Sinter infiltration of TI-6A1-4V

Govender, Preyin (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Ti-6Al-4V is a widely used titanium alloy within the aerospace and medical industry with powder metallurgy (PM) becoming a fast growing industry within. The press-and-sinter technique is one such PM process that can be used to manufacture parts. In order to improve mechanical properties, dimensional tolerances and machinability, ferrous PM press-and-sintered parts are often infiltrated with molten Cu during the sintering heat treatment process. This project investigates the feasibility of infiltrating sintered Ti-6Al-4V compacts with molten Al, in order to improve its properties. Two Ti-6Al-4V powder blends were mixed, namely a blended elemental (BE) blend consisting of elemental Ti, Al and V powder in a 90:6:4 wt% ratios and a master alloy (CPTi+MA) blend consisting of commercially pure Ti (CPTi) powder with a master alloy (MA) powder of Al and V mixed in a 90:10 wt% ratio. These powder blends were compacted in 10 mm right cylinders with a relative green density (ρg) of approximately 75%. The compacts were sintered under high vacuum at temperatures of 1100 ºC and 1200 ºC, respectively, to achieve relative sintered densities (ρs) of < 92%. At ρs > 92 %, open pore channels close off preventing infiltration. Infiltration disks were compacted from pure Al spherical powder. The mass of the disks was calculated by taking the residual porosity of the sintered Ti-6Al-4V compacts, and providing enough molten Al to infiltrate the pores exactly. Infiltration took place under a nitrogen atmosphere at various temperatures between 700 °C - 900 °C, above Al melting point (660.6 ºC) to ensure melting, and for various dwell times. Slices of wrought Al bar stock were also evaluated for infiltration. Neither the Al powder compacts nor the wrought Al slices melted, with the result that all attempts to infiltrate the Ti-6Al-4V with Al failed. Characterization of the sintered and infiltrated samples was performed. Optical microscopy as well as energy dispersive X-ray spectroscopy (EDS) analysis was used to view the microstructures and elemental distribution in the microstructures, respectively. From these analyses, it was confirmed that infiltration with Al did not occur for any of the samples. CPTi+MA samples showed, on average a change in relative density from 74.5% to 90.2%, while BE samples showed negligible change from 73.6% to 74.9% relative density. The microstructure of both blends were observed with the CPTi+MA having a ɑ+β microstructure; while the BE having a predominantly ɑ-microstructure. EDS imaging for the CPTi+MA samples showed a fairly homogenous elemental distribution for both sintering temperatures. The BE blends showed an inhomogenous distribution at both sintering temperatures. Large pores were visible as Al particles melted and diffused into Ti and V, leaving high Al concentrations surrounding the pores. The results indicate that Ti and V diffuse into the Al disks during the infiltration heat treatment process, raising the melting temperature of the Al disks. By observing the concentration of Ti in Al disks, we see on the Ti-Al phase diagram that an intermetallic may have formed, thus increasing the melting temperature above the temperatures used for infiltration.

AFRIKAANSE OPSOMMING: Ti-6Al-4V is ʼn titanium allooi wat gereeld gebruik word in die ruimtevaart- en mediese- bedryf, met poeiermetallurgie (PM) wat ʼn groeiende industrie daarin is. Een proses om PM komponente te vervaardig is die pers-en-sinter tegniek. Meganiese eienskappe, afmetingstoleransies en masjienbaarheid van ysterallooi pers-en-sinter onderdele word gereeld verbeter deur dit met gesmelte Cu te infiltreer gedurend die sintersproses. Die projek ondersoek die haalbaarheid om sinter Ti-6Al-4V te infiltreer met Al, om eienskappe te verbeter. Twee Ti-6Al-4V poeiermengsels is gemeng, spesifiek ʼn gemengde elementêr (BE) mengsel, wat uit Ti, Al, en V poeiers in ʼn 90:6:4 gewigspersentasie verhouding bestaan, en ʼn meesterallooi (CPTi+MA) mengsel van ʼn suiwer Ti (CPTi) poeier en ʼn meester allooi (MA) poeier van Al en V, wat in ʼn 90:10 gewigspersentasie verhouding gemeng word. Hierdie poeiermengsels is in ø10 mm reghoekige silinders gepers tot ʼn relatiewe groen dightheid (ρg) van ongeveer 75%. Die gepersde silinders is onder hoë vakuum by temperature van 1100 ºC en 1200 ºC, onderskeidelik, gesinter tot ʼn relatiewe sinterdigtheid (ρs) van < 92%. By ρs> 92% word oop porie kanale afgesluit en dus is infiltrasie onmoontlik. Infiltrasie skywe is uit suiwer Al sferiese poeier gepers. Massa van skywe is bereken deur oorblywende porositeit van die gesinterde Ti-6Al-4V silinders te vat en dan net genoeg gesmelte Al te gebruik om hul porieë presies te infiltreer. Infiltrasie het onder ʼn stikstof atmosfeer plaasgevind by verskillende woontye, en verskeie temperature (700 °C - 900 °C), wat bo Al se smeltpunt (660.6 ºC) is, om smelting te verseker. Snye van gesmeede Al staafvoorraad was ook vir infiltrasie geëvalueer. Nie die Al poeier of die gesmeede Al skywe het gesmelt nie, en gevolglik was alle pogings om Ti-6Al-4V te infiltreer onsuksesvol. Die gesinterde en geinfiltreede monsters is gekarakteriseer. Optiese mikroskopie en ʼn energiedispergerende X-straalspektroskopie (EDS) analise is gebruik om mikrostrukture en elementêre samestelling daarvan waar te neem. Analises het bevestig dat infiltrasie met Al nie voorgekom het in enige van die monsters nie. CPTi+MA monsters toon ʼn gemiddelde verandering in relatiewe digtheid van 74.5% na 90.2% terwyl BE monsters se relatiewe digtheid toon weglaatbare verandering vanaf 73.6% na 74.9%. Mikrostrukture van beide poeiermengsels is waargeneem, waar CPTi+MA ʼn ɑ+β-mikrostruktuur, en BE hoofsaaklik ʼn ɑ-mikrostruktuur getoon het. EDS beelding vir die CPTi+MA monsters het redelike homogene elementêre verspreiding getoon vir beide sinter temperature. BE monsters het nie homogene verspreiding getoon nie. Groot porieë was sigbaar, aangesien Al partikels gesmelt en in die Ti en V gediffundeer het, wat porieë wat met hoë konsentrasies Al omring word gelos het. Die resultate dui aan dat Ti en V in die Al skywe diffundeer gedurende die infiltrasie hittebehandelingsproses, wat die smeltpunt van Al skywe verhoog. Deur die gewigsverhoudingspersentasie van die Ti in die Al skywe waar te neem, sien ons op Ti-Al fasediagramme dat intermetaalverbindings dalk gevorm het, wat smeltpunte bo die infiltrasietemperature verhoog.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108040
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