CERES :: Browsing by Author "Williams, Stewart W."

Browsing by Author "Williams, Stewart W."

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Open Access
Analysis of fracture toughness properties of wire + arc additive manufactured high strength low alloy structural steel components
(Elsevier, 2020-08-14) Dirisu, Philip; Ganguly, Supriyo; Mehmanparast, Ali; Martina, Filomeno; Williams, Stewart W.
The uncertainty surrounding the fracture behaviour of CMT-WAAM deposited steel, in terms of crack tip condition (J and CTOD) needed to cause crack tip extension, has made this manufacturing technique unpopular to date. Fracture toughness parameters are crucial in the structural integrity assessment of components and structures in various industries for assessing the suitability of a manufacturing process and material. In the offshore wind industry, the EN-GJS-400-18-LT ductile cast grade for the mainframe and hub has lower fracture toughness resistance for its high strength grade. Its high weight level affects the Eigen frequency of the tower and imposes high installation cost incurred from heavy lifting equipment usage. Poor fracture toughness is currently a challenge for wind turbine manufacturers in the quest for a cleaner and cheaper energy in the form of offshore wind. In this study, CMT-WAAM is used in depositing steel components with an oscillatory and single pass deposition strategy. The effects of microstructural variation, as a result of layer by layer deposition and the layer band spacing, on the fracture resistance in the build and welding direction was shown here. The fracture mechanics and failure mode of the WAAM deposited parts were investigated. The microstructural variation, again as a result of the layer by layer deposition and the layer band spacing, are the key parameters that control the fracture toughness of WAAM steel. Anisotropic behaviour in the values was observed between both fracture orientations. The constructive transformation mechanism of the WAAM oscillatory process made way for intragranular nucleation of acicular ferrite on the Ti containing inclusion, thereby improving the toughness of the ER70S-6 deposit with a unique microstructure and Jq value of 640kJ/m2.
Open Access
Analytical model for distortion prediction in wire plus arc additive manufacturing
(Materials Research Forum, 2018-10-05) Hönnige, Jan Roman; Colegrove, Paul A.; Williams, Stewart W.
An analytical model was developed to predict bending distortion of the base-plate caused by residual stresses in additively manufactured metal deposits. This avoids timeconsuming numerical simulations for a fast estimation of the expected distortion. Distortion is the product of the geometry factor K, which is determined by the cross-section of substrate and deposit, and the material and process factor S, which is the quotient of residual stress and the Young’s Modulus. A critical wall height can be calculated for which the structure distorts the most. This critical height is typically less than 2.5 times the thickness of the substrate. Higher walls increase the stiffness of the cross-section and reduce the distortion with increasing height.
Open Access
Analytical process model for wire + arc additive manufacturing
(Elsevier, 2018-04-03) Ríos, Sergio; Colegrove, Paul A.; Martina, Filomeno; Williams, Stewart W.
An analytical process model for predicting the layer height and wall width from the process parameters was developed for wire + arc additive manufacture of Ti-6Al-4V, which includes inter-pass temperature and material properties. Capillarity theory predicted that cylindrical deposits were produced where the wall width was less than 12 mm (radius <6 mm) due to the large value of the surface tension. Power was predicted with an accuracy of ±20% for a wide range of conditions for pulsed TIG and plasma deposition. Interesting differences in the power requirements were observed where a surface depression was produced with the plasma process due to differences in melting efficiency and/or convection effects. Finally, it was estimated the impact of controlling the workpiece temperature on the accuracy of the deposit geometry.
