Evaluation of polychromatic x-ray radiography defect detection limits in a sample fabricated from Hastelloy x by selective laser melting

Selective laser melting is a rapidly maturing additive manufacturing technology ideally suited to the net-shape fabrication of high value metallic components with complex shapes. However, if the processing conditions are poorly controlled, internal defects such as cracks or pores filled with metal powder may be present and impair the properties. As a result, a non-destructive defect detection method needs to be found that is suited to this application. In this work, a staircase sample was designed and fabricated from Hastelloy X by selective laser melting with step thicknesses ranging from 0.8 mm to 10 mm and with each step containing the same series of custom-made spherical, rod-shaped and coin-shaped defects arranged in different orientations and ranging from 0.2 mm up to 2 mm in size. The sample was exposed to various X-ray radiography testing and analysis methods. In particular, a theoretical and experimental evaluation of defect detection limits by polychromatic X-ray absorption radiography was performed based on the measurable contrast, which depends on both defect size and shape and slab thickness. The experimental data suggest that the minimum detectable contrast is about 1-2% when using X-rays with a very broad spectrum. This equates to a minimum detectable defect size of about 0.2 mm for a Hastelloy X slab thickness of <2 mm. The experimental findings are in good agreement with theoretical expectations. The theoretical framework provides a criterion for estimating contrast, which is useful for optimising the experimental conditions. Polychromatic X-ray absorption radiography represents a simple and effective non-destructive investigation technique. Methods for further improving the defect detection limits are also discussed and examples relative to computed tomography are reported

Shape memory effect in Ti-Ni alloy thin sheets/foils produced via ultrafine laminates

ln the present paper, we describe a new processing method to produce thin foils of TiNi shape memory alloys starting from ultrafine laminates of pure Ti and Ni. The final thickness of the Ti/Ni laminate is 100 or 50 $\mu$m after hot and cold rolling. The laminates are diffusion treated at 1073-1173 K to obtain TiNi B2 phase. Some samples are further subjected to low temperature aging at 673-773 K in order to study two-way shape memory effect. The shape memory strain in foils with 50 $\mu$m thickness is about $1.25 \times 10^{-2}$ after diffusion treatment. Two-way memory of $2 \times 10^{-4}$ was also found in 50 $\mu$m foils

The effect of hydrogenation on the ECAP compaction of Ti–6Al–4V powder and the mechanical properties of compacts

The effect of hydrogen content on the compaction of Ti&ndash;6Al&ndash;4V powder at low temperatures, namely 500 &deg;C, using equal channel angular pressing (ECAP) with back pressure has been investigated. The properties of the compacts before and after a heat treatment and de-hydrogenation cycle have been determined. Compaction of powder by ECAP (500 &deg;C and 260 MPa) has shown maximum levels of relative density of 99.3% and 99.4% when charged with 0.05&ndash;0.1 wt.% and 0.61&ndash;0.85 wt.% of hydrogen, respectively. After the de-hydrogenation heat treatment the diffusion bonding between individual powder particles was completed and the microstructure was altered, depending on the level of hydrogen content. Two local maxima of 99.2% and 98.1% were observed in the measured density of consolidated compacts for hydrogen contents between 0.05 wt.% and 0.1 wt.% and between 0.61 wt.% and 0.85 wt.%, respectively. However, the mechanical properties of the compacts within these two ranges of hydrogen content were significantly different due to a difference in the observed microstructure. An exceptionally high ductility of 29%, in combination with a relatively high strength of ~560 MPa, was measured in a shear punch test on specimens which had a prior hydrogen level of 0.05 wt.% before the heat treatment. It was shown that material consolidated from powder hydrogenated to low levels of hydrogen before compaction has the potential to offer substantial improvements in mechanical properties after a suitable heat treatment.<br /

Evolution of nanoscale porosity during equal-channel angular pressing of titanium

Small-angle neutron scattering (SANS) analysis and transmission electron microscopy evidence suggest the occurrence of nanoscale porosity in commercial-purity titanium processed by equal-channel angular pressing (ECAP). SANS data were produced at two different facilities (GKSS, Germany; and Los Alamos, USA) and were analysed using three different methods. The results are consistent and yield a conclusive picture of the distribution of the scattering centres, which are believed to be associated with nanoporosity. Back pressure applied during ECAP tends to reduce the average pore size, which also depends on the processing route used. The results of the study strongly suggest that ECAP leaves a footprint in titanium in the form of a population of polydispersed nanovoids, which may play an important role in subsequent processing of the material

The influence of temporary hydrogenation on ECAP formability and low cycle fatigue life of CP titanium

It is generally believed that thermo-hydrogen processing has a beneficial effect on tensile ductility and fatigue properties of titanium. This study was concerned with investigating whether this also applies to titanium of commercial purity (CP) with an ultrafine-grained structure obtained by equal-channel angular pressing (ECAP). It was shown that despite the possibility to manipulate the microstructure of titanium the thermo-hydrogen processing offers, temporary hydrogenation was not able to improve ductility and low cycle fatigue life of CP titanium over the levels achievable by straight ECAP

A uniaxial tensile stage with tracking capabilities for micro X-ray diffraction applications

First results are presented for a uniaxial tensile stage designed to operate on a scanning micro X-ray diffraction synchrotron beamline. The new tensile stage allows experiments at typical loading cycles used in standard engineering stress&ndash;strain tests. Several key features have been implemented to support in situ loading experiments at the intragranular length scale. The physical size and weight of the load cell were minimized to maintain the correct working distance for the X-ray focusing optics and to avoid overloading the high-resolution raster scan translation stages. A high-magnification optical microscope and image correlation code were implemented to enable automated online tracking capabilities during macroscopic elongation of the sample. Preliminary in situ tensile loading experiments conducted on beamline 12.3.2 at the Advanced Light Source using a polycrystalline commercial-purity Ti test piece showed that the elastic&ndash;plastic response of individual grains could be measured with submicrometre spatial resolution. The experiments highlight the unique instrumentation capabilities of the tensile stage for direct measurement of deviatoric strain and observation of dislocation patterning on an intragranular length scale as a function of applied load