Nicholas P Hoye

Characterization of in-situ alloyed and additively manufactured titanium aluminides

Titanium aluminide components were fabricated using in-situ alloying and layer additive manufacturing based on the gas tungsten arc welding process combined with separate wire feeding of titanium and aluminum elements. The new fabrication process promises significant time and cost saving in comparison to traditional methods. In the present study, issues such as processing parameters, microstructure, and properties are discussed. The results presented here demonstrate the potential to produce full density titanium aluminide components directly using the new technique.

Measurement of residual stresses in titanium aerospace components formed via additive manufacturing

In the present study gas tungsten arc welding (GTAW) with automated wire addition was used to additively manufacture (AM) a representative thin-walled aerospace component from Ti-6Al-4V in a layer-wise manner. Residual strains, and hence stresses, were analysed quantitatively using neutron diffraction techniques on the KOWARI strain scanner at the OPAL research facility operated by the Australian Nuclear Science and Technology Organisation (ANSTO). Results showed that residual strains within such an AM sample could be measured with relative ease using the neutron diffraction method. Residual stress levels were found to be greatest in the longitudinal direction and concentrated at the interface between the base plate and deposited wall. Difficulties in measurement of lattice strains in some discrete locations were ascribed to the formation of the formation of localised texturing where α-Ti laths form in aligned colonies within prior β-Ti grain boundaries upon cooling. Observations of microstructure reveal basket-weave morphology typical of welds in Ti-6Al-4V. Microhardness measurements show a drop in hardness in the top region of the deposit, indicating a dependence on thermal cycling from sequential welds.

Prediction of welding stresses in WIC test and its application in pipelines

In the present study, the Welding Institute of Canada (WIC) restraint test was used to simulate the restraint conditions of full-scale girth welds on energy pipelines to ascertain the influence of welding process parameters on welding stresses. Finite element models are developed, and validated with neutron diffraction measurements, to evaluate the welding stresses for under-matched, matched and over-matched welds. The effects of heat input, wall thickness and variable restraint lengths of WIC sample are systematically investigated. As a practical outcome, this work can help in selection of the appropriate restraint length for WIC tests to simulate the specified stress conditions in the pipeline, and, ultimately, reduce the risk of Hydrogen Assisted Cold Cracking (HACC) in high strength low alloy. This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.

Resistance heated pressing (RHP): A novel technique for fabrication of titanium alloys

A novel powder metallurgical technique for the fabrication of titanium alloys has been developed by utilizing a pressure-assisted, resistance-heating sintering technique. In this technique, the high electrical resistance of oxide layers present on the surface of powder particles has been exploited to ensure effective resistance heating of green compacts. Ti-6Al-4V pre-alloyed powders of 100 µm size were compressed while being heated under a variety of conditions of sintering temperature, pressure and time. The outcomes of our experiments have proven that resistance heating can be a very effective means of heating during powder consolidation. The results have indicated that the required sintering time and temperature in the new resistance-heated sintering technique are much reduced in compared to sinter-press and/or hot isostatic pressing techniques, resulting in a refined microstructure with a concomitant improvement in mechanical properties.