Open Access
Application of laser in seam welding of dissimilar steel to aluminium joints for thick structural components
(Elsevier, 2015-04-01) Martins Meco, Sonia Andreia; Rodrigues Pardal, Gonçalo; Ganguly, Supriyo; Williams, Stewart W.; McPherson, Norman
Laser welding-brazing technique, using a continuous wave (CW) fibre laser with 8000 W of maximum power, was applied in conduction mode to join 2 mm thick steel (XF350) to 6 mm thick aluminium (AA5083-H22), in a lap joint configuration with steel on the top. The steel surface was irradiated by the laser and the heat was conducted through the steel plate to the steel-aluminium interface, where the aluminium melts and wets the steel surface. The welded samples were defect free and the weld micrographs revealed presence of a brittle intermetallic compounds (IMC) layer resulting from reaction of Fe and Al atoms. Energy Dispersive Spectroscopy (EDS) analysis indicated the stoichiometry of the IMC as Fe2Al5 and FeAl3, the former with maximum microhardness measured of 1145 HV 0.025/10. The IMC layer thickness varied between 4 to 21 μm depending upon the laser processing parameters. The IMC layer showed an exponential growth pattern with the applied specific point energy (Esp) at a constant power density (PD). Higher PD values accelerate the IMC layer growth. The mechanical shear strength showed a narrow band of variation in all the samples (with the maximum value registered at 31.3 kN), with a marginal increase in the applied Esp. This could be explained by the fact that increasing the Esp results into an increase in the wetting and thereby the bonded area in the steel-aluminium interface.
Open Access
Arc instabilities during split anode calorimetry with the TIG welding process
(Springer, 2018-03-29) Egerland, Stephan; Colegrove, Paul A.; Williams, Stewart W.
Split anode calorimetry investigations of tungsten inert gas (TIG) arcs were conducted using a large-diameter shielding gas nozzle. Some conditions displayed arc symmetry whilst others proved distinctly asymmetric. The variation of welding current and electrode tip to workpiece distance (ETWD) was studied. Decreasing the ETWD was found to increase the current density towards the arc axis but similar to a previous study of the authors, Gaussian distributions were not observed. The gas nozzle was designed to produce laminar gas flow and sound shielding behaviour; however, anode surface oxidation was found after welding, presumably caused by shielding gas contaminated with oxygen through the welding sequence. Therefore, axial arc symmetry was influenced by random effects. The conditions and reasons for the observed phenomena are explored within this paper.
Open Access
Arc welding of high strength aluminium alloys for armour systems applications
(Cranfield University, 2011) Pickin, Craig Graeme; Williams, Stewart W.
The ternary Al-Cu-Mg system 2xxx series aluminium alloys were examined as construction materials for armour system applications based upon comparable ballistic properties to the currently employed Al-7xxx series alloys. Utilising MIG welding solidification cracking was evident when welding constrained Al-2024 candidate base material using Al-2319 filler, the only available consumable wire for this series. A previously developed thermodynamic model suggested that an incompatible weld chemistry resulted when welding with this filler which would result in hot cracking due to a wide weld pool freezing range and a low volume fraction of eutectic liquid. As this filler wire was the only commercially available Al-2xxx filler this was seen as the principal limiting factor for exploiting this alloy series. The solution was to vary and control weld chemistry. Two approaches were taken. Firstly advanced arc welding was used to control weld dilution with the base material. A clad layer exhibiting a less crack susceptible composition was deposited using the Cold Metal Transfer process and the binary Al-2319 filler wire. Onto this layer the same filler could then be deposited to provide a structural joint. Although not fully validated, by limiting weld dilution with the base material this technique showed potential as an alternative method for suppressing solidification cracking. The second approach, which forms the core of this work, adapted the conventional tandem MIG welding process to mix different series consumable fillers in a single weld pool to control weld composition. A range of ternary weld mixtures were produced which resulted in the development of a robust thermodynamic model. Validation using this system resulted in weld cracking being eradicated. The concept was then further developed to weld using three filler wires; this expanded the mixing range and allowed further model validation. A range of crack free compositions were produced with differing mechanical properties. An optimum weld composition was determined that was then used for characterisation of the weldment. By varying heat input, base material HAZ softening was controlled with joint failure confined to the weld / base material interface. This was attributed to grain boundary liquation due to the welding temperatures involved resulting in solute rich grain boundaries. These areas did not deform easily under tensile loading initiating fracture of the joint. Acceptable joint strengths were realised however ductility was reduced due to the identified failure mode. Although not tested to military specifications, acceptable mechanical test values were recorded which were closely compliant with the minimum requirements for armour system specifications. As a consequence a filler wire composition was recommended for future prototype development.
