High strength titanium alloys are generally used in widespread applications ranging over, but not limited to biomedical, aerospace, automotive, marine, oil and gas, and energy. Besides other ...manufacturing processes, forming is one of the common manufacturing process used to produce components out of these alloys. Forming processes generally involve significant plastic deformation of material under complex multiaxial loading conditions. Titanium alloys undergo considerable plastic deformation before failure while later is governed by the mechanisms of void nucleation, growth and coalescence. A number of titanium alloys used for high strength applications are multiphase alloys having α and β phases. It has been reported in the past that the voids tend to nucleate on the phase boundaries. This study is focused on understanding the growth of the nucleated voids at two selected locations in a dual phase titanium alloy (Ti-10V-2Fe-3Al); globular α phase (hexagonal closed pack, HCP) and at the interface of lamellar α and β phases (α - HCP and β – body centred cubic, BCC). This is one of the very few 3D representative volume element (RVE) study of void growth in single crystal titanium (HCP), carried out using crystal plasticity finite element modelling (CPFEM) at higher triaxialities (ranging 1/3-3) and the first one on the interface of bicrystals with different crystal symmetry. The effects of initial porosity, crystal orientation and the Lode parameter on void growth in single crystal (α-HCP) has been studied and it is found that they affects void growth considerably. An effort has been made to explain the physics behind it. In the second part, growth in a void at the interface of two distinct single crystals (α - HCP and β –BCC) was studied. The effects of Burgers orientation relationship (BOR) variant of the two phases, initial porosity, and phase boundary inclination (PBI) on void growth is investigated. It is found that the PBI has a very strong impact on the void growth. The effect of initial porosity is similar to the void growth in single crystals. Choice of BOR variant affected the void growth in moderate triaxialities.
•A 3D CPFEM RVE void growth study in a dual phase Ti alloy is presented.•Two cases of nucleated voids in α-β Ti alloys are studied; α and α-β phase boundary.•A very first void growth study at interface of dissimilar α-β crystals is presented.•Initial porosity, loading & crystal orientation have strong effect on void growth.•Phase boundary inclination was found to greatly influence void growth in bicrystal.
In this work, the effect of pulsed laser used during the powder bed fusion (L-PBF) additive manufacturing (AM) process on Inconel 718 (IN718) material properties has been investigated. Argon gas ...atomised (AGA) IN718 powder is characterised in terms of flow, density, particle size distribution and morphology. Powder shows mostly spherical morphology with Hausner ratio of 1.17 indicating good flow characteristics. Density optimisation trials are carried out by varying laser power and exposure time. Fabricated samples are characterised in terms of porosity by area fraction analysis using light microscopy and volume fraction analysis using X-ray microcomputed tomography (micro-CT). Minimum porosity of 0.16% is achieved for laser power of 200 W and exposure time of 110 μs Microstructural analysis using the Electron Backscatter Diffraction (EBSD) technique shows limited columnar grain structure in the Z direction and more equiaxed type grains in the XY direction (normal to the Z direction). Tensile test results show 754 MPa yield strength, 1070 MPa ultimate tensile strength and ~24% elongation. Finally, hole drilling residual stress measurements show increase from ~0 MPa to over 450 MPa in tensile stress up to a depth of 1 mm from the top surface of the as-build L-PBF IN718 sample. It has been found that laser pulsing produces higher homogeneity in grain structure and better mechanical properties than that by the continuous laser method.
The potential to improve the mechanical properties of adhesive joints via micro-structured interlocking features is investigated. The micro-structured surfaces were fabricated in polycarbonate via ...injection moulding from a master template. The specimens were then bonded in an interlocking configuration to form single lap joints and tested to failure in tension. Planar untreated (i.e. un-abraded) and planar roughened (i.e. abraded) samples were also tested to provide benchmarks. Compared to the planar roughened case, results show that micro-structuring the interface can yield up to a 95.9% increase in strength and up to 162% increase in work to failure. Increases in strength and work to failure beyond the planar roughened level are attributed to mechanical interlocking of features. As deformation proceeds, progressive bending of each pair of interlocking features develops an increasing resistive load which allows the total load to significantly exceed that of the planar roughened case. Work to failure is increased via a combination of increased maximum force, increased displacement enabled by microfeature bending and a more torturous crack path. Low clearances between interlocking features were found to be favourable for mechanical properties owing to reduced bending stiffness of the repeating periodic unit at the interface.
The optimisation of processing parameters to produce high densification AlSi10Mg parts by laser powder bed fusion (LPBF) has received considerable attention in recent years. Nonetheless, it is ...important to consider the potential presence of as-built large pores in real world applications, e.g. due to limitations of the available LPBF system, time and cost constraints associated with producing near-perfect density and so on. In this work, recycled powder was used to fabricate AlSi10Mg specimens with sub-optimal densification by LPBF and an experimental investigation into the evolution of specimen porosity occurring under increasing tensile load was performed. A combination of high-resolution X-ray micro computed tomography (XμCT) and an in-situ micro-testing stage was employed to acquire 3D images at different loading stages. Specimens were tested in the as-built condition and following hot isostatic pressing (HIPping) or HIPping with T6. As-built porosity did not change markedly in the lead-up to brittle-like fracture. Pores within ductile HIPped specimens were uniformly elongated up to the onset of damage propagation and pore coalescence. Pore shape change occurred largely without volume change at small extension. HIPping plus T6 produced a compromise between as-built and HIPped conditions in terms of the extent of pore modification observed prior to failure.
