Effect of welding mode on selected properties of additively manufactured AA5087 aluminium alloy parts

Wire and arc additive manufacturing (WAAM) is a popular direct energy deposition (DED) method for producing large-scale metallic components. The main advantages of the technique are a high deposition rate and low cost. Furthermore, the utilization of the WAAM is prevalent in the aerospace industry. The AA5087 aluminium alloy with 4.5 wt.% of magnesium has been investigated because of its excellent properties. The present research deals with the study of thermal cycles and fields developed in the alloy during additive manufacturing with two different Cold metal transfer (CMT) modes, namely conventional (CMT) and cycle-step (CMT-CS). The welding system was equipped with a Fronius TransPulse Synergic 3200 CMT power source, a Fanuc Arc Mate 1000iC 6-axes robot with an R 30iA control unit, a welding torch, and a 1-axis positioner. The AA5087 aluminium alloy welding wire with a diameter of 1.2 mm was deposited onto the AA5083 aluminium alloy plate with dimensions of 70 mm x 200 mm x 3 mm during the experiment. The thermal cycles were documented using an Ahlborn Almemo 5690-2 measuring station equipped with K-type thermocouples. The thermal fields were monitored with a FLIR E95 thermography camera. The results showed the evident influence of arc mode on the temperatures developed in manufactured aluminium alloy parts during the process of WAAM.


Introduction
Wire and arc additive manufacturing (WAAM) is a promising additive manufacturing process that utilizes an electric arc to melt and deposit metal wires, layer by layer, to build three-dimensional objects.It is a type of 3D printing technology suitable for producing large metal parts or structures with complex geometries and complex and near-net shape components [1,2].The production of large parts with WAAM technology has offered many advantages compared to subtractive conventional manufacturing methods.These benefits include increased material efficiency, reduced production and lead times, the ability to repair components, and the utilization of numerous materials [2][3][4][5][6][7].Moreover, cost efficiency is often considered an advantage, but this is not always true.It is necessary to evaluate the entire production process, including post-processing steps such as machining and surface finishing, to determine whether cost efficiency is achieved [2,8].
The versatility of WAAM has made it possible to process a wide range of metal materials and manufacture parts for various engineering industries, particularly in aerospace, automotive, marine, and construction [1,2,9].In addition to steel and titanium, aluminium components are produced in these industries, such as aircraft structural components, engine parts, ship propellers, fixtures, and decorative and architectural features [1,2].
The principle of WAAM involves depositing metal wire feedstock layer by layer onto a substrate.An electric arc is the thermal energy source, which moves along the workpiece and locally heats and melts the aluminium alloy [3,8,10,11].Different variants of the WAAM process have emerged depending on the type of heat source used [11].Cold metal transfer (CMT) welding-based WAAM supports ultra-low heat input and more precise arc length control, and the deposition rate is high compared to other heat sources [6,[10][11][12].The resulting mechanical properties of aluminium alloys fabricated by WAAM are comparable to wrought materials [2,13].Despite this, defects such as porosity formation, residual stresses, cracking, dimensional inaccuracy, and through-thickness anisotropy can occur in produced parts [1-2, 9, 14].The thermal cycle affects the thermomechanical and structural behaviour of aluminium alloys.Therefore, it is crucial to investigate the impact of heat source and heat accumulation during the process [15,16].This can be provided by measuring and monitoring the thermal fields, thermal gradient, molten pool, cooling rates, and residual heat to adjust process parameters properly [3,15,17].
Xu et al. [18] studied the effect of thermal cycles on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloy.The results showed the presence of defects in the produced walls and the evolution of the precipitated phase, which caused microhardness differences.Yuan et al. [19] investigated the elemental Mg loss in 5356 aluminium alloy during WAAM.Zhang et al. [20] studied the effect of different CMT-WAAM modes on Al-6Mg alloy.Tensile tests revealed that the material had a higher tensile strength than the wrought one.However, it exhibited anisotropic behaviour in different tensile directions.Su et al. [21] focused on CMT -WAAM of Al-Mg alloys and found that varying heat input resulted in refined equiaxed grains.Nonetheless, the interlayer regions experienced the occurrence of pores and cracks.Yang et al. [22] employed the same technology to produce thin walls of AlSi7Mg0.6alloy and proposed heat treatment to avoid defects.Their results indicated that heat treatment can reduce the non-uniform grain structure.Cadiou et al. [23] simulated the CMT -WAAM process and obtained good agreements with experimental data.The accurate 3D numerical model is a promising way to predict wire and melt pool behaviour.
In summary, the behaviour of aluminium alloys during WAAM is influenced by thermal cycling, solidification behaviour, microstructural changes, residual stresses, and anisotropy.Researchers are continuously studying and optimizing these factors to improve the reliability and predictability of WAAM for aluminium alloys in the industry.Furthermore, the thermal fields and thermal cycles of different CMT-WAAM input parameters are studied on produced AA5087 aluminium alloy walls.

