This thesis presents results from the application of cryogenic cooling on multiple-pass friction stir processing and the subsequent orthogonal machining on friction stir processed and as-received Mg AZ31B-O disks, and shows their combined effects on microstructure and microhardness values. A simple friction stir tool, a specially designed fixture and liquid nitrogen are used to perform multiple-pass friction stir processing experiments on Mg AZ31B-O alloy. The friction stir processed and as-received sheets are then made into disks for the orthogonal machining experiments. This study analyzes the microhardness, microstructure changes by cryogenic friction stir processing and the effect of machining conditions such as dry, MQL and cryogenic and cutting parameters on the Mg AZ31B-O alloy. Four different speeds and three different feed rates are used for the orthogonal machining experiments. The effects of stirring parameters such as the translational feed, rotational speed, cooling conditions and the machining parameters are studied. The resulting microstructure and microhardness from these processes hold a key to the mechanical properties of the alloy. This analysis would help to understand and evaluate the specific aspects of grain size and microhardness that influence the fatigue life of a component.

This work is concerned with the design, development and performance evaluation of an inertance Pulse Tube Cryocooler (PTC). The main components of a PTC are the compressor, regenerator, pulse tube and inertance tube coupled to a reservoir. The inertance tube is a key component that affects the pressure and mass flow and phase shift between them and hence the performance. In conjunction with the compressor, it also plays a strong role in determining the frequency of operation. The PTC is designed based on system level numerical models (SAGE and DeltaE), component level thermo-acoustic models (DeltaE) of inertance tube and regenerator and experimental data of earlier fabricated Stirling coolers. As a starting point, an inertance tube with a diameter of 3 mm and 3.1 m long was chosen through component level analysis that provides phase shift of around 50 degrees at a pressure ratio of 1.1 for an acoustic power of about 4 W (in order to achieve 1 W of net cooling at 80 K) at 25 bar mean pressure and 60 Hz. From this inertance tube geometry, an estimate of the mass flow rate at the cold heat exchanger is obtained. Based on this mass flow rate, the initial dimensions of the pulse tube and regenerator are arrived at. A parametric study using system level model is carried out to obtain the maximum COP by varying inertance tube length and regenerator diameter. A flexure bearing compressor consisting of moving coil linear motor coupled to a piston is designed for the above cold head. Based on the above design considerations, the PTC compressor and cold head are fabricated and assembled. The PTC is charged with helium at mean pressure of 25 bar and instrumented with pressure and position transducers, temperature sensors and a skin-bonded heater for simulating the heat load on the cold head. Experimental data for the PTC were obtained with two different inertance tube lengths for different frequencies of operation. The cold head temperature exhibited a minimum with respect to the frequency. This optimum frequency shifts towards lower frequency with increased length of the inertance tube. The experimental data clearly shows that with different inertance tube lengths the optimum frequency locates itself for obtaining zero phase shift at the middle of the regenerator. It is observed that the optimum frequency is closely linked to the natural frequency of the pressure wave in the inertance tube suggesting a standing wave within the inertance tube with the pressure node at the reservoir. Thus the inertance tube is found to be analogous to a quarter wave resonator in a thermo-acoustic device. It may thus be possible to pre-fix an operating frequency for a given PTC cold head by choosing an inertance tube length close to quarter wave resonator length. This study has given insights on the phase shift between pressure and mass flow rate governed by the inertance tube and the connection between the optimum and natural frequencies which can be used for better design of PTCs.