Open Access
Assessing the effect of TIG alternating current time cycle on aluminium wire + arc additive manufacture
(Elsevier, 2017-10-09) Ayarkwa, Kwasi; Williams, Stewart W.; Ding, Jialuo
The effect of electrode positive time cycle (% EP) of the alternating current TIG process has been investigated for aluminium wire + arc additive manufacture of linear walls. The study considered the effect on oxide removal, linear wall dimensions, microstructure, mechanical properties as well as the effect on electrode wear. The results showed that the effective wall width was minimum at 20%EP with a corresponding maximum in layer height. It was also observed that increasing the% EP increased the electrode wear rate, which in turn affected the arc stability. Microstructure analysis showed a noticeable increase in the grain size for higher% EP. The study also showed that% EP had no significant effect on mechanical properties. From a heat input analysis, a direct correlation was observed between the arc voltage and the% EP. The study also indicated that there could be other contributing factors to wall dimensions. For aluminium wire + arc additive manufacture of linear walls, minimum cleaning ranged between 10%EP and 20%EP.
Open Access
Bead shape control in wire based plasma arc and laser hybrid additive manufacture of Ti-6Al-4V
(Elsevier, 2021-07-16) Wang, Chong; Suder, Wojciech; Ding, Jialuo; Williams, Stewart W.
Wire based plasma transferred arc (PTA)-laser hybrid additive manufacture has the potential to build large-scale metal components with high deposition rate and near-net shape. In this process, a single bead is the fundamental building block of each deposited component, and thus the bead shape control is essential for the deposition of different geometries. However, how to control the bead shape by manipulating various process parameters is still not understood. In this study, the effect of different process parameters, including laser power, energy distribution between the PTA and laser, wire feed speed, travel speed, and laser beam size on the deposition process and bead shape was investigated systematically. The results show that the optimum operating regime for the hybrid process is with the wire being fully melted by the PTA and the melt pool being controlled by the laser, which gives a good bead shape as well as a stable deposition process. The bead shape is significantly affected by the laser power and travel speed due to the large variation in energy input. The effect of wire feed speed is more complex with the bead width initially increasing to a maximum and then decreasing as the wire feed speed increases. The laser beam size has a minor effect on the bead shape, but a small beam size will result in an irregular bead appearance due to the unstable process caused by the high power density. In addition, a procedure for controlling the bead shape in the hybrid process was proposed, which provided a reference for selection of different process parameters to achieve required bead shapes. The feasibility of this proposed procedure was demonstrated by the two deposited multi-layer single-pass walls.
Open Access
Bead shape control using multi-energy source (mes) for wire-based directed energy deposition (ded) process.
(Cranfield University, 2021-11) Chen, Guangyu; Ding, Jialuo; Williams, Stewart W.
Independent control of layer width and height is essential to achieve a simultaneous high build rate with precision net shape and thermal control independent from deposition shape in the wire-based directed energy deposition (w-DED) process. Bead shape control using a multi-energy source (MES) method was studied to achieve independent control of layer width height of a bead for the w-DED process. This study was carried out in three stages: First, a plasma transfer arc (PTA) energy density measurement was conducted. A split anode calorimeter (SAC) was applied for the measurement of PTA energy density. A laser beam with a well-defined energy profile was used to calibrate the calorimeter without the complications of arc instability. An optimised centred grounded SAC device was introduced to reduce arc distortion. More symmetric arc profiles were obtained. The dynamic thermal characterisation of a scanning laser (SL) was then studied using both experimental and numerical approaches. SL experiments were conducted with different oscillation frequencies and laser beam sizes. An innovative solution dependent convection boundary (SDCB) method was introduced to reduce the element amount of the finite element (FE) model. Results show that the quasi-steady state SL can be applied as an equivalent stationary energy source. Finally, an SL-PTA MES system was introduced to study the MES bead shape control, a PTA was employed in the front to create an initial melt pool and melt the feedstock wire, and an SL was used behind the PTA to reshape the melt pool and precisely control the bead width. A bead shape control strategy was proposed by using the wire feeding rate to control the layer height and the scanning width to control the layer width. The experiment results verified that the SL-PTA MES has independent control of layer width and height.
Open Access
Characterisation of residual stress state and distortion in welded plates stress engineered by local mechanical tensioning
(Transtec Publications; 1999, 2014-01-01T00:00:00Z) Ganguly, Supriyo; Wescott, A.; Nagy, T.; Colegrove, Paul A.; Williams, Stewart W.
