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Tool Wear in Turning of Titanium Alloy Ti–6Al–4V Challenges and Potential Solutions for Crater Wear, Diffusion and Chip Formation

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Tool Wear in Turning of Titanium Alloy Ti–6Al–4V Challenges and Potential Solutions for Crater Wear, Diffusion and Chip Formation Examensarbete 30 hp Oktober 2016 Tool wear in turning of titanium alloy Ti–6Al–4V Challenges and potential solutions for crater wear, diffusion and chip formation Erik Bamford Abstract Tool wear in turning of titanium alloy Ti–6Al–4V Erik Bamford Teknisk- naturvetenskaplig fakultet UTH-enheten Titanium alloys are major materials used in the airplane industry, and prospects show that airplane production will double in the next 20 years. Consequently, the demand Besöksadress: for cutting tools for machining of titanium alloys will increase. The primary problem Ångströmlaboratoriet Lägerhyddsvägen 1 when machining titanium alloys is their low thermal conductivity. Crater wear is the Hus 4, Plan 0 main factor limiting tool life, and is generally caused by thermal diffusion due to high temperatures in the tool-chip interface. Postadress: This master’s thesis was performed in collaboration with Sandvik Coromant, with the Box 536 751 21 Uppsala prospect to increase knowledge of how diffusion and chip formation influences crater wear progression. The aim was to study tool wear of cutting tools when turning Telefon: Ti–6Al–4V. This was done by testing two different rake face geometries, both coated 018 – 471 30 03 and uncoated, at cutting speeds of 30–115 m/min. Diffusion was investigated to learn Telefax: about the impact it has on crater wear. Chips were examined to investigate chip 018 – 471 30 00 formation and shear strain. The coated modified rake face insert showed less crater wear only for the initial few Hemsida: seconds of machining. Uncoated inserts with a modified rake face showed higher http://www.teknat.uu.se/student diffusion rate and faster crater wear progression than did standard inserts. The standard inserts showed twice as long tool life as did the modified inserts. No significant differences in the chip formation mechanism were found between modified and standard inserts. Cracks were found within shear bands that were thinner than usual, which suggest that the generation of cracks allows less shear deformation. Handledare: Roland Bejjani Ämnesgranskare: Staffan Jacobson Examinator: Erik Lewin ISSN: 1650-8297, UPTEC K 16021 “Man is a tool-using animal. Without tools he is nothing, with tools he is all.” –Thomas Carlyle Page | III Page | IV POPULÄRVETENSKAPLIG SAMMANFATTNING Verktygsslitage vid svarvning av titanlegeringen Ti–6Al–4V Utmaningar och möjliga lösningar för gropförslitning, diffusion och spånbildning Titanlegeringar används i stor utsträckning i flygplansindustrin, och prognoser tyder på att flygplansproduktionen kommer att fördubblas under de kommande 20 åren. Följaktligen kommer efterfrågan på skärande verktyg för bearbetning av titan att öka. Några gemensamma fördelar med titanlegeringar är att de har låg densitet, hög styrka och är korrosionsbeständiga. Ett stort problem är däremot att de har en mycket låg värmeledningsförmåga, vilket gör att värmen koncentreras i skäreggen och leder till att skäreggen snabbt slits ut. För att kunna forma ett arbetsmaterial måste skäret som bearbetar det vara hårdare. Vid svarvning av titanlegeringar används ofta skär utan beläggning tillverkade av en metall- och kerambaserad komposit av volframkarbid och kobolt. I andra skärprocesser använder man vanligtvis skärverktyg med skyddande ytskikt för att ytterligare reducera nötningshastigheten av det bearbetande skäret. Det finns fortfarande stora problem med att applicera tunna ytskikt på skär för svarvning i titanlegeringar eftersom beläggningen snabbt slits bort. En vanlig utslitningstyp vid svarvbearbetning av titanlegeringar är gropförslitning, vilket är den faktor som huvudsakligen påverkar verktygets livslängd vid konventionella skärhastigheter inom industrin. Gropförslitning orsakas av diffusion, det vill säga rörelse av atomer mellan det bearbetade materialet och skärverktyget, vilket orsakas av höga temperaturer i skäreggen. Detta leder till att bindningar mellan atomer försvagas och att skärverktyget nöts ut. Det är således viktigt att undersöka denna nötningsmekanism för att kunna utveckla nya skärande verktyg med längre livslängd. Detta examensarbete utfördes i samarbete med Sandvik Coromant, ett världsledande företag som är specialiserade på skärverktyg för bearbetning i metaller. Syftet var att öka kunskapen om hur kemisk diffusion och spånbildning påverkar gropförslitning. Svarvtester genomfördes på ett cylinderformat ämne av titanlegeringen Ti–6Al–4V med skär Page | V med och utan ytbeläggning vid skärhastigheter mellan 30–115 m/min. Eggen på de undersökta skären hade en modifierad spånsida, och jämfördes mot standardskär med en plan