Efficient Build-Up of High-Strength Aluminum Structures Using Friction Stir Additive Manufacturing

Friction Stir Additive Manufacturing (FSAM) is a novel process with which large-scale aluminum structures can be produced from high-strength alloys such as the 7xxx series. Due to the prevalence of these alloys in airplanes and rockets, the process offers high application potential, for example in fabricating stringers and stiffeners. The building process in FSAM is characterized by sequentially stacking and friction stir lap welding (FSLW) metal sheets. Before adding the next layer, the surface is machined (i.e., by milling). So far, this is a necessary step to enable gap-free welding of the layers, which results in increased costs and reduced layer heights. The investigations described in this paper were aimed at improving the weld surface quality to enable defect-free FSAM without the additional machining step. For this, FSLW was conducted using different welding tools. The resulting welds were evaluated based on superficial and internal characteristics as well as the mechanical properties (shear strength). With a welding tool in which both a rotating and a stationary shoulder were combined, defect-free weld seams with a mean underfill and a mean flash height of 0.07 mm were produced. In a subsequent study, it was proven that defect-free FSAM without surface machining is possible up to the fifth layer using the combined welding tool.


Introduction
Friction Stir Additive Manufacturing (FSAM) is an innovative additive manufacturing process in which metal sheets (e.g., aluminum [1] or magnesium [2]) are subsequently stacked on a platform and joined by friction stir lap welding (FSLW). Thereby, a rotating tool, consisting of a probe and a shoulder, is plunged into the stacked sheets. Frictional heat is generated, which causes the material in the process zone around the tool to be softened. In combination with the rotary movement (tool rotational speed n), a material flow is generated and the stacked sheets are joined. After plunging, the tool remains in the dwell phase to increase the energy input into the weld. Subsequently, the tool is moved along a welding trajectory at a welding speed v, while it is pressed against the workpiece with an axial force Fz. The weld produced is asymmetric: on the advancing side (AS), the tool's direction of rotation coincides with the direction of the welding trajectory. The other side of the weld is called the retreating side (RS). At the end of the trajectory, the tool is retracted, leaving a negative of the probe and the shoulder on the surface of the workpiece (exit hole). In preparation for building the next layer and to ensure a gap-free welding process, the surface is machined [2].
Only a few studies have investigated the new FSAM process. Table 1 shows a summary of the build heights achieved using FSAM. Considering the height of the final additive build ha, the sheet thicknesses h sheet, i for each layer i as well as the total number of layers n, the material utilization ratio with regards to the heights pmur, h is calculated by The highest pmur, h (93.3%) was achieved by Yuqing et al. [3], who produced a nine-layer build (platform included) of AA7075-O, while the lowest pmur, h (79.4%) was obtained by Palanivel et al. [1] who built four layers (platform included) of the magnesium alloy Mg-4Y-3Nd-T5.
The amount of material, which has to be removed through machining, depends on the weld topography. The surface properties result from the material softening and flow beneath the shoulder. Factors influencing these are the process parameters (e.g., the tool's tilt angle α [4]) as well as the geometry (e.g., the diameter ds [4] and the shape [5]) of the shoulder area that is in contact with the workpiece [5]. Values taken from the literature are listed in Table 1. Aside from conventional tools, stationary shoulder tools can be used to further improve the surface of friction stir lap welds [6].  4 Identified by the authors of the present publication from a photograph of the tools 5 Measured by the authors of the present publication from a photograph of a cross section and compensating the platform thickness The state of the art can be summarized as follows: ▪ In FSAM, the surface of each welded sheet is machined in order to ensure zero gap for the subsequent welding process. This reduces the layer height and efficiency of the process. ▪ Factors influencing the weld surface are the process parameters and the geometry of the FSW tool.
With stationary shoulder tools, smooth surfaces can be produced for FSLW [6]. ▪ Previous investigations for FSAM [1,3,7] were performed using conventional FSW tools with a concave featureless shoulder. The shoulder diameters ds ranged from 10 mm to 30 mm and the tilt angles α from 1.5° to 2° (Table 1).

Methodology
Objective. The objective of the investigations was to achieve improved weld surfaces in FSAM, which reduces the effort for the machining process step before building the next layer. This leads to a more efficient process and an increased material utilization rate regarding the build heights pmur, h.