Local mechanical tensioning is one of the most efficient and industrially relevant stress engineering techniques to modify weld residual stress field and subsequently reduce buckling distortion. However, application of rolling load and its magnitude need to be optimised for an energy efficient rolling process. In the present study gas metal arc butt welded plates of low carbon mild steel were rolled by a dual roller in different rolling configuration (top and reverse side rolling) and with different magnitude of rolling load. All the plates were rolled post welding. Residual strain profiles of the post weld rolled plates were measured, using the SALSA strain scanner, and the inplane stress were characterized. Average distortion of the rolled plates was correlated with the residual stress state. Reverse rolling was found to be more effective in removing distortion while the stress profile did not show any significant reduction of the peak stress.
Open Access
Characterisation of the cold metal transfer (CMT) process and its application for low dilution cladding
(Elsevier Science B.V., Amsterdam., 2011-03-01T00:00:00Z) Pickin, Craig Graeme; Williams, Stewart W.; Lunt, M.
The process characteristics of the synergic cold metal transfer (CMT) process have been examined for welding aluminium alloy. Utilising a simple backlighting system and through the arc monitoring the droplet transfer modes were identified. Whilst the modified short circuit mode was evident for the lower parameter range, a two part transfer mode based upon a combination of spray and short circuit transfer was observed for the mid to upper parameter range. The technology was also explored as a cladding process for applying to ternary alloyed (Al–Cu–Mg) aluminium plate. This alloy system is known to be susceptible to solidification cracking when MIG welded using the binary Al-2319 (Al–Cu) filler wire, this being due to the wide element freezing range of the weld resulting from mixing with the base material. Utilising this filler, weld dilution ratios for both CMT and pulsed welding were identified across the examined parameter range. The CMT process exhibited greater control of dilution that enabled deposition of a quasi-binary (Al–Cu) layer exhibiting a less crack susceptible composition. Onto this layer conventional MIG welding could be applied which could potentially eradicate cracking using a binary fil
Open Access
A comparison framework to support the selection of the best additive manufacturing process for specific aerospace applications
(Inderscience, 2020-05-31) Garcia-Colomo, Alberto; Wood, Dudley; Martin, Filomeno; Williams, Stewart W.
Additive Manufacturing (AM) is a cutting-edge technology that provides up to 100% of material efficiency and significant weight reduction which will positively impact aircraft fuel consumption in addition to high design freedom. Consequently, many aerospace companies are considering implementing AM thanks to these benefits. Therefore, the aim of this research is to assist aerospace organisations with a selection among different AM technologies. To enable this, primary data from (8) experts in the field of AM was collected through semi-structured interviews and cross-referenced with secondary data to identify the key factors for consideration in the selection of AM equipment for aerospace applications. Four AM technologies Laser Powder Bed Fusion (LPBF), Electron Beam Powder Bed Fusion (EBPBF), Wire Arc AM (WAAM) & Laser Metal Deposition (LMD) were highlighted by the experts as the most appropriate for aerospace applications. The main outcome of this study is the development of a comparison framework that helps companies select their AM technology depending on their main business or specific application.
Open Access
Comparison of joining efficiency and residual stresses in laser and laser hybrid welding
(2011-04-01T00:00:00Z) Suder, Wojciech; Ganguly, Supriyo; Williams, Stewart W.; Paradowska, A. M.; Colegrove, Paul A.
Laser welding is a high energy density process, which can produce welds with less energy input and thereby lower residual stress generation compared to arc welding processes. However, the narrow beam dimension makes it extremely sensitive in terms of fit up tolerance. This causes a problem in achieving high quality welds. Laser with arc hybrid process overcomes such issues. In this paper, longitudinal residual strains were compared for autogenous laser welding and laser/TIG hybrid processes. Joining efficiency, which is defined by the penetration depth achieved per unit of energy input, was correlated with the residual strain generation. It has been shown that to achieve a specific penetration depth, there is an optimum welding condition for each of the welding processes, which will give minimum tensile residual stress generation. The results imply that for the same penetration depth, hybrid process resulted in similar to 50% higher tensile longitudinal domain compared to autogenous laser.