Experimental Investigation of Welding Stresses in MWIC Weldability Test

The use of high-strength steels in the manufacture of energy pipelines, coupled with the transition to larger pipe diameters and greater wall thicknesses, has led to an increased potential for cracking including hydrogen assisted cracking of energy pipelines due to higher constraint induced stresses. In the present study, a modified version of the Welding Institute of Canada (MWIC) restraint test was used to simulate the constraint conditions of full-scale girth welds on energy pipelines, allowing the influence of welding process parameters on crack formation to be assessed. MWIC test samples of X70 grade high-strength low alloy pipeline steel were manually welded using two different welding processes, namely shielded metal arc welding (SMAW) and modified short-arc welding (MSAW). Residual strains, and hence stresses, in these samples were analysed quantitatively using neutron diffraction technique. Overall, results indicate that the modified WIC restraint test produces significant residual stresses and so is effective in constraining the root run and in consequence studying the hydrogen assisted cracking of high-strength pipeline steels. Introduction The construction of Australian oil and gas pipeline networks is carried out using high strength low alloy (HSLA) steel line pipe and employing Shielded Metal Arc welding (SMAW) in conjunction with hydrogen rich cellulosic consumables. The application of cellulosic electrodes at ambient temperature ensures good weld penetration and shorter construction lead time with huge cost saving [1]. The drawback however is the risk of hydrogen cracking emanating from high hydrogen content of cellulosic electrodes and the high levels of restraint as a result of clamping and lifting stresses the pipeline is subjected to during construction. The Welding Institute of Canada (WIC) test was developed more than 30 years ago to provide a simple and economical way for the assessment of weldability and risk of hydrogen assisted cold cracking. Over the past three decades it was extensively utilised by the industry to qualify pipeline welding procedures [2-5]. However, the geometry of WIC test has some shortcomings associated with the difficulty for instrumentation of the WIC for the physical measurements of strain and temperature distribution during welding. This is further exacerbated with a low success rate of achieving industry acceptable welds in the WIC test environment. The original WIC design has been modified to better represent stovepipe welding of pipelines, as shown in Fig.1. The modifications enabled better control of welding mechanics and thus achieving reasonable success rate in depositing an industry acceptable weld [6]. The new designed geometry enables easier access for instrumentation and more reproducible production of test specimens. To ensure that the modifications did not modify significantly the thermal and restraint Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 557-562 doi: http://dx.doi.org/10.21741/9781945291173-94 558 conditions of the test and the results are comparable with those achieved in other studies using the WIC test, physical and Finite Element studies for the original and modified test geometries have been undertaken [6, 7]. The results have shown that the modifications to the design have little or no influence on the thermal and mechanical properties of the test, but improve the ease with which consistently high quality samples are produced. The results have shown the modifications to the design have little or no influence on the thermal and mechanical properties of the test, but improve the ease with which consistently high quality samples are produced. The modified test procedure is now accepted for a wide range of investigations. Due to the novelty of this weldability test, there is no report on the prediction and measurement of residual stresses, which is one of the influential factors in weld metal cracking including hydrogen assisted cold cracking (HACC), in the MWIC test. Moreover, no direct link has been established so far between the welding processes of the MWIC test and the welding stresses. To address the above issues, the current paper is focused on an investigation of the effects of welding process on residual stresses in the MWIC test. Residual stress measurements in these samples were analysed quantitatively using neutron diffraction techniques on the KOWARI strain scanner at the OPAL research facility operated by the Australian Nuclear Science and Technology Organisation (ANSTO). Experimental procedure Details of weld deposition procedure The weld consumable was specified to be a E6010 electrode for the shielded metal arc welding (SMAW) with a diameter of 3.2 mm while for the MSAW the root pass was completed with a ER70s-6 electrode. The chemical composition of both electrodes is shown in Table.1. Four samples were fabricated, two samples was used to measure the lattice spacing (d0,hkl) in a stress free mode for both welding processes and the other two samples were used to evaluate the residual stresses for SMAW and MSAW process. The yield strength of the parent metal is 490-520 MPa. Table 1: Chemical composition of welding consumables and parent metal

Residual Stress and Critical Crack Size before and after Post-Weld Heat-Treatment

Post-weld heat-treatment (PWHT) is performed to reduce residual stress, but is not always possible to perform. The residual stresses on a thick section weld on a gas pipeline were determined before and after PWHT to assess residual stress and critical defect sizes.

