The results from this study, on a few commercial and model metastable beta titanium alloys, indicate that the growth restriction factor (GRF) model fails to interpret the grain growth behavior in the additively manufactured alloys. In lieu of this, an approach based on the classical nucleation theory of solidification incorporating the freezing range has been proposed for the first time to rationalize the experimental observations. Beta titanium alloys with a larger solidification range (liquidus minus solidus temperature) exhibited a more equiaxed grain morphology, while those with smaller solidification ranges exhibited columnar grains. Subsequently, the printability of two candidate beta titanium alloys containing eutectoid elements (Fe) that are prone to beta fleck in conventional casting, i.e., Ti-1Al-8V-5Fe (wt%) or Ti-185, and Ti-10V-2Fe-3Al (wt%) or Ti-10-2-3, is further investigated via two different AM processing routes. These alloys are used for high-strength applications in the aerospace industry, such as landing gears and fasteners. The Laser Engineered Net Shaping and Selective Laser Melting (the two AM techniques) results show that locally higher solidification rates in AM can prevent the problem of beta fleck and potentially produce β-titanium alloys with significantly enhanced mechanical properties over conventionally cast/forged counterparts. Further, the detailed investigation of microstructure-mechanical property relationships indicates that the precipitation or formation of non-equilibrium secondary phases like α or ω in these commercial systems can be advantageous to the mechanical properties. The influence of process parameters on the evolution of such secondary phases within the β matrix grains has also been rationalized using a FEM-based multi-physics thermo-kinetic model that predicts the multiple heating-cooling cycles experienced by the layers during the LENS deposition. Overall, the results indicate that Ti-1-8-5 and Ti-10-2-3 are promising β-Ti alloys for AM processing. Further, the results also demonstrate the ability to tune the microstructure (secondary phase precipitation and grain size) via changes in the process parameters to achieve desirable mechanical properties, obviating the need for any secondary post-processing. The understanding obtained through this work can be coupled with the concept of β-phase stability prediction, via parameters like bond order (Bo), the energy level of metal d-orbital (Md), Mo equivalency, etc., to design novel beta titanium alloys with the desired microstructures tailored via AM for structural applications.

Yes / Resistance spot welding of aluminum (Al5754) to magnesium (AZ31B) alloys results in the formation of a variety of solidification microstructures and intermetallic compounds that may affect the in-service performance of the weld. This study evaluates the relationship between the welding parameters and the properties of the weld nugget that is formed, and clarifies the morphological and microstructural evolutions within the weld regions during the low-current “small-scale” resistance spot welding of Al5754 to AZ31B. The investigations included a combination of microstructural characterization and thermodynamic analysis of the weld region. The results show that the welding time and clamping force parameters have significant effects on the properties of the nugget formed. The optimal welding parameters were found to be 300 ms welding time and 800 N clamping force. Weld nuggets formed with lower welding time and clamping force were undersized and contained extensive porosity. Meanwhile, a clamping force above 800 N caused gross deformation of the test samples and the expulsion of the molten metal during the welding process. The most significant microstructural changes occurred at the weld/base metal interfaces due to the formation of Al17Mg12 and MgAl2O4 intermetallic compounds as well as significant compositional variation across the weld pool. The thermal gradient across the weld pool facilitated the formation of several microstructural transitions between equiaxed and columnar dendrites.

Ferritic stainless steel is typically used in the automotive industry to fabricate welded tube that is plastically deformed for flanging, bending and necking. The effect of welding parameters during autogenous gastungsten arc welding (GTAW) of thin sheet on the weld metal structure and tensile properties were determined. Two grades of ferritic stainless steels, a titanium-containing Grade 441 and a titanium-free molybdenum-containing Grade 436, were used as base metal. Statistical analysis was used to determine the influence of welding parameters on the microstructure of autogenous GTAW welds. The results of Grade 441 indicated that the welding speed and peak welding current had a statistically significant influence on the amount of equiaxed grains that formed. For Grade 436, the same welding parameters (welding speed and peak welding current) had a statistically significant influence on the grain size of the weld metal grains. The ductility of a tensile test coupon machined parallel to the weld direction, for both base metal grades, was unaffected by the welding parameters or the weld metal microstructure. The elongation was determined by the amount of weld metal in the gauge area of a tensile coupon. The titanium content of the base material seems to have the most significant effect on the formation of equiaxed grains. / Dissertation (MEng)--University of Pretoria, 2019. / Materials Science and Metallurgical Engineering / MEng / Unrestricted

The aluminium alloys for electrical conductivity applications are generally not grain refinedsince the addition of grain refiners drops the electrical conductivity by introducing impuritiesinto the melt. Non-grain refined aluminium may lead to bar fracture and cracks during themetalworking process. The present study focuses to find an optimum balance between the grain refiner addition andthe electrical conductivity of commercial EC grade 1070 aluminium alloy for electricalapplication. In order to reach this goal, the electrical conductivity and the macrostructure ofcommercial EC grade 1070 aluminium (commercial pure aluminium) have been studiedunder a series of controlled lab scale trails. Specific addition levels of different grain refiners(TiBloy, Al-5Ti-1B, Al-3Ti-0.15C, and Al-3Ti-1B) were added to the metal melt and sampleswere taken at specific time intervals. The collected samples were sectioned, ground andmacro-etched. Thereafter, the macrostructure was analysed by the use of a digital camera andthe electrical conductivity was measured at temperature. The obtained result was expressed asa percentage of the International Annealed Copper Standard (IACS %). The macro-structuralanalysis showed that TiBloy, Al-5Ti-1B, and Al-3Ti-1B, with the maximum addition level of0.1%, cannot grin refine commercial pure aluminium. However, at higher grain refiner levelsthe number of columnar grains increased and their size decreased. The Al-3Ti-0.15C master alloy, with the same addition level as the once chosen for the othergrain refiners (up to 0.1%), showed significantly better grain refining. By the addition of0.1% of this grain refiner the macrostructure became very equiaxed already after 30 minutesof grain refiner addition. The fading of the Al-3Ti-0.15 master alloy was, however, observedfor samples with a long holding time. Nevertheless, by maximum addition level (0.1%) and a90 minutes holding time the macrostructure remained as equiaxed grains. The electrical conductivity results showed that none of the applied grain refiners (TiBloy, Al-5Ti-1B, Al-3Ti-0.15C, and Al-3Ti-1B), with the maximum addition level of 0.1%, decreasedthe electrical conductivity of commercial pure aluminium.

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.