Open Access
Compensation strategies for robotic motion errors for additive manufacturing (AM)
(University of Texas, 2016-08-10) Bandari, Yashwanth K.; Charrett, Thomas O. H.; Michel, Florent; Ding, Jialuo; Williams, Stewart W.; Tatum, Ralph P.
It is desirable to utilise a robotic approach in additive manufacturing as Computer Numerical Control (CNC) is expensive and it has high maintenance costs. A robotic approach is relatively inexpensive compared to CNC and can provide much more flexibility, enabling a variety of configurations and easier parallel processing. However, robots struggle to achieve high positioning accuracy and are more prone to disturbances from the process forces. This paper attempts to characterise the robot position and velocity errors, which depend on the build strategy deployed, using a laser speckle correlation sensor to measure the robotic motion. An assessment has been done as to whether these errors would cause any problem in additive manufacturing techniques, where the test parts were built using the Wire+Arc Additive Manufacture (WAAM) technique. Finally, different compensation strategies are discussed to counter the robotic errors and a reduction of 3 mm in top surface profile irregularity by varying the wire feed speed (WFS) during the path has been demonstrated.
Open Access
Conduction mode: broadening the range of applications for laser welding
(2012-11-14) Assuncao, D. E.; Ganguly, Supriyo; Yapp, David; Williams, Stewart W.; Mustafa, Kocak
Conduction laser welding opens up a range of innovative applications for laser welding. This relatively novel mode of laser processing expands the application potential significantly beyond what is normally achieved today. The main reason for this could be attributed to the different characteristics of conduction process when compared to keyhole laser welding. An example is the higher stability of conduction which results in welds of higher quality and better control of the welding process. Despite the advantages of conduction laser welding, it is yet to be exploited significantly for industrial applications and there are very few applications for which this mode of operation is used. This paper is aimed at presenting different varieties of applications for conduction laser welding using a fibre laser. This ranges from high quality aluminium welds to laser brazing of stainless steel to metal foams. The objective of this paper is to highlight the main features of conduction laser welding process and exemplify some conduction laser welding applications.
Open Access
Control of residual stress and distortion in aluminium wire + arc additive manufacture with rolling
(Elsevier, 2018-06-25) Honnige, Jan; Colegrove, Paul A.; Ganguly, Supriyo; Eimer, Eloise; Kabra, S.; Williams, Stewart W.
The aluminium alloy wire 2319 is commonly used for Wire + Arc Additive Manufacturing (WAAM). It is oversaturated with copper, like other alloys of the precipitation hardening 2### series, which are used for structural applications in aviation. Residual stress and distortion are one of the biggest challanges in metal additive manufacturing, however this topic is not widely investigated for aluminium alloys. Neutron diffraction measurements showed that the as-built component can contain constant tensile residual stresses along the height of the wall, which can reach the materials' yield strength. These stresses cause bending distortion after unclamping the part from the build platform. Two different rolling techniques were used to control residual stress and distortion. Vertical rolling was applied inter-pass on top of the wall to deform each layer after its deposition. This technique virtually elimiated the distortion, but produced a characteristic residual stress profile. Side rolling instead was applied on the side surface of the wall, after it has been completed. This technique was even more effective and even inverted the distortion. An interesting observation from the neutron diffraction measurements of the stress-free reference was the significantly larger FCC aluminium unit cell dimension in the inter-pass rolled walls as compared to the as-build condition. This is a result of less copper in solid solution with aluminium, indicating greater precipitation and thus, potentially contibuting to improve the strenght of the material.
Open Access
Crack path selection at the interface of wrought and wire + arc additive manufactured Ti–6Al–4V
(Elsevier, 2016-05-12) Zhang, Jikui; Zhang, Xiang; Wang, Xueyuan; Ding, Jialuo; Traoré, Yéli; Paddea, Sanjooram; Williams, Stewart W.