Experimental setup
During the experiment, the ESAB OK Autrod 5087 filler wire (AlMg4.5MnZr)with a diameter of 1.2 mm was utilized for deposition onto the surface of an AA5083 (AlMg4.5Mn)aluminium plate.The plate dimensions were 70 mm × 200 mm × 3 mm.The chemical composition of the filler wire and the plate can be found in Table 1 and Table 2. Before the deposition process, the plate surface underwent cleaning using a steel brush and degreasing with acetone.
The Wire and arc additive manufacturing (WAAM) workplace comprised a Fronius TransPulse Synergic 3200 CMT welding power source.The source is based on the Cold metal transfer (CMT) process and supports the use of three different droplet transfer modes, namely conventional (CMT), pulse (CMT-P), and cycle-step (CMT-CS).Furthermore, a Fanuc Arc Mate 1000iC 6-axe robot with an R30iA control unit, a welding torch, a shielding gas delivery system (99.996%pure argon), and a 1-axis positioner were part of the setup (Figure 1a).The shielding gas flow rate remained constant at 18 l/min, the wire feed speed ranged from 3.9 m/min to 5.5 m/min, and the contact tip-to-work distance was maintained at 15 mm.The dwell time after each deposition was set to 120 seconds.Throughout the experiment, 100 mm length walls with approximately a height of 80 mm were manufactured using the CMT and CMT-CS modes.The difference between the two utilized modes lies in the variation in the welding current and voltage waveforms.The walls were fabricated according to the alternating direction strategy (Figure 2).The deposition parameters are outlined in Table 3.Heat input values were calculated using the formula  =  × (∑   ×  )

𝑇𝑆
. Ui and Ii represent the arc voltage (V) and current (A) for each mode, respectively.TS stands for travelling speed in m/min, and η is the arc thermal efficiency coefficient (0.8 for CMT) [24].The values of heat input are given in Table 3.
The temperature changes during the deposition of individual layers were monitored using K-type thermocouples, which were fixed to the surface of the aluminium plate near the beginnings and ends of the deposits (Figure 1c).The thermocouples designated TC1 to TC4 were attached to the Ahlborn Almemo 5690-2 measuring station and utilized during the measurements.
The thermographic analysis of the aluminium walls during fabrication was performed using a FLIR E95 infrared camera (Figure 1b), with a maximum temperature limit of 1500 °C.The camera possesses a measurement accuracy of ± 2 %, a thermal sensitivity of 50 mK, a refresh rate of 30 Hz, and a resolution of 640 × 480 pixels.
Table 3. Deposition parameters.Because the emissivity is a temperature-dependent value, especially in the context of high temperatures generated during fusion welding, an emissivity value of 0.09 was chosen in accordance with scientific literature and was confirmed through thermocouple validation.Data processing was carried out using the FLIR Thermal Studio Pro software to determine the temperature at any given point during the deposition process.

Measurement of thermal cycles
Figure 3 shows the thermal cycles of individual deposits during the fabrication of the aluminium walls with CMT and CMT-CS processes recorded using K-type thermocouples.The thermocouples (TC1 to TC4) were fixed to the base plate, with their respective positions indicated in Figure 1c.The thermal cycles were recorded under a specific set of process parameters (Table 3).The thermocouples were inserted into 1 mm diameter holes drilled into the base plate and subsequently bonded with manual spot resistance device.Prior to the measurements, a specialized high-temperature paste was utilized.The thermal cycles corresponding to the deposition of at least 14 layers are clearly discernible.The highest temperature of 407 °C was observed after the deposition of the 3rd layer with the CMT-CS process.It was monitored with the thermocouple TC4, positioned near the end of the deposited layer and approximately 10 mm from the axis of the layer and 20 mm from the start/end of the deposited weld bead.The length of the deposited weld beads was 100 mm.As the process of aluminium wall fabrication continued, the maximum temperature decreased due to the increasing distance of the welding torch from the position of the TC4 thermocouple.The measured cooling cycles are like those encountered in welding operations [1,2].All thermal cycles gradually returned to room temperature.In the case of the CMT process, the maximum temperature reached 386 °C after deposition of the 3rd layer.The cooling rates of TC3 and TC4 thermocouples are identical.Furthermore, the repeatability and reliability of the measurement technique were confirmed.When depositing the 1st layer with the CMT-CS process, the maximum temperature reached at the distance of 10 mm from the weld axis and 20 mm from the weld end was 375.3 °C.The temperature slightly increased during the deposition of the 2nd layer to 390.8 °C.The maximum temperature was recorded during the deposition of the 3rd weld bead, reaching a value of 407 °C.After this point, the maximum measured temperature by the thermocouple started to decrease to 333.3 °C during the deposition of the 5th weld bead.The mentioned trend continued to the 9th deposited layer when the temperature reached 203.7 °C.A decrease below 200 °C was observed during the deposition of the following layers.The maximum measured temperature during the deposition of the 10th layer was 184.3 °C.The minimum value of Tmax was recorded during the CMT-CS deposition of the 12th layer, i.e., 123.9 °C.The decrease in the maximum measured temperature is associated with an increased wall height.Thus, the distance between the position of the thermocouple and the moving heat source increased during the WAAM process.
Likewise, the CMT method was employed to deposit the wall made of 5087 aluminium alloy.During the 1st layer deposition with the CMT process, the temperature at the distance of 10 mm from the weld axis and 20 mm from the end of the weld was measured to be 377.9°C, which was higher than the temperature measured during the first weld bead deposition by the CMT-CS process.The temperature slightly increased to 370.1 °C during the deposition of the 2nd layer.Similarly, the highest temperature was recorded during the deposition of 3rd weld bead, reaching a value of 386 °C.Subsequently, the maximum measured temperature decreased to 300.5 °C during the deposition of the 5th weld bead, which is lower than the temperature recorded during the deposition of the same layer using the CMT-CS process.The decreasing temperature trend continued until the 9th deposited layer, where the temperature dropped to 195.2 °C.A temperature below 200 °C was observed from this point onwards, i.e., during the deposition of the following layers.The maximum temperature measured during the deposition of the 10th layer was 175.4 °C, and a value of 162.2 °C was documented during the CMT deposition of the 12th layer.The measured value for the CMT weld bead was higher compared to the same layer produced with the CMT-CS.