spånsida. Gropförslitningen undersöktes med mätinstrumentet Alicona för att få en tydlig bild av skärytans topografi, vilket visade hur ytan påverkats efter svarvning vid olika tidpunkter och skärhastigheter. Titan fastnar i gropen på skäret och diffusionen leder till att bitar avlägsnas från skärverktyget, vilket bidrar till förslitningsprocessen. Detta studerades genom att titta på tvärsnitt av skären med svepelektronmikroskop. Diffusionen av titan och kol studerades med elektronmikrosond för att mäta halten av titan och kol vid gränssnittet av titanskiktet och skärverktyget, samt för att undersöka skärets nötningshastighet. Under bearbetningsprocessen bildas svinn i form av spånor. Tvärsnitt av spånor kan studeras för att få en inblick i hur spånbildningsprocessen påverkar gropförslitningen. Titanspånor har en segmenterad geometri, det vill säga att de är sågtandade. Interna spänningar under bearbetningsprocessen bidrar till att materialet skjuvas och kan avslöja hur materialet påverkas av deformationen under bearbetningen. Skjuvning är ett mått på hur mycket materialet deformeras utan volymförändring och uppkommer genom att två motsatta spänningar påfrestar materialet, vilket leder till att materialet töjs ut. Specifika dimensioner och vinklar i titanspånorna uppmättes med hjälp av ljusoptiskt mikroskop och graden av segmentering, segmentfrekvensen och skjuvningen i spånorna beräknades. Skjuvningen kan avbildas med ljusoptiskt- och svepelektronmikroskop och visar sig som smala skjuvningsband mellan segmenten. Energiröntgenspektroskop användes för att studera diffusion av partiklar från skärverktyg till titanspånor. Tillämpning av beläggning på spånsidan av skärverktyget bidrog till en minskad kemisk diffusion, men beläggningen kan orsaka andra förslitningsmekanismer som påskyndar gropförslitning. Belagda skärverktyg med modifierad spånsida visade på en mindre gropförslitning de första sekunderna av bearbetning, vilket troligtvis beror på att beläggningen var kvar på en större yta än för standardskär. Skär utan beläggning med en modifierad spånsida visade högre diffusionshastighet och snabbare gropförslitningshastighet än standardskär. Mätning av fasförslitning visade att standardskär hade dubbelt så lång livslängd som modifierade skär. Förändringar i spånbildningen kan påverka gropförslitningen och bestäms genom att undersöka spångeometrin. Dock kunde inga signifikanta skillnader bekräftas mellan modifierade och standardskär. Page | VI TABLE OF CONTENTS ABSTRACT ............................................................................................................................................................... I POPULÄRVETENSKAPLIG SAMMANFATTNING........................................................................................................ V ACKNOWLEDGEMENTS ......................................................................................................................................... XI LIST OF ABBREVIATIONS ....................................................................................................................................... XII LIST OF NOMENCLATURE..................................................................................................................................... XIII LIST OF FIGURES .................................................................................................................................................. XIV IN CHAPTER 1: INTRODUCTION .................................................................................................................. XIV IN CHAPTER 2: THEORY .............................................................................................................................. XIV IN CHAPTER 3: LITERATURE REVEIW ........................................................................................................... XIV IN CHAPTER 4: METHODOLOGY................................................................................................................... XV IN CHAPTER 5: RESULTS AND DISCUSSIONS ................................................................................................. XV IN APPENDIX B ........................................................................................................................................... XVI IN APPENDIX C ........................................................................................................................................... XVI IN APPENDIX D .......................................................................................................................................... XVI IN APPENDIX F .......................................................................................................................................... XVII IN APPENDIX G ........................................................................................................................................
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