Stress in Thin Wall Structures Made by Layer Additive Manufacturing

Manufacturing of thin wall structures is one of the main applications of additive manufacturing, where it has significant advantages over traditional milling and machining techniques or welded analogues. Such thin walled structures are common in structural aerospace components, and are also frequently made from titanium alloys. For such large-scale components, layer deposition strategy is more advantageous rather than a pixel-wise deposition approach due to the demand for high productivity and size requirements. Several techniques can be used to produce layer-wise buildups, including laser-powered Direct Metal Deposition (DMD) process or gas tungsten arc welding (GTAW). Although, in the general case of arbitrary thin wall structures the stress distribution is complex, for some simple geometries, the stress state is simple and can be well characterized within a model by a single parameter representing a layer deposition stress in the steady-state regime. The model calculations were verified by experimental results on a thin-walled sample component that was manufactured from Ti-6Al-4V by GTAW with the residual stresses measured using KOWARI neutron strain scanner at the OPAL research reactor (ANSTO). Introduction Titanium based alloys are very widely used in aerospace industry due to their high specific strength, fatigue properties and excellent corrosion/oxidation resistance [1], and frequently need to be shaped into thin-wall structures, such as wing ribs and spars, various structural elements, gear boxes, etc. Traditional metal forming (e.g. rolling, extrusion) and machining methods are difficult, labour/cost intensive [2] and frequently extremely wasteful, especially when thin-wall structures are to be fabricated, resulting in unacceptably high buy-to-fly ratios. In comparison, additive manufacturing (AM) of titanium components seems to be the most attractive manufacturing technology, dramatically improving manufacturing costs and reducing waste to minimum with almost no limitation on the component shape. While some components for aerospace applications are small scale (<300 mm) and can be readily manufactured by powder-bed techniques such as Selective Laser Melting (SLM) or Electron Beam Melting (EBM), large-scale major airplane components present challenges due to size limitations of the build chamber. In this case, other techniques such as blownpowder based Direct Metal Deposition (DMD) or wire-fed based Wire-Arc Additive Manufacturing (WAAM) are used. The latter is also often called Wire-Arc Additive Layer Manufacturing (WAALM) since fabrication route involves multiple-pass deposition to build wall-like structures in a layer-by-layer manner to produce the ‘near net shape’ profile. Development of these techniques into industrial scale for high quality production of engineering components from titanium alloys in an economically efficient way is a general challenge for AM technology at present. Although many technological process parameters such as deposition energy and speed, feedstock deposition rate, built-up trajectory, clamping system, resulting microstructure and defect structure are Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 497-502 doi: http://dx.doi.org/10.21741/9781945291173-84 498 to be considered when optimising product quality and production costs, the residual stress is one of the most serious issues, being the core reason for significant bowing, bending and deflection, often resulting in compromised dimensional tolerances. For experimental studies, a single wall structure (T-shape sample) of a constant thickness and height, built on a rectangular base plate appears to be a standard choice [3-6]. Depending on the material, exact dimensions of the build-up and the process, the deflection of the back side of the base plate can vary greatly, but a typical reported deflection of the base plate in case of Ti-6Al-4V is 14 mm per 1000 mm longitudinal base length [6]. In addition to causing distortion, residual stresses may have detrimental effects on mechanical properties, especially fatigue behaviour, thereby degrading the performance of the component in service. Considering AM process of a T-shape sample, two distinct steps or episodes in the overall stress formation can be isolated. When first layer is deposited a stress distribution in the base plate is created which is essentially very similar in nature and stress distribution to a single-bead welding path [5]. Although second and third passes can modify the stress distribution created by the first pass, all consecutive passes outstanding several mm from the base plate do not affect the base stress distribution. Instead they build-up stress in a different manner dictated by the geometry of the thinwall structure. If the wall thickness is much smaller than the wall height and length, the zero plane stress condition is applicable with transverse stress (through thickness) being equal to zero. The aim of the current study is to investigate residual stress build-up in a T-shaped sample made from Ti-6Al-4V by WAALM in order to quantify residual stress process within an empirical model. One of the multiple possibilities is to consider different scenarios for clamping and its influence on the resultant residual stress. This quantification will allow certain conclusions to be drawn about the mechanism(s) of residual stress formation, hence allowing the prediction of residual stresses in samples of different dimensions and, possibly, to evaluate residual stress mitigation strategies. Sample