Hybrid manufacturing of components from Ti-6Al-4V by metal forming and wire-arc additive manufacturing
Introduction
The production of titanium based high-temperature components is of great importance for the transport and energy sector and especially for the aerospace industry (Boyer, 1996). High-performance components made of titanium alloys are traditionally produced by thermo-mechanical treatment in multistage forging operations and require expensive dies. Besides, to obtain net-shape parts, extensive machining operations are needed which may entail a material yield of less than 10 % (Dutta and Froes, 2016). One driving force for the development of new processing routes for titanium is the reduction of the costs and scrap rates. For many applications, net-shape technologies such as additive manufacturing could enable a more resource-efficient production (Gibson et al., 2014) than conventional processing routes. However, manufacturing costs and process time in additive manufacturing rise rapidly with part size. Thus, the disadvantages of additive manufacturing and forging operations could be levered by mating both processes to new process chains, allowing to reduce the number of processing steps and to avoid high material waste. By combining AM and metal forming, two possible process sequences can be put into practice.
One possible hybrid technology is the combination of forging with AM. An AM process can be used to add the required features, i.e. cooling or reinforcement ribs or other structural or functional elements to the forged workpiece. This hybrid process chain is investigated intensively currently. Bambach et al. (2017) showed that AM can be used to create local reinforcements on sheet metal parts. Extensive work on additively manufactured features that are added to formed parts has been performed by Merklein et al. (2016a), (b). Different AM processes such as laser beam melting, laser metal deposition, as well as gas metal arc welding, were investigated regarding their application potential in hybrid processing. It was shown that hybrid manufacturing offers a high potential for industrial material utilization. Ahuja et al. (2014) investigated the laser beam melting process of thin walls made of Ti-6Al-4 V and characterized the resulting material properties. Butzhammer et al. (2017) analyzed the tensile bonding strength of hybrid parts produced by the combination of laser beam melting and warm bending of Ti-6Al-4 V. It was consistently confirmed that the interface between the sheet and the additively manufactured part is most important for the strength of the components. Hirtler et al. (2018) investigated the use of AM for generating or modifying stiffening ribs on pre-forms created by hot forging. The authors produced a semi-finished product from the EN-AW 6082 aluminum alloy containing a rib geometry under typical hot working conditions, and the rib height was increased by WAAM, adding AlSi12 material. While most of the existing work on Ti-6Al-4 V focuses on AM of features on pre-formed sheet metal parts, the present study investigates the use of AM for generating or modifying stiffening ribs on pre-forms created by hot forging. First, a semi-finished product containing a rib geometry is produced from Ti-6Al-4 V under typical hot working conditions, and the rib height is increased by WAAM. According to (Williams et al., 2016), the WAAM process is superior to other AM techniques regarding the manufacturing time and deposition rate, power efficiency, and investment cost.
In an alternative process sequence, the AM process can be used to generate a pre-shaped semi-finished part, which will be then forged using a single forming step to achieve the final contour. Such process chains have been patented recently with a focus on titanium alloys (Busch et al., 2015; Di et al., 2015). The first investigation of Semiatin et al. (2001) on hot forming of laser deposited Ti-6Al-4 V showed that the flow stress, as well as the developed microstructures of heat-treated and HIPed laser-deposited Ti-6Al-4 V, are very similar to those produced by ingot-metallurgy. Hot formability of Ti-6Al-4 V produced by SLM, EBM, and WAAM was investigated in previous studies of the authors (Sizova and Bambach, 2018; Bambach et al., 2019, 2018; Sizova et al., 2019). It was shown that AM materials show good formability as well as lower flow stresses and activation energies for hot forming compared to conventional wrought material with a lamellar microstructure usually used in conventional forging of Ti-6Al-4 V. The application of rolling during WAAM (wire-arc additive manufacturing) of Ti-6Al-4 V was also considered in the literature (Martina et al., 2016). The authors reported grain refinement of the primary β-grains as well as a reduction of residual stresses and an improvement of mechanical properties. Colegrove et al. (2017) showed that rolling can be applied either independently between welding passes at room temperature or immediately following the deposition head. Both methods of application lead to a significant refinement in the microstructure and improvement of mechanical properties. Similar results were observed by Yang et al. (2018) by applying ultrasonic impact treatment to Ti-6Al-4 V WAAM material, i.e. significant reduction in residual stresses and improvement of mechanical properties. Some studies on steels were also presented in the literature. In work of Ambrogio et al. (2019) the combination of selective laser sintering and single point incremental forming of stainless steels (AISI 304 and 630) was investigated. The authors