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Article

Realization of Friction Stir Welding of Aluminum Alloy AA5754 Using a Ceramic Tool

by
Toni Sprigode
1,*,
Andreas Gester
1,
Guntram Wagner
1 and
Ulrich Degenhardt
2
1
Chair of Composites and Material Compounds, Chemnitz University of Technology, 09111 Chemnitz, Germany
2
QSIL Ingenieurkeramik GmbH, 96528 Frankenblick, Germany
*
Author to whom correspondence should be addressed.
Metals 2024, 14(9), 1089; https://doi.org/10.3390/met14091089
Submission received: 2 August 2024 / Revised: 10 September 2024 / Accepted: 18 September 2024 / Published: 23 September 2024
(This article belongs to the Special Issue Recent Trends in Friction Stir-Related Manufacturing Technologies)
Browse Figures
Versions Notes

Abstract

:
When engaging in the friction stir welding of aluminum/aluminum joints, the conventional use of tools made of hard metal and steel involves a complex and costly production process. These tools experience wear over welding distances and require frequent replacement to ensure the consistency of the welded seams. The exploration of silicon nitrite as a tool material emerges as a promising alternative in this scenario. The heightened hardness of non-oxide ceramics anticipates a diminished wear rate compared to traditional welding materials, translating into an extended operational lifespan. Nevertheless, the adoption of ceramics introduces challenges initially perceived as detrimental to friction stir welding. The inherent brittleness of silicon nitrite makes it susceptible to breakage under specific loads, and thermal stresses within the component can lead to failure. To mitigate these vulnerabilities, a ceramic material with high thermal shock resistance and a low proportion of sintering additives was used. Employing these accurately designed tools friction stir welding (FSW) was performed on sheets of AA5754, followed by a comprehensive examination of their microstructural and mechanical properties. It was demonstrated that a joint efficiency of 88% can be achieved, and that an increase in hardness within the stir zone occurred as a consequence of grain refinement. Furthermore, the Portevin–Le Chatelier effect, which is characteristic of this alloy, was influenced by the FSW process.

1. Introduction

Friction stir welding (FSW) is a process that is widely used in aerospace [1,2], automotive [3,4], and general construction industries [5]. It is used to join similar aluminum structures by mixing the materials together. One of the key advantages of this process is that the joining temperature is below the melting point of the base material. Furthermore, the process does not result in brittle joints due to overheating and does not require the use of additional shielding gases or heat treatments.
When joining aluminum alloys by FSW, high-strength joints could be achieved. For example, Barlas and Ozsarac [6] varied the tilt angle, rotational speed, and rotation direction of the tool while welding the aluminum alloy AA5754. This resulted in an ultimate tensile strength (UTS) of 86% compared to the as-received material. Ahmed et al. [7] joined 5 mm thick sheets of the same aluminum alloy, obtaining an efficiency of 97% by varying the tool travel speed and testing different eccentricities of the tool probe. Furthermore, Flippis et al. [8] demonstrated that it was possible to enhance the UTS of the AA5754 FSW joints above that of the base material by adjusting the travel and rotational speed.
For FSW, rotationally symmetrical tools, consisting of a shoulder and a probe, are used to join the aluminum structures while rotating and moving along the seam. During the process, the tools are exposed to wear as well as thermal and mechanical stress, which can lead to tool failure after several welds. To circumvent this issue, tools are manufactured from resistant steels (AISI 4340 [9], AISI H13 [10,11]) or hard metals (WC-Co [10,12]). The tools made from these materials require complex subtractive manufacturing, which often results in high costs.
The utilization of ceramic materials may offer a promising alternative for addressing the discrepancy between process stability and the elevated costs, due to their reduced material, manufacturing, and processing costs. Mylsiwiec and Śliwa [13] were able to weld 0.5 mm thick sheets of AA2024-T3 by using ceramic FSW tools that were whisker-reinforced. The authors did not specify the ceramic or the type and amount of reinforcement. However, a design of experiments with the subsequent analysis of variance resulted in a UTS of 95% compared to the base material. No signs of tool wear were observed. In a study conducted by the same authors and Buszta and Ostrowski [14], FSW tools made of ceramic compounds of Si3N4, in which Si-N bonds were partially replaced by Al-N and Al-O bonds, were used to weld thin aluminum alloy sheets. The utilization of these ceramic tools resulted in an asymmetrical cross-section of the macrostructure with a poorly visible heat-affected zone (HAZ). In comparison to the hard metal tools, the dwell time could be eliminated and the travel speed significantly increased, while the efficiency of the joint remained at 80%. Emmel et al. [15] explored the potential of ceramic materials for use as an FSW tool material during the welding of similar steel joints. This study demonstrated that SiC, Al2O3, and ZrO2 are unsuitable for the FSW of steels due to the presence of thermal stresses during plunging or welding. It was found that the non-oxygen ceramic Si3N4 was the only material investigated that could resist the thermal stresses that occurred during welding and produce defect-free welds. Nevertheless, it became evident that considerable tool wear had occurred after just a few meters of welding. In a study conducted by Zifčák et al. [16], the tool wear behavior was analyzed when welding steel S235 with WC and Si3N4 as tool material. The ceramic tool exhibited the greatest lifetime, achieving a weld length of approximately 2.1 m. Additionally, an asymmetrical microstructure and hardness profile was observed in the weld seam produced by welding steel with a ceramic tool. Another investigation into the welding of stainless steel with FSW tools composed of Si3N4 revealed that the wear mechanism is dependent on the rotational and travel speed of the tool [17]. Furthermore, it was demonstrated that a tensile strength similar to that of the base metal could be achieved [18]. It is to be expected that the thermal and mechanical stresses are significantly lower when welding aluminum alloys than when welding steel. Consequently, FSW tools made of Si3N4 should be able to weld aluminum alloys with low wear.
In this study, the joining of similar sheets made of the aluminum alloy AA5754 via FSW was realized by the use of the non-oxygen ceramic Si3N4 as a tool material. The resulting solid welds were subjected to mechanical property analysis, including tensile testing and hardness measurement. Additionally, the microstructure was examined using light microscopy (LM) and scanning electron microscopy (SEM) accompanied by energy-dispersive X-ray spectroscopy (EDS).