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The aerospace industry actively pursues innovation, especially in materials and their use in new applications, to improve their aircraft as well as their competitive position. Ti-6Al-4V has been available now for more than 50 years. Yet, in the new generation of aircraft using structural composites, a dramatic increase in the proportion of Ti-6Al-4V will be seen along with emerging application in automotive and chemical industries. This material possesses superior material properties compared to conventional materials such as steel and aluminium, although it is at the expense of machinability. Researchers are therefore actively searching for improved cutting technologies to improve production rates for Ti-6Al-4V. At higher cutting speeds than the industry norm of 60 - 90 m/min, machining becomes a challenge, resulting in low productivity on titanium parts. The limiting factor in the machining of Ti-6Al-4V is high tool temperatures of the order of 1000oC, caused by its resistance to absorb heat and good mechanical strength at elevated temperatures. The result is extreme temperatures that are concentrated on the cutting edge of the tool. The challenge to improve the tool life is therefore focused on removing heat from the insert. Liquid nitrogen was identified as a good candidate as coolant with the additional advantage of being environmentally friendly. The research presented investigates the use of a gravity feed enclosed liquid nitrogen cooling system to improve the tool life of the cutting inserts. The liquid nitrogen is contained on the insert rake face by means of a tool cap. To improve the effectiveness of the cooling method, a polycrystalline diamond (PCD) insert was used. This insert has a considerably higher thermal conductivity that aids in cooling the cutting edge. Tungsten carbide inserts are used for benchmark testing. The round tungsten carbide inserts with conventional cooling performed exceptionally well for machining titanium compared to square inserts, yielding exceptional tool life improvements while significantly increasing the material removal rate. Positive results were recorded with the liquid nitrogen cooling system when used with the polycrystalline diamond cutting insert. A number of far reaching performance issues are identified relating to the design of the tool cap that hindered clear scientific outputs. From a research perspective, the project makes a contribution to the knowledge base in this field. Additionally a new approach in cooling was investigated, resulting in clear indications of design changes required. / AFRIKAANSE OPSOMMING: Die lugvaart industrie streef aktief innovasie na, veral op die gebied van materiale en hul gebruike, om hul vliegtuie en kompeterende posisie in die mark te verbeter. Ti-6Al-4V is al vir meer as 50 jaar beskikbaar. ‘n Drastiese verhoging in die aanvraag na Ti-6Al-4V deur die lugvaart, motor en chemiese industrieë word verwag wanneer die volgende geslag vliegtuie wat koolstofvesel as strukturele materiaal begin gebruik, in produksie gaan. Die materiaal het beter materiaaleienskappe as konvensionele materiale soos staal en aluminium, maar dit kom egter teen die prys van masjieneerbaarheid. Ti-6Al-4V se masjienering bo die industrie norm van 60 – 90m/min is ‘n groot uitdaging. Navorsers soek daarom deurentyd na verbeterde sny tegnologieë om die produksie tempo van Ti-6Al-4V te verbeter. Die beperkende faktor vir Ti-6Al-4V masjienering is die temperatuur wat genereer word. Die weerstand van die materiaal om hitte te absorbeer en sy goeie meganiese eienskappe veroorsaak dat temperature in die beitel 1000oC bereik. Hierdie temperature word egter op die snykant van die beitel gekonsentreer. Die uitdaging is dus om hierdie temperature in die beitel te beheer. Vloeibare stikstof is geïdentifiseer as ‘n goeie kandidaat vir verkoeling met die bykomende voordeel dat dit omgewingsvriendelik is. Die navorsing wat hier uiteengesit word, ondersoek die gebruik van ‘n geslote kamer beitelverkoelingstelsel wat deur gravitasie met vloeibare stikstof voorsien word om die beitel leeftyd te verbeter. Die oppervlak van die beitel word in hierdie konsep direk aan die vloeibare stikstof blootgestel. Om die effektiwiteit van die stelsel te verbeter word van PCD beitels gebruik gemaak. Die beitel se verbeterde hittegeleidingsvermoë help om die beitel se snykant koel te hou. Tungstenkarbied beitels word gebruik om ‘n standaard te stel vir eksperimentele analise. Die ronde tungstenkarbied beitels en konvensionele verkoeling het verstommend goed presteer vir Ti-6A-4V masjienering in vergelyking met vierkantige beitels. Die materiaalverwyderingstempo is aansienlik verhoog sonder om die beitel se leeftyd in te boet. Positiewe resultate is waargeneem met die vloeibare stikstof sisteem saam met die PCD beitels. ‘n Aantal verreikende uitdagings is geïdentifiseer wat suiwer wetenskaplike afleidings bemoeilik. Hierdie probleme kan almal aan die ontwerp van die toerusting toegeskryf word. Die werk lewer egter steeds ‘n bydrae tot die kennis in die veld. ‘n Bykomende benadering vir verkoeling is ondersoek wat duidelik ontwerp-veranderings aandui.

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