Alpha case removal from titanium alloys by machining with tungsten carbide cutting tools

Conradie, Francois Willem (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The use of titanium is rising steadily. This surge is due to the metal’s favourable biocompatibility, corrosion resistance and high specific strength. Despite the increasing demand, titanium production is currently limited by outdated manufacturing processes. New processing techniques are therefore under investigation so that raw material may be produced at lower cost. One of the manufacturing processes under review, is the use of chemical milling for the removal of the hard and brittle oxide layer (alpha case) which forms at production temperatures above T = 600°C. Chemical milling facilities currently used in alpha case removal demand high workplace and environmental safety standards which would incur high capital cost if constructed in South Africa. Alternatively, the already established South African machining industry can be expanded to economically remove alpha case using existing infrastructure (milling machines). No machining guidelines are available and such a process is currently deemed uneconomical due to tooling cost. This study therefore investigated the performance of tungsten carbide indexable cutters in the removal of alpha case through machining, and developed guidelines for the economical removal of alpha case. Background experiments determined the hardness-depth profile, composition and grain structure, which were used to aid in the experimental setup and design of primary experiments. The primary objective investigated the feasibility of replacing the acidic solutions of chemical milling, with tungsten carbide cutting tools in machining. The wear of the tungsten carbide cutters and the effect of alpha case on their performance were documented and used to measure the feasibility of machining. At high cutting speeds the carbide cutting tools experienced excessive chipping and notching, which resulted in short tool life and low material removal. Alpha case removal was, however, readily achieved at low cutting speeds where traditional flank wear was experienced. At low cutting speeds, tool life also more closely resembled that which is observed with traditional titanium machining. Furthermore, feed rate had a negligible effect on tool wear and tool life at low cutting speeds. The most effective cutting strategy for alpha case removal with tungsten carbide indexable cutters, therefore involves the employment of low cutting speeds in combination with high feed rates. This will ensure long tool life while still realising reasonable material removal rates. The secondary objective investigated is the scope of feasibility of alpha case machining removal in the context of the South African manufacturing industry. The already established machining industry in South Africa would profit from the expanded titanium machining industry, which could in turn hold further downstream manufacturing benefits. It is hypothesised that at low annual production volume, the tungsten carbide cutting tools used in machining removal of alpha case are the more economical option. Owing to the high cost of constructing new chemical milling facilities, only at high production volume would chemical milling become a viable option for long term manufacturing.

AFRIKAANSE OPSOMMING: Titaanbenutting is besig om wêreldwyd te styg danksy die materiaal se gunstige eienskappe soos bioaanpasbaarheid, weerstand teen roes en hoë spesifieke sterkte. Ten spyte van die groeiende aanvraag, word titaanproduksie tans beperk deur verouderde produksieprosesse. Nuwe verwerkings-tegnieke word dus ondersoek om rou materiaal te vervaardig teen laer koste. ’n Voorbeeld van so ’n produksieproses onder hersiening, is die gebruik van chemiese ets in die verwydering van die bros en harde oksiedlaag (alpha-laag) wat gevorm word by produksie-temperature bo T = 600°C. As titaanbevoegdheid bereik wil word deur middel van programme soos die Titanium Centre of Competency, dan is die verwydering van die alpha-laag een van die kwessies wat aangespreek moet word. Chemiese ets fasiliteite vereis hoë werkplek- en omgewingsveiligheidstandaarde, wat op hoë kapitaalkoste sou uitloop indien dit in Suid-Afrika gebou sou word. As alternatief kan die reeds-gevestigde Suid-Afrikaanse masjineringsbedryf uitgebrei word om ekonomiese verwydering van die alpha-laag te bewerkstellig. So ’n proses word tans beskou as onekonomies en geen riglyne is beskikbaar vir masjineringsverwydering nie. Hierdie studie ondersoek dus die vermoë van wolfram-karbied geïndekseerde beitels in the verwydering van die alpha-laag deur masjinering, en ontwikkel riglyne vir die ekonomiese verwydering van die alpha-laag. Agtergrondeksperimente het die hardheid-diepte profiel, samestelling en greinstruktuur van die alpha-laag bepaal. Dit is gebruik om met verdere ontwerp van eksperimente te help. Die primêre doel, was die ondersoek van die lewensvatbaarheid om die suuroplossing van chemiese ets te vervang met wolfram karbied-geïndekseerde beitels in masjinering. Verskeie metingstegnieke is gebruik om die werksverrigtingvermoë, asook die effek wat die alpha-laag op die karbied-beitels hou, vas te stel. Teen hoë snyspoed het die karbied-beitels oormatige afsplintering en slytasie ervaar. Dit het gelei tot kort beitel lewensverwagting en lae materiaalverwydering. In lae snyspoedgevalle is die alpha-laag effektief verwyder met min slytasie en snyleeftyd nader aan wat waargeneem word met tradisionele titaanmasjinering. Teen lae snysnelhede het die voerkoers ook ’n geringe uitwerking op die karbied-beitel se lewensduur. Die effektiefste snystrategie vir die verwydering van alpha-laag met wolfram-karbiedbeitels behels dus die gebruik van lae snysnelhede in kombinasie met hoë voertempo. Die sekondêre doel was ’n ondersoek na die omvang van die lewensvatbaarheid van alpha-laagmasjinering in die konteks van die Suid-Afrikaanse vervaardigingsbedryf. Die reeds-gevestigde masjineringsbedryf sal voordeel trek uit die uitgebreide titaan masjineringsbedryf, wat verdere voordele vir die vervaardigingsbedryf kan hou. Daar word gepostuleer, dat masjinering met wolfram-karbied snybeitels vir die verwydering van die alpha-laag, die mees ekonomiese opsie is indien lae jaarlikse produksievolume gehandhaaf word. Weens die hoē konstruksiekostes, sal chemiese meulfasiliteite net op die langtermyn vatbaar wees indien hoë produksie-volumes bereik kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98782
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