presented part manufacturing with variable rigidity, as well as complex parts combining 3D geometries on flat sheets. Investigations of incremental sheet forming of deposited and commercial sheets from stainless steel were performed by Pragana et al. (2019). It was reported that the formability of deposited 316 L stainless still is lower than that of conventional sheet material, but is still appropriate to withstand large plastic deformations that are typical of metal forming. Silva et al. (2017) investigated the mechanical and formability characteristics of an aluminium alloy AA5083 sample produced by WAAM. The results show that the deposited aluminium alloy has excellent ductility and that its final stress response can significantly be improved as a result of strain hardening. The existing studies hence confirm that hybrid technologies combining AM and forging are promising. Moreover, by applying of forming operations to additively manufactured Ti-6Al-4 V material the microstructures, as well as the mechanical properties, can be influenced and improved. In accordance with the literature, during the WAAM process, the microstructure represents a basket weave (α-phase) microstructure inside a coarse columnar (several millimetres) prior β-grain structure (Martina et al., 2016). Heat treatment can alter the morphology of the α-phase, but it is hard to change the β-grain morphology without plastic deformation Semiatin et al., 1997. Wang et al. (2013) presented data on mechanical properties of Ti-6Al-4 V samples produced by WAAM. The authors showed that the mechanical properties were anisotropic, with higher strength and lower ductility being observed in the horizontally orientated tensile specimens. In addition, a small number of samples contained small gas pores that caused premature failure of fatigue samples. While the tensile strength of WAAM samples reaches the specification of forged material (>900 MPa), the tensile elongation is often found to be lower (6 %) than required by specification (>10 %). Consequently, as was mentioned above, additional thermomechanical treatments can be used to refine the microstructure, to remove the porosity, and to improve the mechanical properties of WAAM material.
In summary, the proposed hybrid processing routes appear to allow for similar properties as conventional forming process chains for Ti-6Al-4 V but may reduce the number of processing steps and increase the material yield. The main goals of the present study are: (i) to analyze the production of stiffening ribs using WAAM on pre-forms created by hot forging and to investigate the microstructure evolution and mechanical properties of the produced parts, (ii) to investigate the microstructure evolution during hot working of pre-forms made of Ti–6Al–4 V produced by WAAM, and (iii) to compare the resulting microstructures and mechanical properties to that of WAAM material before and after hot forming.
The paper is structured as follows: Section 2 gives an overview of the manufacturing of hybrid parts using hot forming and WAAM. Also, the procedures used for microstructural analysis and for determining the mechanical properties are presented. Section 3 details the results of metallographic examinations of WAAM material compared to hot forged WAAM material. In addition, the mechanical properties are investigated and the results are discussed. Finally, conclusions and outlook are presented.
Section snippets
Material and methods
The most widely used titanium-alloy Ti–6Al–4 V was selected for the investigations in the present work. The geometry used in the current study for the representation of the hybrid processing chain ‘hot forming + AM’ is shown in Fig. 1 (left) and represents a forged T-section with a length of 99 mm. The width of the rib is 10 mm. Higher ribs cause much higher tool wear and increase the appearance of forging defects. Using WAAM, the height of the rib was extended to a total of 108 mm. A six-axis
Results and discussion
Fig. 1 (right) shows the hybrid part with a 66 mm WAAM-produced wall which exhibits a shiny surface without signs of oxidation. The horizontally deposited layers can be seen clearly due to the surface undulation of each deposited layer. Fig. 3 shows a typical microstructure of Ti-6Al-4 V produced by WAAM. The macrostructure of the arc-deposited Ti-6Al-4 V is characterized by epitaxial growth of large columnar prior β-grains which stretch through the deposited layers. The β-grain size of WAAM
Conclusions
The present study confirms that hybrid technologies combining WAAM and forging are very promising for Ti-6Al-4 V part production regarding cost and material savings. They seem especially interesting for complex parts which require multiple forging steps and for large scale structural components which are difficult to produce both by forging (large forces, expensive dies) and by WAAM (distortion, mechanical properties and process time). The following conclusions are relevant for this work:
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The
Acknowledgments
The authors gratefully acknowledge the financial support provided by Federal Ministry for Economic Affairs and Energy (BMWi) for the LUFO SAMT64 Project - ‘Forging and additive manufacturing as a process combination for the resource-efficient production of aerospace structural components made of TiAl6V4 on flexible production scales’ (20W1719D). The authors are also grateful to the colleagues from the Chair of Physical Metallurgy and Materials Technology of BTU Cottbus-Senftenberg for the
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