¨ojligt att producera gjutgods med k¨arnor upp till en fyllhastighet p˚a30 ms−1. Fyllhastigheten var den avg¨orande parametern. Men l¨asaren b¨or komma ih˚agatt denna gr¨ans endast g¨aller f¨or en given uppst¨allning. Varje konfiguration m˚astetestas separat med den inf¨orda modellen. Slamm- ningsf¨orloppet vid sm¨altans f¨orsta inverkan visade sig vara icke-kritiskt om sprickfria k¨arnor anv¨andes. Det har ocks˚avisat sig att metoden att utv¨ardera endast toppkraften inte g˚artillr¨ackligt l˚angt.Effekter senare i processen kan ha en viktigare effekt p˚agrund av st¨orre krafer summerade ¨over tiden. Till skillnad fr˚ande ursprungliga antagandena, f˚arden v¨armen som ¨overf¨ors fr˚an sm¨altan till k¨arnan inte f¨orsummas, trots att fyllningstiden ¨ar mindre ¨an 0.1 s. Att definiera allm¨anna numeriska begr¨ansningar f¨or f¨orh˚allandenun- der vilka saltk¨arnor ¨ar en genomf¨orbar teknik ¨ar mycket sv˚arteftersom geo- III metriska f¨or¨andringar ocks˚aspelar en viktig roll. Detta understryker styrkan och anv¨andbarheten hos den presenterade modellen ¨annu mer, eftersom in- genj¨oren nu kan testa varje uppst¨allning var f¨or sig. Det blev snart klart att utvecklingen av en komplett modell beh¨over ut- vecklas av framtida forskare att utveckla. Det visade sig att det inte ¨ar viktigt att inkludera skotthylsan p˚agjutningsmodellen med nuvarande tillg¨anglig CFD-teknik i ¨oppen k¨allkod. Den presenterade strategin i denna avhandling tillsammans med de utvecklade CCM-verktygen kan d¨arf¨or ge ett kraftfullt verktyg f¨or gjutning- eller CAD-ingenj¨orer f¨or att fr˚anfall till fall avg¨ora huruvida ett koncept f¨or gjutning kommer att vara producerbart eller ej. Verktygen str¨acker sig fr˚anett begr¨ansat CFD-tillv¨agag˚angss¨att f¨or att ut- v¨ardera endast krafterna till en helt kopplad FSI-metodik som beskriver k¨arn- deformationen ¨over tiden. Alla modeller har testats och validerats med press- gjutningsexperiment vid h¨oga tryck och resultaten st¨ammer ¨overens med ti- digare publicerade resultat med som mest avvikelser p˚a5-10%. Nyckelord: pressgjutning, f¨orlorade saltk¨arnor, ber¨akningskontinuum- mekanik, tv˚a-fasfl¨ode, OpenFOAM, CFD, fluid-struktur interaktion, volume- of-fluid metoden IV Acknowledgments As it is always the case in projects of the 21st century, the result is never the product of one single person alone. I would therefore like to take the opportunity to appreciate the contribution of several other stakeholders to the project. First and foremost to name are Dr. Michael Vynnycky and Prof. Dr. P¨arG. J¨onssonas my principal supervisors at KTH's department of Mate- rial Science and Engineering. Their support, guidance, encouragement and fruitful discussions greatly helped to deliver better research work in the end. This work would have been difficult to accomplish without their support. As this was an industrial research project in cooperation with the Volk- swagen AG's Kassel site, I am also indebted to the managers at the foundry there.