production, process and materials WAALM, as an arc-based deposition process can be realised in several ways, it can utilize either the gas metal arc welding (GMAW) or the gas tungsten arc welding (GTAW). The latter version has been developed in the University of Wollongong as a practical AM method with high deposition rate enabling production of large components [7]. In this process, build-up of a three-dimensional near-net shape freeform is achieved through deposition of a single row of successive weld beads onto a substrate to produce a component in a layer-wise manner. The process utilizes robotic automation that can be programmed to suit the design of the future component [8]. Using the reported WAALM, a thin-wall of Ti-6Al-4V was built to the full length along the centre line of a Ti-6Al-4V base plate (L250xW100xT12, mm). The resulting build-up thin wall was approximately 8 mm in thnessi and 40 mm in height. The as-deposited was further machined via conventional milling to a wall of high precision dimensions with 5 mm thickness, 36 mm height and 178 mm length. The final shape of the component is shown in Fig. 1. Parameters of deposition GTA welding process have been already reported [9]. The base plate, attached to a linear actuator, was moved with a travel speed of 150 mm/min, while the welding torch and wire feed were held stationary. Deposition was conducted using a current-controlled power source operating at a steady state current of 110 A and giving an average arc energy of 485 J/mm. Filler wire of 1.0 mm diameter was fed at a rate of 2000 mm/min to provide a specific energy input of approximately 10 kJ per gram of deposited material. Another sample was produced at conditions Fig. 1. Design of the thin-wall structure produced by WAALM with indication of the principal directions. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 497-502 doi: http://dx.doi.org/10.21741/9781945291173-84 499 of 20 kJ per gram of deposited material by reducing the travel speed by a factor of two Both, the base plate and the filler wire materials were commercially sourced and produced to ASTM B265 and B863 standards respectively. Neutron diffraction experiment and data analysis Neutron residual stress measurements of the wall-structure were performed on the KOWARI neutron diffractometer at OPAL research reactor at ANSTO [10]. For the measurement the Ti(103) reflection was used at 90°-geometry employing a neutron wavelength of approximately λ = 1.7 Å. Three principal directions were measured, the normal, transverse and longitudinal across the wall height (from base plate to the top of the wall) in the middle portion of the 250 mm long sample with variable density of the measurement point (1 mm close to the base plate and 2 mm away from the base plate). More than 20 experimental points along the wall height were obtained by scanning. To use efficiently sample geometry, a gauge volume with size of 2×2×20 mm was used for measurements of the normal and transverse strain components, while this gauge volume was reduced to 2×2×2 mm when the longitudinal component was measured. To acquire better grain statistics, the gauge volume was moved during measurements of the longitudinal component to cover equivalent volume of 2×2×20 mm. For the given experimental conditions, an average accuracy of ~100 μstrain was achieved providing stress accuracy of ~20 MPa in terms of calculated stresses (errors only due to the neutron counting statistics). With respect to determining the sample d0, the more standard approach of cutting small coupons that can be assumed stress free proved unreliable due to poor statistics associated with the comparatively large prior β-Ti grain size. As only the stress distribution in the thin wall section was of interest, an alternative approach was employed where the through thickness stress was assumed to be zero in the thin wall section. This assumption of a plane stress condition is considered valid due to the 5 mm wall thickness being of similar size to the ~3 mm gauge volume (or spatial resolution), and was seen to give results with great accuracy. Based on this condition, three d-spacings for three directions could be resolved into two stress components (longitudinal and normal) and d0. Modelling of stress profile in a wall A simple model can be considered base

Weldability of Ti-6Al-4V Alloys Formed via Various Powder Consolidation Techniques

This study considers the weldability of Ti-6Al-4V alloys formed by various powder consolidation methods. Samples were prepared from commercially sourced pre-alloyed Ti-6Al-4V powder using both conventional press-and-sinter (PS) and the new novel resistance-heated pressing (RHP) methods. Fusion welding was executed by the gas tungsten arc (GTA) process with arc stability assessed in-situ by observations of the arc as well as monitoring of transient arc voltage. Results indicated equivalent arc stability between samples of RHP and commercially sourced wrought material while samples formed by PS showed high instability in arc initiation, attributed to high levels of porosity. Post weld analysis of mechanical hardness in powder based samples revealed no significant deviation in weld metal properties from welds conducted on commercially sourced wrought material. In all cases weld microstructures typical of Ti-6Al-4V alloys were observed with significant grain growth in the fusion and heat affected zones. Samples prepared by PS methods showed internal porosity due to gas evolution upon solidification, which may again be attributed to the highly porous initial microstructure.