2. Materials and Methods

For the purpose of joining, aluminum alloy sheets with dimensions of 100 mm × 60 mm × 1.25 mm were utilized, which were made of the alloy AA5754 and subjected to a rolling process, followed by annealing to quarter hard (H22). The composition (data sheet) and mechanical properties (experimentally examined) of these sheets are detailed in Table 1. The sheets were fixated in all directions on a cut-out of AISI 304 stainless steel foil in order to prevent them from adhering to the backing plate.
The welding tests were conducted within a DMU80T four-axis universal machining center from DMG Mori (Wernau, Germany), which had been configured for the FSW process and had instruments to record the process forces in the x, y and z directions. Additionally, thermocouples (TC) of type K were integrated into the backing plate (17.5 and 67.5 mm distance to the plunge point) to measure temperatures during the welding process.
Two distinct welding parameters were utilized to illustrate the process stability of the FSW methodology when operating with a ceramic tool. The travel speed was 50 mm/min for both experiments, while the rotational speed and process force were set to 1500 × 1/min and 2000 N for Experiment 1 and 1250 × 1/min and 2500 N for Experiment 2. These experiments were subsequently designated as FSW1 and FSW2, respectively. The tool was inserted into the butt configuration of the sheets to a depth of 0.3 mm at a rate of 5 mm/min in both setups. Additionally, the tool was tilted by 2° in the welding direction and rotated in a counterclockwise direction. In order to perform the welding process, a ceramic tool insert was secured in an adapter that was fixed to the spindle. The conical tool shape is illustrated in Figure 1. Its diameter at the shoulder is 11 mm, while the diameter of the probe decreases from 8 mm to 4 mm over a length of 1.15 mm. No additional features were implemented.
In this study, a special grade of silicon nitride (Si3N4) was utilized as the tool material, SN-PU [19], manufactured by QSIL Ingenieurkeramik GmbH (Frankenblick, Germany). The low sintering additive content of only 3 wt.% in comparison (Table 2) to the standard silicon nitride qualities (SN-GP, 8–15 wt.%) results in enhanced high-temperature, oxidation, and corrosion resistances. In a previous study on the FSW of steel [15], the material demonstrated significantly enhanced performance and lifetime. Consequently, this ceramic should be able to resist the stresses for the FSW tests of the aluminum alloys described here.
The molding of the silicon nitride components was accomplished through the use of cold isostatic pressing (CIP) of granulates, followed by near-net shape green machining of the bodies. The sintering process was conducted in a gas pressure sintering furnace at approximately 1800 °C in 10 bar nitrogen atmospheres. To ensure complete densification and the absence of microstructural defects, which are essential for the attainment of high-performance properties, a subsequent hot isostatic pressing (HIP) was conducted at approximately 1700 °C with a nitrogen pressure of 2000 bar. Subsequently, hard machining with diamond tools was performed in order to achieve precise dimensions, tolerances, and surface quality.
Following welding, the joints were subjected to mechanical and microstructural examination. This was conducted using a tensile testing machine, ProLine from ZwickRoell (Ulm, Germany), which was equipped with a load cell of 10 kN. The tensile tests were conducted at a testing speed of 2.5 × 10−3 s−1, in accordance with the standards set DIN EN ISO 6892-1 [19]. Due to the sheet thickness of 1.25 mm, the tensile test samples were of type H of the DIN 50,125 standard and were water jet-cut from the welded joints and the base metal sheets. The Vickers hardness of the welded joints was quantified via nanoindentation, which was performed on a FISCHERSCOPE HM2000 XYm from Helmut Fischer GmbH (Sindelfingen, Germany) with a measurement force of 0.1 N. Light microscopic (LM) images of the cross-sections of the welds were obtained using an Olympus GX51 from Carl Zeiss AG (Jena, Germany), following the etching of the samples with 5% hydrofluoric acid (HF) in accordance with Flick’s method. In addition, Barker’s electrolytic etchant was employed to visualize the grain structure of the aluminum alloy. This was achieved by cooling the HBF4 to 4 °C and exposing the sample to 25 V for 30 s. Furthermore, the microstructure was also investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) using a ZEISS LEO1455VP (Carl Zeiss AG, Jena, Germany). All recordings were conducted at an acceleration voltage of 25 keV.