Research Approach.
Three experimental studies were conducted with different FSW tools. In preceding screening experiments, different shoulder geometries (concave and flat) and features (with/without scrolls) were investigated to identify a suitable tool and parameter window. Then, using the identified tool, two parameter studies were conducted in which the welding temperature Tp, the welding speed v, and the axial force Fz were varied (studies 1 and 2). A second parameter study (study 2) was conducted in which the tool from study 1 was combined with a stationary shoulder. In a subsequent study (study 3), a five-layer FSAM build was produced using three different parameter sets and without the machining process step.
All weld surfaces produced were inspected visually and by topographical as well as metallographic analysis. In addition, shear testing was performed in studies 1 and 2 to determine cause-effectrelationships between the weld quality and the mechanical properties. The experimental setup and methods that were used are described below.

Key Engineering Materials Vol. 926
Experimental Setup. The welding experiments were conducted using an industrial robot (KR 500-2 MT, KUKA AG, Augsburg, Germany), which was equipped with an FSW spindle (RS XXXX000-0784, CyTec Zylindertechnik GmbH, Jülich, Germany). The axial force Fz was measured by load cells above the spindle and was internally controlled. Sheathed thermocouples (type K, 443-7967, RS Components GmbH, Frankfurt am Main, Germany) with a diameter of 0.5 mm were glued (COT Resbond 989-1, Polytec PT GmbH, Karlsbad, Germany) into the FSW tool to acquire the temperatures inside the weld during the process (Fig. 2). A temperature measuring tool [9] was used to process the thermocouple signals. To compensate for the cooling effect of the stationary shoulder [6] and to ensure more similar welding conditions when applying different tools [10], closed-loop temperature control was applied as described by Bachmann et al. [11]. The control algorithm was implemented in a real-time system (MicroLabBox, dSPACE GmbH, Paderborn, Germany). Further information regarding the research machine setup is given in [12].
In the experiments, aluminum sheets (300 mm × 50 mm × 4 mm, Fig. 1) of the high-strength alloy EN AW-7075-T6 were friction stir lap welded. The aluminum sheets were clamped on each short side with the same screw torque (40 Nm). The stacking configuration for studies 1 and 2 was chosen so that lower shear strengths were to be expected. The literature [13] suggests that this is the case when the RS lies on the wide side of the upper sheet (Fig. 1a). The decision was based on the assumption that the overall strength in later FSAM builds ( Fig. 1b) is determined by the strength of the weakest side. For study 3, the sheets were stacked so that they fully overlapped (Fig. 1b). To prevent the collision of the tool with the clamping claws, the weld lengths in studies 2 and 3 were reduced. The exit holes in study 3 were filled with epoxy resin (aluminum repair stick, Engelbert Strauss GmbH & Co. KG, Biebergemünd, Germany) in order not to leave them empty for the buildup of the next layer and thus to prevent instabilities in the force controlled process. For studies 1 -3, a standard FSW tool, consisting of a rotating shoulder and a threaded conical probe with three flats, was used. The diameter of the shoulder ds (16 mm) was determined from the base diameter of the probe dp (8 mm) in accordance with the design guidelines given by FULLER [14], who suggests a ratio between the shoulder diameter ds and the probe diameter dp from 2 to 3. Due to the negative geometric influence of the shoulder diameter on the heel plunge depth, the lowest ratio from this interval (2) was chosen. The shoulder shape (flat and double-scrolled) was selected based on screening experiments. For studies 2 and 3, the tool was combined with an additional stationary shoulder with an outer diameter dss of 20 mm (Fig. 2). . The experiments in studies 1 and 2 were conducted following a central composite design (CCD), which is a combination of a cube-and a star-shaped experimental plan. This enables the efficient experimental investigation of nonlinear effects. Table 2 shows the process parameters chosen for the conducted experiments. In study 3, the additive builds were produced with three process parameter sets chosen from study 2. Temperature control was activated during the dwell phase of the welding process. The feed motion was started once the set temperature Tp had been reached. The probe length lp (6 mm), the rotational speed during the plunging of the FSW tool n0 (2000 r/min), and the tilt angle α (0.5°) were determined during the screening experiments. A low tilt angle value α was chosen because it geometrically influences the plunge depth of the tool heel [4] and therefore the seam underfill. Due to the low stiffness of the welding robot and the higher forces when welding with a stationary shoulder, a higher tilt angle of 1° and a lower welding speed v were necessary in studies 2 and 3 compared to study 1. The larger diameter of the stationary shoulder (20 mm) also resulted in a higher axial force Fz for studies 2 and 3. Visual Inspection. All weld seams were visually inspected and categorized into four classes (Fig. 3): a) "flawless", b) "acceptable with flash", c) "acceptable with minor imperfections" and d) "unacceptable" (with major defects like surface lack of fill). Topography Analysis. All welds from classes a and b (studies 1 -3) were scanned using a threedimensional profilometer (VR-3100, Keyence Corporation, Osaka, Japan). The weld surfaces were captured with a 12-fold magnification and a measurement accuracy of μm in the z-direction and 5 μm in the x-and y-directions. Distortions during welding were removed from the data as described by Hartl et al. [5]. Afterwards, the reference plane was set as the sheet surface surrounding the weld. With the exception of the exit hole area, the whole weld surface was evaluated (Fig. 1). The mean (h ̅ f and h ̅ su ), the maximum (ĥf [5] and ĥsu) and the standard deviation (h f, sd [5] and h su, sd [5]) values of the flash heights as well as the maximum seam underfills along the weld centerline were used as indicators for evaluating the weld seam quality.

