An investigation of the impact of selected cooling strategies on milling of difficult-to-cut materials with an emphasis on titanium alloys and hardened steel

Thesis (MScEng)--Stellenbosch University, 2013.

ENGLISH ABSTRACT: The aerospace- and automotive industries have an urgency to save space and reduce weight, as well as a need to increase fuel efficiency and reduce emissions. This has led to the use of lightweight structural materials, such as Ti6A14V alloy, which is the most widely used titanium alloy in the aerospace industry. This alloy has an exceptional strength-to-density ratio. The work also covers studies on tool steel 40CrMnMo7 that is used in applications in the tooling-, aerospace and automotive industry. In the quest for improved performance new alternative methods of efficiently machining these materials are investigated. One of the important criteria during machining of these materials is their machinability. This study discusses current research in high performance machining strategies and techniques for advanced materials such as Ti6Al4V and 40CrMnMo7. The properties that make these materials advantageous for the use in the aerospace- and automotive industry also make them difficult to cut. The widespread application of Ti6Al4V in the aerospace industry has encouraged investigations into cooling strategies or -techniques to maintain and improve tool life. Ti6Al4V has a low thermal conductivity causing the heat generated during machining to accumulate on the cutting edge of the tool. During various experiments the application of external compressed air blow cooling (dry cutting), flood cooling, high pressure through spindle cooling (HPTSC) and modifications thereof were investigated. The research project also evaluated the performance of a coating (TiAlN) and various coating treatments. The objectives of the HPTSC modifications were to improve the coolant stream impingement on the tool surface, effectively compressing the thermal barrier, and to reduce the chip-tool contact area. This would lead to a decrease in tool heating and wear. The modified techniques failed to increase tool life but showed signs of increased heat removal capability under the given conditions. It was observed that air blow cooling (dry cutting) delivered the best results when considering cutting materials, coating, coating treatment and cooling strategies or –techniques throughout the experiments conducted.

AFRIKAANSE OPSOMMING: Die Ruimte-en motor-industrie het 'n dringendheid om ruimte te bespaar en gewig te verminder, sowel as 'n behoefte om brandstofdoeltreffendheid te verbeter en emissies te verminder. Dit het gelei tot die gebruik van liggewig strukturele materiale, soos Ti6A14V Allooi, wat die mees gebruikte titanium allooi in die Ruimte is. Hierdie allooi het 'n uitsonderlike krag-tot-digtheid-verhouding. Die studie dek ook gereedskapstaal 40CrMnMo7 wat in die gereedskap, Ruimte-en motor-industrie aangewend word. In die soeke na verbeterde prestasie word nuwe alternatiewe metodes om effektief bewerking van hierdie materiaal ondersoek. Een van die belangrikste kriteria tydens bewerking van hierdie materiaal is die bewerkbaar daarvan. Hierdie studie bespreek die huidige navorsing in hoë prestasie bewerking strategieë en tegnieke vir gevorderde materiale, soos Ti6Al4V en 40CrMnMo7. Die eienskappe wat hierdie materiaal voordelig maak vir die gebruik in die lug-en Ruimte-en motor-industrie, maak dit terselfdetyd moeilik om te sny. Die wydverspreide toepassing van Ti6Al4V in die lug-en Ruimte industrie moedig ondersoeke aan na koelstrategieë of -tegnieke om die instrumentlewe te handhaaf en te verbeter. Ti6Al4V het lae termiese geleidingsvermoë wat veroorsaak dat die hitte, wat gegenereer word tydens bewerking, versamel op die voorpunt van die instrument. Tydens verskillende eksperimente was die toepassing van eksterne saamgeperste lugblaas-verkoeling (droë sny), vloed verkoeling, hoë-druk-deur-die-spil-afkoeling (HPTSC) en aanpassings daarvan geondersoek. Die navorsingsprojek het ook die prestasie van 'n bedekkingslaag (TiAlN) en verskeie bedekkingslaagbehandelings geëvalueer. Die doelwit van die HPTSC aanpassing was om die koelmiddelstroom beklemming op die instrument oppervlak te verbeter, en effektiewelik die termiese versperring saam te pers, asook die skerf-teenoorinstrument kontak te verminder. Dit sou lei tot 'n afname in die instrumentverwarming en -slytasie. Die gewysigde tegnieke het daarin misluk om die instrumentlewe te verhoog, maar het tekens getoon van 'n toename in hitte verwydering vermoë onder die gegewe omstandighede. Dit is dus waargeneem dat lugblaasverkoeling (droë sny) die beste resultate gelewer het in die oorweging van sny materiale, bedekkingslaag, bedekkingslaagbehandelings en verkoeling strategieë of -tegnieke wat regdeur die eksperimente uitgevoer was.