Measurement of thermal fields
Thermal fields measured by an IR camera on the surface of the weld bead immediately after CMT-CS WAAM deposition process are given in Figures 4a -h.The brightest area, located around the deposited wall, was excluded from temperature evaluation.In this zone, the thin layer of magnesium oxide was produced on the surface of 5083 aluminium alloy base material, which is why the highest temperatures are indicated, as the emissivity is close to 1 due to the presence of the mentioned dark oxide film.Thermal fields after the deposition of the 1st layer are presented in Figure 4a, where the deposition strategy from left to right was chosen.The temperature fields after deposition of the 2nd layer are given in Figure 4b, where the mentioned weld bead was deposited from right to left.The thermal fields after the deposition of the layers 3rd to 8th are given in the Figures 4c -h  Figure 5 displays the temperatures measured on the surface of the weld bead that was deposited by the CMT-CS process.The surface temperatures were measured in two locations -Box 1 and Box 2. The additive manufacturing process was carried out from left to right (as shown in Figure 5).Right after the deposition of the first layer, the temperature at Box 2 reached 348 °C.The second layer was deposited in the opposite direction (from right to left), and hence, the temperature at Box 2 decreased to 158 °C.It should be considered that a dwell time of 120 seconds was set to cool down the weld beads before the deposition of the next layer.After the deposition of the 3rd layer, the temperature at Box 2 increased again and reached 351 °C.Since the opposite direction of deposition of layers was applied, the temperatures measured at Box 2 were alternating.Thus, during of deposition of the 1st, 3rd, 5th, and 7th weld beads, the temperatures were higher at Box 2, ranging from 348 °C to 368 °C.This was observed when the deposition direction from left to right was applied.In this case, the time between the movement of the welding torch from Box 2 to the end of the weld was much shorter, and hence the heat dissipation was slower.On the other hand, when the deposition strategy from right to left was utilized, as in the case of 2nd, 4th, 6th, and 8th weld beads, the temperature measured at Box 2 was lower, ranging from 158 °C to 180 °C.This was associated with the fact that the distance between Box 2 and the end of the deposition was greater, which allowed more time for heat dissipation towards the lower parts of the deposited wall.A similar situation was observed for location Box 1, but the directions were opposite to the previous one.When referring to surface temperatures before the deposition of layers, the temperatures were low, ranging from 33.7 °C to 59.6 °C.It is worth noting that these temperatures measured before the deposition of the layer, i.e., 120 seconds after the deposition of the previous weld bead.

Conclusions
Based on the measurements of thermal cycles and temperature fields during Wire and arc additive manufacturing (WAAM) of 5087 aluminium alloy, the following observations can be made: • the highest measured temperature was recorded during the deposition of the 3rd layer using the CMT-CS process, reaching a value of 407 °C, • the maximum temperature measured during the CMT additive manufacturing was lower, reaching only 386 °C, • a similar situation was also observed in the case of the deposition of the 5th and 10th layers, where the temperatures were lower in the case of the CMT process, • a different situation was observed in the case of the 12th deposited weld bead, where the temperature for the CMT deposit was higher, • during measurements of the thermal fields by an IR camera, higher temperatures were observed in the area designated as Box 2 when the deposition strategy from right to left was applied, • in the case of the deposition of the 1st, 3rd, 5th, and 7th weld beads, the temperatures were higher at Box 2, ranging from 348 °C to 368 °C.

Figure 1 .
Figure 1.Experimental setup for WAAM a) robotic workplace, b) arrangement for thermographic analysis, and c) position and designation of utilized thermocouples.

Figure 3 .
Figure 3. Thermal cycles of individual layers after a) conventional CMT and b) CMT-CS. .

Figure 5 .
Figure 5.The temperatures measured at the beginning and the end of the deposition of each layer.