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 90deg geometry employing a neutron wavelength of approximately l 1.7 A. 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 2x2x20 mm was used for measurements of the normal and transverse strain components, while this gauge volume was reduced to 2x2x2 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 2x2x20 mm. For the given experimental conditions, an average accuracy of ~100 mstrain 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 b 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