Metallographic Analysis.
A cross-section sample was taken from each weld (studies 1 and 2) and each additive build (study 3) as shown in Fig. 1 using a wet cut-off grinder. The samples from studies 1 and 2 were embedded in epoxy resin for stabilization. All samples were ground, polished with 1 μm cotton cloth, and micro-etched with Kroll's rea ent. The cross-section characteristics were recorded using a microscope (MM-40, Nikon, Tokyo, Japan) and the above-mentioned profilometer.
Shear Testing. Three shear specimens were taken from each weld of studies 1 and 2 (Fig. 1a) with a surface quality within classes ac. In contrast to the topographical investigations, welds from class c were also taken into consideration because it was assumed that minor imperfections do not significantly influence the shear strength. The specimens were milled to a width of 14 mm. A material testing machine (Z050, AllroundLine, ZwickRoell GmbH & Co. KG, Ulm, Germany) was used to determine the shear force Fn. The shear tests were performed at room temperature at an initial strain rate of 0.23 × 10 -3 1/s. The mean (F ̅ n), the maximum (F n ) and the standard deviation (Fn, sd) values of the three measurements were determined for each weld.

Results and Discussion
A summary of all collected data is given in Table 3 and 4 (see Appendix). The first digit of the experiment number (exp. no.) corresponds to the study, the second to the part and the last digit in study 3 to the layer number. In the following, the data resulting from the conducted experiments are elaborated on and discussed. 10

Achievements and Trends in Material Forming
Weld Surfaces in Studies 1 and 2. In Fig. 4, the achieved surface qualities for each conducted experiment of studies 1 (Fig. 4a) and 2 (Fig. 4b) are displayed. The horizontal axis is used for depicting the welding temperature levels Tp. The two vertical axes show the levels of the welding speed v (left) and of the axial force Fz (right). Each of the 15 data points corresponds to one conducted experi-ment. The shape and color of the data points are assigned to the surface quality classes ad (Fig. 3). An example (exp. no. 1.3) with the corresponding welding parameters is shown in Fig. 4a. In study 1 (Fig. 4a), only four out of the 15 welds presented a completely filled surface, which was accompanied by flash (class b). These welds were produced by applying low to medium welding speeds v, combined with medium to high axial forces Fz. The welding experiment with the lowest welding speed v (50 mm/min, exp. no. 1.10) had to be stopped prematurely because of an increasing plunging of the tool. Unacceptable weld surface qualities (class d) resulted when applying low to medium axial forces Fz and/or low to medium welding temperatures Tp. Weld surfaces with minor imperfections (class c) were produced by using higher welding temperatures Tp in combination with medium axial forces Fz. No welds with a surface quality class a could be produced in study 1. A topographical analysis was therefore only conducted for welds of class b. The maximum seam underfill ĥsu (0.85 mm) and the maximum flash height ĥf (4.32 mm) were lowest in exp. no. 1.13 and 1.3 respectively.
In contrast, with the stationary shoulder tool in study 2 (Fig. 4b), almost all welds were classified as "flawless" with respect to their surface quality (class a). The seam underfills ĥsu ranged from maximum values of 0.36 mm (exp. no. 2.6) to 0.55 mm (exp. no. 2.5). Only one of the weld surfaces (exp. no. 2.2, class c) displayed minor imperfections. This was welded with a parameter set in which a higher welding speed v (80 mm/min), a lower welding temperature Tp (480 °C), and a lower axial force Fz (7.5 kN) were combined, which could have resulted in an insufficient compression of the weld and therefore caused the imperfection.