3. Results and Discussion

3.1. Process

During the welding process, the temperature of the backing plate and the process forces were recorded. The vertical force (Fz, Figure 2) increased to approximately 2000 N during the plunging step until the aluminum alloy was softened by frictional heat, at which point it increased a second time to the maximum of approximately 3500 N when the tool was inserted further into the sheets. Upon initiation of the tool’s movement in the x-direction, the forces remained at a constant level. The vertical force remained at approximately 2000 N, which was specified by the force control, while the horizontal forces (Fx, Fy) varied slightly around 0 N. As illustrated in Figure 2, the temperature graphs of the backing plate increased to a maximum when the tool reached the position of the thermocouples (x1 = 17.5 mm and x2 = 67.5 mm) and subsequently decreased at a more gradual rate following the tool’s departure from them. Furthermore, the peak temperature of the second curve (approximately 400 °C) was higher than that of the first (approximately 340 °C). This is attributable to the heat capacity and thermal conductivity of the aluminum alloy sheets and the backing plate. In addition, it is recommended that the maximum welding temperature is maintained within the material, as demonstrated in numerous studies [24,25], although it cannot be directly measured. Nevertheless, the quality of the weld seam remained unaffected by the temperature increase.
Overall, the results of the welding temperature and process force recordings show that the ceramic tool can withstand high vertical forces (approximately 2000 N) and temperatures of at least 400 °C at the same time.
As illustrated in Figure 3, the FSW process, when utilizing a ceramic Si3N4 tool, results in an even and highly refined scaling of the welded seam. Apart from a few millimeters caused by the plunging into the sheets, no flash was observed. Moreover, no defects could be identified on the surface of the weld seam. The tool shape remained unaltered throughout the FSW process. No evidence of tool wear was visible after a total welding distance of 2 m. In contrary, adhesions of the aluminum alloy were present on the tool surface.