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•A combination of X-ray tomography and micro-testing has demonstrated a powerful solution for characterizing in-situ microvoid growth and coalesce in aluminium alloy AlSi10Mg.•Process conditions (as-built, hot isostatic pressing (HIP) and HIP+T6) have shown a significant impact on 3D pore evolution within AlSi10Mg.•Under tensile deformation, microvoids in the as-built condition do not change markedly while the most substantial change occurs in the HIPped material
The aim of this work is to determine the impact of post-thermal treatments on the mechanical properties and microstructure of alloy 718 superalloy manufactured through modulated laser powder bed ...fusion. Three post-thermal treatments at 980 °C and 1200 °C with and without hot isostatic pressing are utilised, followed by standard ageing procedure. The tensile mechanical properties, microstructure, crystallography, morphology, fracture impact and hardness after each heat treatment are determined. It has been found that the heat treatment procedure at 980 °C significantly affects the tensile properties while leading to no change in grain size or orientation, and formation of twined grains. A temperature of 1200 °C with and without pressure causes considerable grain growth in comparison with the as-built and 980 conditions and significant formation of twinned grains. Hot isostatic pressing was found to produce the tensile mechanical properties close to those of wrought alloy 718.
The contour method is one of the most prevalent destructive techniques for residual stress measurement. Up to now, the method has involved the use of the finite-element (FE) method to determine the ...residual stresses from the experimental measurements. This paper presents analytical solutions, obtained for a semi-infinite strip and a finite rectangle, which can be used to calculate the residual stresses directly from the measured data; thereby, eliminating the need for an FE approach. The technique is then used to determine the residual stresses in a variable-polarity plasma-arc welded plate and the results show good agreement with independent neutron diffraction measurements.
An approach to producing interfaces with tailored and repeatable normal contact stiffness using micropatterned surfaces is developed. A finite element model is first used to design square wave ...interfaces having a range of stiffnesses, and these are fabricated in polycarbonate via a microfabrication process. Results demonstrate that the contact stiffnesses of the fabricated interfaces are both tailorable and repeatable. The approach can be broadened to other materials and is useful for applications requiring specified interface stiffness. Finally, even with these deterministic interfaces, we show that low levels of roughness on the surface features are sufficient to produce a load-dependent contact stiffness at lower loads. Therefore, tailorability is mostly applicable above this limit where total contact stiffness converges to a load-independent value.
Graphic Abstract
The dry friction of surfaces with nanoscale roughness and the possibility of using micropatterning to tailor friction by manipulating contact area is investigated. Square wave patterns produced on ...samples from silicon wafers (and their unstructured equivalent) were slid against unstructured silicon counter surfaces. The width of the square wave features was adjusted to vary the apparent feature contact area. The existence of nanoscale roughness was sufficient to ensure Amontons’ first law (F = μP) on both structured & unstructured samples. Somewhat counterintuitively, friction was independent of the apparent feature contact area making it difficult to tailor friction via the feature contact area. This occurred because, even though the apparent feature contact area was adjusted, the surface roughness and nominal flatness at the contact interface was preserved ensuring that the real contact area and thereby the friction, were likewise preserved. This is an interesting special case, but not universally applicable: friction can indeed be adjusted by structuring provided the intervention leads to a change in real contact area (or interlocking)– and this depends on the specific surface geometry and topography.
•Investigates dry friction of flat & structured surfaces with nanoscale roughness.•Studies micropatterning for tailoring of friction via manipulation of contact area.•Confirms validity of Amontons' first law for surfaces with nanoscale roughness.•Highlights special case with friction invariant of apparent feature contact area.•Tailoring friction via this approach requires changing the real contact area.
Abstract
Ductile metals undergo a considerable amount of plastic deformation before failure. Void nucleation, growth and coalescence is the mechanism of failure in such metals.
α
–
β
titanium alloys ...are ductile in nature and are widely used for their unique set of properties such as specific strength, fracture toughness, corrosion resistance and resistance to fatigue failures. Voids in these alloys have been reported to nucleate on the phase boundaries between
α
and
β
phase. Based on the findings of crystal plasticity finite element method investigations of the void growth at the interface of
α
and
β
phases, a void nucleation, growth, and coalescence model has been formulated. An existing single-phase crystal plasticity theory is extended to incorporate underlying physical mechanisms of deformation and failure in dual phase titanium alloys. Effects of various factors stress triaxiality, Lode parameter, deformation state (equivalent stress), and phase boundary inclination on void nucleation, growth and coalescence are used to formulate a phenomenological constitutive model while their interaction with a conventional crystal plasticity theory is established. An extensive parametric assessment of the model is carried out to quantify and understand the effects of the material parameters on the overall material response. Performance of the proposed model is then assessed and verified by comparing the results of the proposed model with the RVE study results. Application of the constitutive model for utilisation in the design and optimisation of the forming process of
α
–
β
titanium alloy components is also demonstrated using experimental data.
When defects reach a critical size, failure occurs in engineering components. The criticality assessment of defects is hence a key aspect of the structural integrity of gas pipelines in service. ...Fitness-For-Service (FFS) assessments are generally employed for evaluating the criticality of a crack-like flaw in structures using simplified assumptions in relation to geometry and material properties. Whilst the errors resulting from these modelling simplifications prove acceptable in many cases, there are situations where it will be necessary to take into account the nonlinearities in geometry and/or material behaviour. This can be either to avoid excess conservatism or, on the contrary, to ensure the results are safe. In such cases it becomes essential to develop a finite element model of the structure to account for such real-engineering complexity. Welding, the most prevalent technique to join pipe, often brings about a misalignment between two pipes and hence complex crack shape is formed. The aim of this study is to develop an elastoplastic finite element model of a gas pipeline possessing a crack in a misaligned weld. The remaining life of the pipeline is determined using a Failure Assessment Diagram (FAD) and the Paris law. The results obtained from the finite element method to determine the stress intensity factors are compared to results derived using the API-579 for stress intensity factors calculations.
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