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Residual stresses in Ti-6Al-4V from low energy laser repair welding
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
2018 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesisAlternative title
Restspänningar i Ti-6Al-4V av lågenergetisk laserreparationssvetsning (Swedish)
Abstract [sv]

Millimeterstora och svårupptäckta defekter kan uppstå internt i stora och komplexa gjutgods av Ti-6Al-4V, ibland går dessa oupptäckta tills detaljen genomgått mekanisk bearbetning och en stor kostnad redan har gått in i den. Dessa defekter och andra industriella olyckshändelser leder till ett behov av additiva reparationsmetoder där den för tillfället rådande metoden är TIG-svetsning. Denna metod reparerar defekterna men leder till oacceptabla restspänningar vilka kan åtgärdas med värmebehandling som i sin tur kan orsaka ytdefekten alpha case. Därav finns ett industriellt behov av reparationsmetoder som leder till mindre eller negligerbara restspänningsnivåer i reparerad detalj.

Detta arbete utfört hos GKN Aerospace – Engine Products Sweden i Trollhättan analyserar eventuella förhållanden mellan parametrarna Effekt, Spot size, och Svetshastighet och de resulterande restspänningarna i ett lågparameterområde på materialet Ti-6Al-4V.

En parameterrymd uppspänd av 17 parameteruppsättningar etablerades, svetsades och analyserades med mikrografi. Ur denna rymd simulerades de 8 yttre parametrarna med hjälpa av Finita Elementmetoden i svetssimuleringsmjukvaran MARC och ett förhållande mellan ingående parametrar och resulterande restspänningar undersöktes.

En statistiskt säkerställd trend erhölls för att en minskad Svetshastighet leder till minskade tvärspänningar i mitten på en 20mm lång svetssträng. Detta är applicerbart för svetsar nyttjande start och stopplåtar.

Det noterades även att en ökning i Effekt eller Spot size, eller en minskning utav svetshastigheten leder till att det av restspänningar utsatta området ökar i storlek. Detta är har implikationer för efterföljande värmebehandling i avgörandet av form och storlek på området som skall värmebehandlas.

Abstract [en]

Minute defects may occur in large complex Ti-6Al-4V castings, sometimes these are unnoticed until after machining and a high cost has been sunk into the part. These defect and other potential manufacturing mishaps render a need for additive repair methods. The state of the art method TIG welding can repair the parts but may leave unacceptable residual stresses, where the state of the art solution of Post Weld Heat Treatment might create a surface defect known as alpha case. Therefore there is a need for a repair weld method that results in lesser or negligible residual stresses.

This thesis, carried out at GKN Aerospace – Engine Products Sweden, Trollhättan analyses the potential relationships between the laser welding parameters Power, Spot size, and Weld speed and the resulting residual stresses in a low energy parameter area on the material Ti-6Al-4V.

A parameter box of 17 parameter sets was established, laid down and analyzed under micrograph, of this box the outer 8 parameter sets were simulated via the Finite Element Analysis welding simulation software MARC and a relationship between the input parameters and their resulting residual stresses was analyzed.

A statistically significant trend was found supporting the claim that a decrease in transversal stresses in the center of a 20mm weld line is caused by an increase in Weld speed. This has implications for welds using run-on & run-of plates.

It was also noted that an increase in Power or Spot size, or a decrease in Weld speed increases the area under residual stress; both as individual parameters and in synergy. This has implications for Post Weld Heat Treatment in determining the size and shape of the area in need of treatment.

Place, publisher, year, edition, pages
2018. , p. 95
Series
TRITA-ITM-EX 2018 ; 622
Keywords [en]
Micro laser welding, Low energy welding, Ti-6Al-4V, Ti64, Residual stress, Response Surface Methodology, Repair welding, Welding simulation
Keywords [sv]
Mikrolasersvetsning, Lågenergetisk svetsning, Lågenergisvets, Ti-6Al-4V, Ti64, Restspänningar, Response Surface Methodology, Reparationssvets, Svetssimulering
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-244322OAI: oai:DiVA.org:kth-244322DiVA, id: diva2:1290046
Educational program
Master of Science in Engineering - Mechanical Engineering
Supervisors
Examiners
Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-02-19Bibliographically approved

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