Weld Morphologies in Studies 1 and 2.
Only one parameter set in study 1 (exp. no. 1.7) produced welds without cavities inside the weld nugget, in contrast to five (exp. no. 2.3, 2.6, and 2.8 -2.10) of the 15 welds of study 2. Hooking was visible in all cross sections (example in Fig. 5). The welds of study 2 without cavities all exhibited a greater flash height (maximum values ĥf above 0.68 mm and mean values h ̅ f above 0.15 mm). It can therefore be assumed that the appearance of cavities is negatively correlated to the flash height.  2). No correlation to the weld topography was detected. Welds that could withstand higher shear forces F ̅ n above 9.5 kN were found for both studies and in each of the tested surface quality classes ac. These welds all displayed internal cavities. The six welds without cavities (exp. no. 1.7, 2.3, 2.6, 2.8 -2.10) exhibited reduced strengths with mean shear forces F ̅ n lower than 8.33 kN. From this analysis, it can be assumed that cavities or minor superficial imperfections do not reduce the overall shear strength of the welds. The fracture pattern (Fig. 6) suggests that the cause for reduced strengths was instead hooking, which facilitated crack initiation. The literature [15,16] supports this hypothesis, as shear strengths increased with rising welding speeds v (Table 3), which results in lower hooking heights [15]. Weld Surfaces and Morphologies in Study 3. Finally, FSAM was performed using the combined tool from study 2 and without the machining process step. Three different parameter sets were chosen: The parameter set that produced the lowest mean seam underfill and flash (exp. no. 2.11 for 3.3) as well as two parameter sets that did not result in any cavities in study 2 (exp. no. 2.8 and 2.9 for 3.1 and 3.2). Fig. 7 shows that the mean seam underfills h ̅ su steadily increased with every layer in an almost linear relation for all parameter sets. The weld surface qualities declined in the higher layers of the additive build (Table 4). The best result with regards to the surface and the internal quality was obtained with a high welding temperature Tp of 500 °C and axial force Fz of 8.5 kN (exp. no. 3.1, Fig. 8). With that, the weld surface quality declined as late as in the final layer (exp. no. 3.1.5) from class a (Fig. 8c) to c (Fig. 8b). Within almost every layer (except exp. no. 5.1.2 and 5.1.3) the lowest maximum seam underfill values ĥsu (0.47 mm -0.74 mm) and flash heights ĥf (0.47 mm -0.60 mm) were achieved with this process parameter set. The lowest mean seam underfills h ̅ su (0.03 mm -0.19 mm) and flash heights h ̅ f (0.07 mm -0.14 mm), however, resulted from a parameter set (exp. no. 3.3) in which a lower welding temperature (490 °C), the highest welding speed (95 mm/min), and a lower axial force Fz (8.0 kN) were combined.
It is therefore assumed that seam underfills can be compensated up to a certain point by welding with higher temperatures Tp and axial forces Fz, as this results in an increased deformation of the material. With slightly lower welding temperatures Tp and axial forces Fz, cavities formed (exp. no. 3.2.4) or the surface quality started to degrade as early as in the fourth layer (exp. no. 3  In contrast to the friction stir lap welds, which were produced with the same process parameters in study 2 (exp. no. 2.11), no cavities were visible in the first three layers of the additive build (exp. no. 3.3.1 -3.3.3). Several reasons for this discrepancy are possible: Cavities could have been irregularly distributed along the weld, which means it cannot be guaranteed that all cavities are detectable in cross sections. Another possibility is that smaller cavities on the upper part of the seam are broken up when restirring the material during the welding of the next layer.
Formation of weld defects in higher layers was also observed by Zhao et al. [7] and Palanivel et al. [2], which they attributed to different thermal conditions in the layers. Because of the use of temperature control in study 3, it is hypothesized that the defects in the higher layers of this study are, however, the result of insufficient compression during welding caused by the increased gap between the metal sheets (Fig. 7).

Conclusions and Outlook
Three studies were conducted to improve the weld surfaces, thereby enabling a direct build-up without surface machining and a material utilization ratio regarding the build heights pmur, h of 100%.
In studies 1 and 2, different FSW tools were investigated in terms of their effect on the weld surface quality, the weld morphology (i.e., internal defects), and the shear strength. Based on that, three fivelayer (excluding the platform) additive builds were produced without surface machining. The most important conclusions are: ▪ The surface and the internal quality of friction stir lap welds can be improved using an FSW tool with both a rotating and a stationary shoulder. ▪ The maximum shear strength of friction stir lap welds is influenced by hooking. Cavities did not reduce the overall weld strength. ▪ It is possible to produce at least four flawless layers using FSAM without surface machining. ▪ It is hypothesized that defects in the higher layers of the FSAM builds were caused by an insufficient decompression due to an increased gap between the aluminum sheets. The data suggests that the effect of these gaps on the weld surface quality and the formation of cavities can be compensated by welding with higher temperatures Tp and axial forces Fz. ▪ For an improved efficiency of the process for flawless builds consisting of more than four layers it is suggested to only machine every fourth layer. Further investigations need to be carried out regarding this process strategy. Table 3: Process parameters and welding results of studies 1 and 2; non-existent data is marked with "-". The best welding results are highlighted in green. The process parameter sets used for study 3 (