Fabricating large scale titanium alloy thin-walled double-sided part by hot-wire arc additive manufacturing

Using traditional process to fabricate large scale titanium alloy thin-walled part such as wings and grid rudder, has the problem of low material utilization. Wire arc additive manufacturing (WAAM) is a flexible manufacturing technology with high deposition efficiency and high material utilization. In addition, a hot-wire device is attached to the WAAM system as auxiliary heat source, and the WAAM system becomes hot-wire arc additive manufacturing (HWAAM) system. In this paper, a large scale titanium alloy drone frame was fabricated by HWAAM. A double-side alternating deposition method was proposed to reduce the thermal deformation of the substrate. Two additional sections of the drone frame was taken off for mechanical analysis. The shape of the whole drone frame is completed and the mechanical properties meet the requirements of the project.


Introduction
Additive manufacturing (AM) is different from the traditional material reduction manufacturing method, which fabricate the part from bottom to top based on the scattering accumulation principle [1] .In recent years, the demands of alloy application and component scale are increasing high [2] .Especially in the field of aerospace, large scale titanium alloy parts are in great demand.However, the high buy-to-fly rate of traditional manufacturing method leads to high cost of material [3] .Therefore, considering the high price of titanium alloy, the use of additive manufacturing of titanium alloy large scale parts can save costs [4] .Common additive manufacturing methods are selective laser additive manufacturing (SLM), electron beam melting (EBM) and wire arc additive manufacturing (WAAM) [5,6] .Compare with the other two, WAAM has the advantages of high deposition rate and high material utilization [7], which are of great benefit to fabricate large scale parts.Therefore, it is very important for the development of aviation industry to study the method of fabricating large scale titanium alloy parts with WAAM.
Large-scale parts fabricated by AM have been widely studied in the past years [8] .In terms of selective laser melting, Liu et.al used multi-laser selective laser melting (ML-SLM) to fabricate largescale alloy steel parts, multiplying manufacturing efficiency while achieving a tensile strength of 1255MPa [9] .As for electron beam melting, Rannar et.al have successfully manufactured more than 50 different types of components using EBM technology showing promising mechanical properties, density and hardness [10][11][12][13] .In the case of WAAM, the efficiency is much higher than SLM and EBM.Cranfield Universitydesigned dual robot cell that can produce parts that are up to 10 m in length [14] .They also built a 24 kg Ti-6Al-4V external landing gear assembly with a buy to fly rate of 1.2 by WAAM, which enabled material savings more than 220 kg [2] .BAE System from UK fabricated a 1.2-meter-long Ti-6Al-4V wing spar structure by WAAM, of which the deposition rate of this part was about 0.75 kg/hour rand the cost could be saved by 29% [2,15] .Ding et.al summarized the main points of WAAM large-scale components building as four parts, including distortion controlling, building style choosing, compromising between buy to fly rate and building complexity, and heat build-up controlling [15] .In addition, to reduce residual stresses and distortion, Wu et.al used multiple machine learning algorithms and a mechanistic model to rank separately parameters as well as thermomechanical variables that affect the evolution of stresses [16] .
In this paper, each of the eight parts of the drone frame was fabricated by WAAM and eventually welded together.The deposition and processing parameters of each component are shown.Machining and welding process are also partially publicized.An additional section of the part was taken off for mechanical analysis.The mechanical properties at room temperature meet the requirements of the project.The principle and overview of HWAAM system is shown in Figure1.(a) and (b).Besides the main devices of WAAM, a power source is used to produce the resistance heat.The positive pole is connected to wire slider and the negative pole is connected to the substrate.When the resistive heat source is working, the wire is heated in advanced, which increases the heat input to the part for a higher deposition rate [17].And Figure1.(c) shows the principle of cooperation between hot-wire device and welding gun.