3.2. Microstructure

The macroscopic overview images of the welded joint (FSW1, Figure 4) reveal the typical zones for the FSW of the aluminum alloys, namely the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and base material (BM), which differ in their distributions of particles, grain sizes, and grain structures. A heat-affected zone (HAZ) that differs from the BM could not be identified. It is evident that there are precipitations in the BM, which have decreased in size and become more homogeneously distributed in the TMAZ and SZ due to the material stirring, especially on the AS. Moreover, the material on the advancing side (AS) is observed to be finer structured in comparison to the retreating side (RS). This could be the consequence of higher temperatures and greater deformation, which results in increased dynamic recrystallization. In particular, Barker’s etched overview image reveals an asymmetric microstructure, characterized by a distinct transition between the SZ and TMAZ on the AS, in comparison to a relatively gradual transition on the RS. This is in line with the literature on welding with ceramic tools, as referenced in [12,16]. The grains in the SZ on the AS are equiaxed and of a fine size, while the grains on the RS are distributed. This can be attributed to the asymmetric material flow of the plasticized material and temperature distribution around the probe during the welding process. Furthermore, there is an oxide line evident in the center of the SZ, which is also present when welding AA5754 with non-ceramic tools [26].
The SEM image of the BM (Figure 5a) shows differently oriented grains and dark and bright precipitates. The EDS analysis (Table 3) confirms that the solid solution (EDS1) consists of AlMg3, which is also a known name for AA5754. In comparison, the analysis of the bright precipitates (EDS2) reveals the presence of silicon, manganese, and iron, while the amount of magnesium and aluminum is reduced. Due to the different detection range of elements while EDS (Kanaya–Okayama range [27]), the bright precipitations are too small to define an exact result, but the investigation compared with the literature indicates that the bright particles should be α-Al(Fe,Mn)Si and Mg2Si [26,28]. The dark precipitates are etched out of the material due to OPS polishing. It is assumed that these are the intermetallic compounds of Al3Fe and Al6Mn [26,28]. Comparing the BM with the SEM images of TMAZ and SZ, the grain size and precipitates decrease, while the precipitates become more homogeneously distributed, as seen in the LM images. Furthermore, no ceramic particles of the Si3N4 tool could be identified in the LM and SEM images.

3.3. Mechanical Properties

The FSW process resulted in a reduction in both the ultimate tensile strength (UTS) and yield strength (YS) to 203.3 MPa and 103.7 MPa for FSW1 and 198.4 MPa and 102.6 MPa for FSW2, respectively, in comparison to the BM (232.0 MPa and 164.0 MPa, respectively), representing the efficiencies of UTS of 88% and 86% (Figure 6). Additionally, the Young’s modulus (E) exhibited a decrease from 86.0 GPa (BM) to 71.7 GPa (FSW1) and 78 GPa (FSW2), while the total strain (A) increased from 8.1% (BM) to 12.3% (FSW1) and 12.5% (FSW2). This phenomenon of the change in the mechanical properties may be attributed to the annealing of the aluminum alloy as a consequence of the heat input associated with the FSW process. This results in work hardening degradation, which consequently leads to a transition in the condition of heat treatment from type H22 to type O. The mechanical properties of type O of AA5754 are comparable to those observed in the welded joints [29].
During the tensile test, all specimens fractured in the middle of the SZ due to the presence of the oxide line (Figure 7). Figure 8 illustrates the stress–strain behavior of the BM and the welded joints, which demonstrates the reduction in UTS and increase in A for the FSW joints in comparison to the BM. Furthermore, the Portevin–Le Chatelier effect (PLC), which occurs in 5xxx aluminum alloys at room temperature, is clearly visible in form of jags in both the BM and FSW joints [30]. The FSW process results in a decrease in the range of the force peaks associated with the PLC. One potential explanation for this phenomenon is the increase in dislocation density resulting from the intense plastic deformation [31]. This results in the formation of additional dislocations, which serve as nucleation and propagation sites for the PLC bands due to the accumulation of mobile solute atoms. Consequently, there are more PLC bands, which results in increased dislocation movement and therefore less stress is required. Further investigations are necessary to verify this thesis.
The hardness of the weld was measured at a step distance of 0.184 mm in the x-direction, with a total of 64 indentations for each line. Five lines were tested across the sheet thickness at a distance of 0.267 mm, starting at 0.1 mm from the bottom of the weld to avoid effects on the sheet edge and the embedding material. In sum, 320 hardness measurements were performed on an area of 12.35 mm2. The average hardness value of the measured area is 73.6 HV.
As illustrated in Figure 9, the hardness values of the FSW joint show a clear distinction between SZ, TMAZ, and BM. The SZ and TMAZ are subjected to deformation and heat input during the welding process, which leads to dynamic recrystallization. This effect results in grain refinement (Figure 5). According to the Hall–Petch relationship, the increased number of grain boundaries maximizes the resistance to dislocation movement, resulting in higher hardness in SZ (Hmax = 103.9 HV) and TMAZ (HTMAZ = ~85 HV) compared to BM (HBM = ~70 HV). Additionally, it is notable that there are localized regions on the RS exhibiting lower hardness values (Hmin = 65.5 HV) in comparison to the BM. This finding suggests the existence of a HAZ resulting from grain coarsening due to heat input without deformation.
The HAZ was not observed in the microstructure, which is consistent with the current literature on this subject [26,32]. However, the hardness on RS is lower than that on AS because of reduced deformation during the process. Furthermore, there is a discrepancy in the values across the SZ. The highest hardness values are observed at the top of the sample, where the most severe stirring occurs. The hardness of the SZ decreases towards the center of the SZ. This is the consequence of a reduction in deformation and a lower heat input compared to the upper SZ, according to the shoulder contact on the surface. The increased hardness values on the bottom of the weld should be the result of the heat input of the in-process-heated backing plate combined with the stirring of the probe, which leads to dynamic recrystallization. The dark area on the upper welded seam of Figure 9 are the result of incorrect measurements of the embedding material due to the reduced sheet thickness of the weld following FSW (Figure 4).