Crack propagation deviation tendency in specimens containing an interface between wrought alloy substrate and Wire + Arc Additive Manufacture (WAAM) built Ti–6Al–4V is investigated from the viewpoints of microstructure, residual stress and bi-material system. It is found that a crack initiated at the interface tends to grow into the substrate that has equiaxed microstructure and lower resistance to fatigue crack propagation. Experimental observations are interpreted by finite element modelling of the effects of residual stress and mechanical property mismatch between the WAAM and wrought alloy. Residual stresses retained in the compact tension specimens are evaluated based on measured residual stress in the initial WAAM built wall. Cracks perpendicular to the interface kept a straight path owing to the symmetrical residual stress distribution. In this case the tangential stress in bi-material model is also symmetric and has the maximum value at the initial crack plane. In contrast, cracks parallel to the interface are inclined to grow towards the substrate due to the mode II (or sliding mode) stress intensity factor caused by the asymmetric residual stress field. Asymmetric tangential stress in the bi-material model also contributes to the observed crack deviation trend according to the maximum tangential stress criterion.
Open Access
Criticality of porosity defects on the fatigue performance of wire + arc additive manufactured titanium alloy
(Elsevier, 2019-01-28) Biswal, Romali; Zhang, Xiang; Syed, Abdul Khadar; Awd, Mustafa; Ding, Jialuo; Walther, Frank; Williams, Stewart W.
This study was aimed at investigating the effect of internal porosity on the fatigue strength of wire + arc additive manufactured titanium alloy (WAAM Ti-6Al-4V). Unlike similar titanium alloys built by the powder bed fusion processes, WAAM Ti-6Al-4V seldom contains gas pores. However, feedstock may get contaminated that may cause pores of considerable size in the built materials. Two types of specimens were tested: (1) control group without porosity referred to as reference specimens; (2) designed porosity group using contaminated wires to build the specimen gauge section, referred to as porosity specimens. Test results have shown that static strength of the two groups was comparable, but the elongation in porosity group was reduced by 60% and its fatigue strength was 33% lower than the control group. The stress intensity factor range of the crack initiating pore calculated by Murakami’s approach has provided good correlation with the fatigue life. The kink point on the data fitting curve corresponds well with the threshold value of the stress intensity factor range found in the literature. For predicting the fatigue limit, a modified Kitagawa-Takahashi diagram was proposed consisting of three regions depending on porosity size. Critical pore diameter was found to be about 100 µm.
Open Access
Deformation microstructures and strengthening mechanisms for the wire+arc additively manufactured Al-Mg4.5Mn alloy with inter-layer rolling
(Elsevier, 2017-11-29) Gu, Jianglong; Wang, Xiaoshu; Bai, Jing; Ding, Jialuo; Williams, Stewart W.; Zhai, Yuchun; Liu, Kun
Applying inter-layer rolling to the wire+arc additively manufacturing (WAAM) process with increasing loads of 15 kN, 30 kN and 45 kN, achieves excellent mechanical properties for 5087 (Al-Mg4.5-Mn) alloys. Compared with the as-deposited alloy, the average micro hardness, yield stress and ultimate tensile strength of 45 kN rolled alloys reached to 107.2 HV, 240 MPa and 344 MPa, which were enhanced by 40%, 69% and 18.2%, respectively. Primary coarse grain structures were found to become greatly refined with an evident rolling texture after deformation. The strengthening mechanisms mainly are deformation strengthening, grain refinement, and solution strengthening. Meanwhile, the elongation of rolled alloys stays over 20%. The plasticity was not obviously diminished compared with the as-deposited alloy. This is two times greater than the commercial wrought Al-Mg alloy with similar composition. The excellent plasticity may be chiefly due to grain refinement, pores closure and reduction, and grain recrystallization during the WAAM re-heating process. The combination process of rolling deformation with WAAM deposition is an effective technique in refining microstructure and improving mechanical properties for AM aluminum alloys.
Open Access
Design and implementation of a wireless sensor communication system with low power consumption for energy harvesting technology
(2012-02-02T00:00:00Z) Marsic, Vlad; Zhu, Meiling; Williams, Stewart W.
This paper presents the design and implementation of a wireless sensor communication system with a low power consumption for integration with energy harvesting technology, that can be employed in energy autonomous wireless sensor communication applications. The design and implementation focus on three levels: hardware, software and data transmission. The resulted system is able to satisfy all the theoretical and practical requirements in order to be included in a wireless sensor structure that is able to give the device self-powered autonomy, due to a smart inter-correlated management of the energy resources.