Experiment
Each part of the drone frame was deposited in an argon atmosphere with a purity of 99.99%, assuring O2 concentration below 100 ppm throughout the whole process.The material used in this paper is TC4(Ti-6Al-4V) and the diameter of the TC4 wire is 1.6mm.The material of the substrate is also TC4.The eight separated parts and the final welded component of the drone frame is shown in Figure2.All of these patterns were deposited separately on both sides of the same substrate.In order to avoid bending the substrate due to thermal residual stress and disturbing deposition of the other side, each part needs to be turned upside down once one layer is finished.When the number of layers on each side reached five, three layers can be deposited continuously in one load.There is no obvious bending deformation of the substrate using this method, which is named double-side alternating deposition method.A diagram of this method is shown in Figure 3.Most of the ribs on the drone frame can be finished with one single beam, some with a double beam, and a small with a block.And the process parameters of single beam, double beam, and block are different, which are shown in Table 1.Overlap distance is the distance between two adjacent beams.Obtaining block and double beam with length over 100mm need to preheat the substrate in advance to get fine overlap performance (porosity and mechanical properties).After the deposition, the excess of the substrate was first removed by the linear cutting machine, including some pre-designed circle to reduce the weight.Then a rough machining of the parts was carried out, which was to preliminarily observe whether there were pores, collapse or not reaching the design dimensions inside the parts, as shown in Figure 4. Porosity and collapse can result in poor mechanical properties, and the wrong dimensions can cause unsuccessful welding of the whole frame.These problems were fixed by cutting and re-WAAM process.The whole frame was welded together after every part was qualified, and the welding process was completed at AECC Beijing Institute of Aeronautical Materials.

Mechanical properties
A thin-walled section and a block section were randomly removed from the drone flame, as shown is Figure 4.The mechanical properties of thin-walled sample were tested longitudinally and transversely, and the mechanical properties of block sample were tested in three groups named X, Y, and Z, corresponding to three orthogonal directions respectively.Each group contains ultimate tensile strength (UTS), yield strength (YS) and elongation rate (EL).The results are shown in Table 2 and Table 3.All of these mechanical properties meet the requirement of the project.For thin-walled sample, all mechanical properties are significantly lower in longitudinal direction, which is typical of the WAAM process [18] .It's worth mentioning that the elongation rate of WAAM parts are much lower than forgings (usually reaching 20% or more).As for block sample, the mechanical properties in the X direction are better than those in the Y direction, and the Y direction is better than the Z direction, which is due to the absence of overlap and remelts.

Conclusion
In this paper, a TC4 alloy drone was separately fabricated and welded together.Conclusions are as follows.
(1) The parameters of single beam, double beam, and block should be designed differently to get fine shape and good mechanical properties of components.The heat input of double beam and block should be less than single beam.The layer thickness and overlap distance should also be increased.
(2) If the part need to be deposited on both sides of the substrate, the proposed method of doubleside alternating deposition method is effective in eliminating thermal deformation.The frequency of turning can be reduced with the increase of the number of layers.

Figure1
Figure1.(a) Schematic of the hot-wire arc additive manufacturing system.(b) The hot-wire arc additive manufacturing system.(c)Schematic of HWAAM process.

Figure 2 .
Figure 2. Overview and separate parts of the TC4 alloy drone flame.

4 4. 5 Figure 3 .
Figure 3. Schematic diagram of the double-side alternating deposition method, (a) Continuous single-side deposition method can lead to serious thermal deformation.(b) Double-side alternating deposition method eliminates thermal deformation.(c) More efficient double-side alternating deposition method after five layers.

Figure 4 .
Figure 4. Welding joint and collapse inside the thin-wall.

Figure5.
Figure5.(a) Thin-walled section.(b) Block section.(c) ~ (d) Schematic illustration of the way to take drawing samples from thin-walled section and block section.(e) Size of the tensile test sample.Table2.Mechanical properties of thin-walled sample.Directions UTS(MPa) YS(MPa) EL(%)

Table 1 .
Process parameters of single beam, double beam, and block.