Effects of Pressure and Number of Turns on Microstructural Homogeneity Developed in High-Pressure Double Torsion

With electron backscatter diffraction and transmission electron microscopy, we study the rate of grain refinement and the uniformity in the evolution of microstructure in commercial purity Cu samples during high-pressure double torsion (HPDT). We aim to identify the processing conditions that would produce a microstructure that is both refined and uniform across the sample in grain size, texture, and intra-granular misorientation with minimal energy input. Two processing variables, pressure and number of turns, are probed. To provide a reference for HPDT, the investigation is also carried out using the standard high-pressure torsion (HPT) technique. For both processes, grain sizes decrease with the number of turns and applied pressure. Under pressure of 600 MPa and 4 torsional turns, HPDT provided a more homogeneous grain structure than HPT. Likewise, we also demonstrate that for the same processing condition, HPDT again produces the more homogeneous grain structure. It is found that a more homogeneous grain structure is achieved after doubling number of turns than doubling the pressure amount to 1.2 GPa. However, the rate of grain refinement substantially increases with doubling the pressure. Considering these results, the HPDT process, compared to HPT, takes better advantage of the role that high pressure plays in shear strain-induced grain refinement and homogenizing the microstructure. Last, analysis of the applied work finds that the least amount of work required for achieving fine and homogeneous microstructure occurs when the applied pressure is maximized and number of turns is minimized.

     

Elimination of porosity in directly fabricated titanium via cold gas dynamic spraying

Cold gas dynamic spray (cold spray) is a deposition technology in which particles achieve supersonic velocities. Commercial purity titanium (CP Ti) is directly fabricated to exploit the potential for cold spray to be utilised as a new fabrication technology. Deposition parameters for elimination of porosity were identified. Results show that using helium as the deposition gas has a significant influence on the elimination of pores. The volume fraction of porosity decreased with a decrease in the distance between the nozzle and the substrate. Furthermore, a decrease in average particle size led to a reduction in porosity of the deposited material. The results show that the directly fabricated CP Ti exhibits a higher hardness compared with wrought CP Ti. Optimisation of cold spray parameters led to a significant reduction of pores in directly fabricated parts.     

Approximation of integrable functions based on \phi -transform

Let \(E\) be a bounded subset of real line which contains its infimum and supremum. In this paper, we have defined the \(\phi -\) transform and its inverse, where \(\phi \) is a function from \(E\) into \((0,1]\) . We will have shown that real-valued integrable functions on \([a, b]\) and real-valued continuous functions on \(E\) can be approximated by this transformation with an arbitrary precision.     

Structures of intermetallic phases formed during immersion of H13 tool steel in an Al–11Si–3Cu die casting alloy melt

The structures of intermetallic alloy layers formed during immersion of H13 tool steel into an aluminium die casting alloy melt have been studied by X-ray diffraction. Energy dispersive spectroscopy (EDS) analysis on the intermetallic phases was also conducted. A thick composite layer away from the H13 steel substrate consisted of irregular intermetallics and solidified cast alloy. A thin intermetallic layer was present between the thick composite layer and an inner compact layer next to the steel substrate. The intermetallic phase in the composite layer was found to have a cubic structure, _bcc-(FeSiAlCrMnCu). The thin layer was identified to be structurally isomorphous with hexagonal _H-Fe₂SiAl₈. The compositional difference between _H and _bcc intermetallic phases was mainly that the latter consisted of a higher amount of Cr+Mn+Cu. This is consistent with the suggestion that chromium, manganese and copper stabilise _bcc phase at the expense of _H phase. The inner compact layer next to the steel substrate was identified to be isomorphous with orthorhombic η-Fe₂Al₅.     

PHEV powertrain co-design with vehicle performance considerations using MDSDO

Structural and Multidisciplinary Optimization - The complexity of plug-in hybrid-electric vehicles (PHEVs) motivates the simultaneous integration of component design and supervisory control...     

Investigating the microstructure and composition of cold gas-dynamic spray (CGDS) Ti powder deposited on Al 6063 substrate

The compositional variation, morphology and microstructure of cold gas-dynamic spray are of great importance for its proper application. This study investigates titanium powder deposition on an Al 6063 substrate using light optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composition was examined using energy dispersive X-ray spectroscopy (EDX), X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF SIMS). Optical and electron microscopy revealed heavily deformed Ti powder particles penetrating 10 to 30 μm into the Al substrate. Examination using TEM did not reveal any evidence of second phases at the interface suggesting a sharp transition between the two metals. The presence of nanocrystals and grain refinement of both the coating and the substrate suggest the formation of a partial hetero-epitaxy condition near the interface. EDX results from a dedicated high-resolution scanning transmission electron microscope showed a sharp compositional change with a maximum inter-diffusion region of about 5 nm. Bonding of the coating to the substrate is therefore thought to be achieved by the particle/substrate interlocking and direct metal to metal bonding. However, it is most likely that the refine crystalline structure near the interface will be beneficial to the adhesion of the coating. XPS and ToF SIMS provided evidence of nitrogen pick-up during the spray process in the form of N and TiN even when utilizing Helium as the gas carrier. The presence of TiN suggests reaction of the Ti with the entrained air during spraying which explains the occurrence of flashing jet outside the nozzle. Investigation of the material properties using nanoindentation showed reasonably consistent hardness and elastic modulus values throughout the titanium coating and at the transition region. The hardness was slightly higher than typical commercially available bulk Ti.     

Die erosion and its effect on soldering formation in high pressure die casting of aluminium alloys

Erosion of the die material during die filling has long been regarded as a possible damage mechanism of dies in high pressure die casting (HPDC) of aluminium alloys. Melt impingement and erosion have also been proposed to be an important step leading to die soldering. However, there is little information in the literature on the direct measurement of any kind of die erosion in HPDC. The present analysis, based on existing erosion theories, has shown that liquid impingement and solid particle erosion is not likely to occur in HPDC in the short-term while soldering does. There is a paucity of data indicating how cavitation erosion may occur in HPDC in the short-term. Pins were examined that were used as soldering targets in specially designed dies during soldering trails using a semi-industrial HPDC machine. Although these pins were subjected to severe melt impingement during the trails, very little erosion occurred before the formation of soldered layers.     

Balanced realizations of singularly perturbed systems

Balanced realizations of linear time-invariant singularly perturbed systems are studied. The reduced model of a singularly perturbed system that is obtained, based on the balanced realization, is compared to that obtained by time-scale considerations. An approximate balancing transformation for singularly perturbed systems is constructed based on balancing transformations of the approximating slow and fast subsystems.     

Adaptive algorithms for sparse system identification

In this paper, identification of sparse linear and nonlinear systems is considered via compressive sensing methods. Efficient algorithms are developed based on Kalman filtering and Expectation-Maximization. The proposed algorithms are applied to linear and nonlinear channels which are represented by sparse Volterra models and incorporate the effect of power amplifiers. Simulation studies confirm significant performance gains in comparison to conventional non-sparse methods.