Influence of powder particle size distribution on press-and-sinter titanium and Ti-6Al-4V preforms

Bosman, Hendrik Ludolph (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: This research focusses on the press-and-sinter manufacturing process through which titanium powders are employed to produce dense titanium and the Ti-6Al-4V alloy; more specifically, the influence of particle size distribution (PSD) on the densification behaviour and material properties are investigated. Commercially pure (CP) titanium powders of -100 and -200 mesh sizes were blended in various proportions and used to conduct compressibility and sintering studies. To produce Ti-6Al-4V, a -200 mesh 60Al-40V master alloy (MA) powder was additionally blended with the CP titanium powders. Powders and powder blend were characterised using scanning electron microscopy and laser diffraction. A vast array of specimens was produced while varying the following production parameters: aspect ratio, compaction pressure, sintering time and sintering temperature. Aspect ratios of cylindrical specimens were varied to produce thin disks (1:3), as well as square (1:1) and long (3:2) cylinders. Compaction pressures were varied from 200 MPa to 600 MPa using double action compaction. Sintering was conducted under high vacuum (<10-4 mbar, or better) with sintering temperatures ranging from 1000°C to 1300°C; typical holding times were two hours, with certain specimens being re-sintered to four, and up to six hours. From the results of the compressibility and sintering studies, a baseline densification pathway was elected: compaction at 400 MPa followed by sintering at 1300°C for two hours. This allowed meaningful comparison of the behaviour of different powder blends. Several CP titanium and MA Ti-6Al-4V powder blends of known weight compositions were considered by creating a model using the precursor powder PSD data to predict the blended powder PSDs. A few promising CP and MA blends were prepared and specimens were produced according to the elected baseline process. The densification behaviour was studied at each process step. Densification trends similar to those indicated in literature for bimodal powder blends were found for the CP titanium blends; however, the effect of the MA powder alloying addition was dominant in the case of the MA Ti-6Al-4V blends’ densification behaviour. Mechanical properties were tested using three point bending and Vickers hardness (HV10), respectively. Transverse rupture bar specimens were pressed (400 MPa) and showed either brittle or ductile fracture after being sintered for two hours at either 1000°C or 1300°C, respectively. The thermal conductivity of specific specimens was measured and showed that the thermal conductivity of sintered titanium is lower than that of the equivalent wrought material. The sintered microstructure of various specimens was investigated to gain insight into differences in pore structures among the blend compositions. A vast range of densification results has been put forth from which to extract data for future research. Recommended future work would include: the procurement of tooling for tensile test specimens, a redesign of the thermal conductivity experimental setup, and the addition of fine -325 mesh CP titanium powders to widen the range of PSDs achievable.

AFRIKAANSE OPSOMMING: Hierdie navorsing fokus op die pers-en-sinter poeiermetallurgiese vervaardigingsproses waardeur titaniumpoeiers gebruik word om digte titanium en die Ti-6Al-4V allooi te produseer. Meer spesifiek word die invloed van partikel grootte distribusie (PGD) op verdigtingsgedrag en materiaal-eienskappe ondersoek. Kommersieel suiwer (KS) titaniumpoeier van -100 and -200 maasgroottes was gemeng in verskeie proporsies en gebruik in saampersbaarheid- en sinteringstudies. In die produksie van Ti-6Al-4V was ʼn -200 maas 60Al-40V meester-allooipoeier (MA) bykomend met die KS titanium poeiers gemeng. Die karaktereienskappe van die poeiers en poeiermengsels is ondersoek deur gebruik te maak van skanderings-elektronmikroskopie en laserdiffraksie. ʼn Groot reeks monsters was geproduseer onderhewig aan die afwisseling van die volgende produksie-parameters: aspek-verhoudings, kompaksiedruk, sinteringstyd en sinteringstemperatuur. Aspek-verhoudings van silindriese monsters is afgewissel om dun skywe (1:3), asook vierkantige- (1:1) en lang-silinders (3:2) te produseer. Kompaksie-druk is gevarieer vanaf 200 MPa tot 600 MPa met behulp van dubbelaksie-kompaksie. Sintering is uitgevoer onder hoë vakuum (<10-4 mbar, of beter) met sinteringstemperature wat wissel vanaf 1000°C tot 1300°C met ʼn tiperende oondtyd van twee ure. Sekere monsters is hersinter tot vier, en selfs tot ses uur. Uit die resultate van die saampersbaarheid en sinteringstudies, was 'n basislyn verdigtingspad gekies: kompaksie by 400 MPa gevolg deur sintering by 1300°C vir twee ure. Dit het die betekenisvolle vergelyking van die gedrag van verskillende poeiermengsels moontlik gemaak. Verskeie KS titanium en MA Ti-6Al-4V poeier-mengsels van gekose gewigsamestellings is oorweeg. 'n Model was geskep deur die PGD-data van die voorloperpoeier te benut om die gemengde poeiers se PGD te voorspel. 'n Paar belowende KS en MA mengsels is voorberei en monsters is vervaardig volgens die gekose basislyn-proses. Die verdigtingsgedrag is ondersoek by elke stap van die proses. Verdigtings-tendense, soortgelyk aan dié wat in die literatuur vir bimodale poeiermengsels beskryf word, is gevind vir die KS titanium mengsels. Die effek van die MA poeier se toevoeging was egter oorheersend in die verdigtings-gedrag van die MA Ti-6Al-4V mengsels. Meganiese eienskappe is getoets met behulp van drie punt buigtoetse en Vickers hardheid (HV10), onderskeidelik. Dwars-breek balkmonsters is gekompakteer (400 MPa) en het bros óf rekbare breuk getoon na sintering vir twee ure teen, onderskeidelik, 1000°C en 1300°C. Die termiese geleidingsvermoë van spesifieke monsters is gemeet en het getoon dat die termiese geleidingsvermoë van gesinterde titanium vêr laer is as dié van die ekwivalent smee-materiaal. Die gesinterde mikrostruktuur van verskeie monsters is ondersoek om insig te verkry oor die verskille in die poreusheid van die verskillende mengselsamestellings. 'n Groot verskeidenheid verdigtingsresultate is aangeteken waaruit data vir toekomstige navorsing onttrek kan word. Aanbevelings vir toekomstige werk sluit in: die verkryging van gereedskap vir die produksie van trektoetsmonsters; die herontwerp van die opstelling van die termiese geleidings-eksperiment; en die toevoeging van fyn -325 maas KS titaniumpoeiers om die omvang van PGDs uit te brei.

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