Metallurgical and Materials Engineering

The main objective of this research was to investigate the effect of soaking time and quenching media on the structure and mechanical properties of aluminium bronze (Cu-10%wt.Al). Sand casting method was adopted in casting the samples. Standard specimens were prepared from the as-cast and heat treated samples for tensile, hardness and impact strength tests as well as microstructural analysis according to standard. The samples were solutionized at temperature of 900 o C for 30, 60 and 180mins and quenched in water, brine and SAE 40 respectively. The tensile, hardness and impact strength test were conducted using JPL tensile strength tester (Model: 130812), dynamic hardness tester and impact testing machine (U1820) respectively. The alloy microstructures were studied using an optical metallurgical microscope (Model: L2003A). Microstructural analysis result indicated coarse sparse distribution of α+ γ2 precipitates in the as-cast specimen and presence of fine pearlite (α + γ2) in a matrix of α dominance in the heat treated specimens. The presence of coarse intermetallic compounds (β`-phase) was revealed as the soaking time increased. The obtained results indicated that heat treatment significantly improved all the tested mechanical properties except the impact strength. Optimum ultimate tensile strength and hardness values of 710MPa and 513MPa respectively were recorded for the specimen solutionized at 900 o C for 30 minutes and quenched in water and brine respectively. Among all the quenching media used, brine gave the best combination of mechanical properties.

Based on very early occurring ruptures found in the first stage high pressure turbine blades of a turbo reactor in a local aviation company, this study has the aim to determine their safe life. The first stage blades are subjected to simultaneous action of gas pressure coming from the combustion chamber, centrifugal forces in the case of the rotor blades and to important temperatures transients, which progress in a very aggressive environment due to hot gases. These combined parameters cause a high state of stress involving several complex mechanisms of damage, such as: fatigue caused by mechanical stress fluctuations, thermo-mechanical fatigue caused by temperature variations and corrosion caused on the stressed elements. Life cycle determination asks for stress evaluation of blades regarding several variables which are approached deterministically in the study. Heat exchange between combustion gases and metal blades is considered. The total stress on two kinds of blades is calculated by the addition of the thermal effect and the mechanical loading. The stress cycle is then calculated for different steps of the engine function during the operation by considering the variation of the thermal and the mechanical properties of the system. Safe life determination is done by two different approaches: the safe life approach by the initiation model and the damage tolerance approach considering the defect growth mechanics and considering the pitting corrosion effect. The calculation is applied for stator and rotor blades of an aero engine high pressure turbine made of NI 738. Since these parts are high risk components from the point of view of potential failure consequences, the risk is assessed as well. The results obtained are studied to determine the solution to the problem, and to propose a safe decision to be taken about the design or maintenance procedures.

The metal 3-D printing market is currently dominated by high-end applications, which make it inaccessible for small and medium enterprises, fab labs, and individual makers who are interested in the ability to prototype and additively manufacture final products in metal. Recent progress led to low-cost open-source metal 3-D printers using a gas metal arc welding (GMAW)-based print head. This reduced the cost of metal 3-D printers into the range of desktop prosumer polymer 3-D printers. Consequent research established good material properties of metal 3-D printed parts with readily-available weld filler wire, reusable substrates, thermal and stress properties, toolpath planning, bead-width control, mechanical properties, and support for overhangs. These previous works showed that GMAW-based metal 3-D printing has a good adhesion between layers and is not porous inside the printed parts, but they did not proceed far enough to demonstrate applications. In this study, the utility of the GMAW approach to 3-D printing is investigated using a low-cost open-source metal 3-D printer and a converted Computer Numerical Control router machine to make useful parts over a range of applications including: fixing an existing part by adding a 3-D metal feature, creating a product using the substrate as part of the component, 3-D printing in high resolution of useful objects, near net objects, and making an integrated product using a combination of steel and polymer 3-D printing. The results show that GMAW-based 3-D printing is capable of distributed manufacturing of useful products for a wide variety of applications for sustainable development.