4. Conclusions

The objective of this study was to demonstrate the use of the non-oxide ceramic Si3N4 as a tool material for friction stir welding of similar AA5754 joints. The results of the study allow for the following conclusions to be drawn:
  • It is possible to weld 1.25 mm thick sheets of AA5754 by using a ceramic FSW tool made of Si3N4. No wear or even fractures of the tool occurred during welding at a vertical force of 2500 N and temperatures of at least 400 °C.
  • The formation of solid joints with an ultimate tensile strength of 88% relative to the base material was successfully achieved.
  • The microstructure of the FSW joint is identical to that of conventional tools, including characteristic zones, grain refinement, and grain distribution. Additionally, no ceramic particles of the tool could be found in the welded seam.
  • The SEM images demonstrate a refinement and more homogeneous distribution of the precipitates in TMAZ and SZ as a result of the FSW process.
  • In contrast to the BM, the PLC is affected by the FSW process. The range between force peaks exhibited a reduction.
  • The grain refinement of the TMAZ and SZ resulted in an increase in the hardness, especially on the upper SZ.
This study demonstrates that it is feasible to weld the aluminum alloy AA5754 with a tool composed of Si3N4. The mechanical properties of the joint and the process behavior are analogous to those observed in the FSW utilizing conventional tool material. Nevertheless, to establish Si3N4 as a standard tool material for joining aluminum alloys, further investigations are required. In particular, the travel speed should be augmented to facilitate the utilization of this ceramic tool in industrial applications.

Author Contributions

Conceptualization, T.S.; methodology, T.S., A.G. and U.D.; formal analysis, T.S., A.G. and U.D.; investigation, T.S.; resources, G.W.; writing—original draft preparation, T.S.; writing—review and editing, T.S., A.G., G.W. and U.D.; visualization, T.S.; supervision, G.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article material, further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Ulrich Degenhardt was employed by the company QSIL Ingenieurkeramik GmbH. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. 3D models of (a) FSW adapter and (b) tool; (c) sintered and machined Si3N4 tool.
Figure 1. 3D models of (a) FSW adapter and (b) tool; (c) sintered and machined Si3N4 tool.
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Figure 2. Process forces [N] and temperatures [°C] during the FSW process of FSW1.
Figure 2. Process forces [N] and temperatures [°C] during the FSW process of FSW1.
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Figure 3. (a) Weld seam of the AA5754/AA5754 joint (FSW1) by using of a ceramic Si3N4 FSW tool and (b) Si3N4 FSW tool after the welding process.
Figure 3. (a) Weld seam of the AA5754/AA5754 joint (FSW1) by using of a ceramic Si3N4 FSW tool and (b) Si3N4 FSW tool after the welding process.
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Figure 4. LM overview images ((a) according to Barker, (b) according to Flick) and (c) detailed images of (A) BM, (B) TMBZ and (C,D) SZ.
Figure 4. LM overview images ((a) according to Barker, (b) according to Flick) and (c) detailed images of (A) BM, (B) TMBZ and (C,D) SZ.
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Figure 5. SEM images including EDX spots of (a) BM, (b) TMAZ, and (c) SZ.
Figure 5. SEM images including EDX spots of (a) BM, (b) TMAZ, and (c) SZ.
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Figure 6. Average values of UTS, YS, A, and E of AA5754 BM and FSW joints.
Figure 6. Average values of UTS, YS, A, and E of AA5754 BM and FSW joints.
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Figure 7. Macrographic image of the cross-section of the AA5754 FSW joint (FSW1) after tensile test.
Figure 7. Macrographic image of the cross-section of the AA5754 FSW joint (FSW1) after tensile test.
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Figure 8. Stress–strain graphs of AA5754 BM and FSW joint.
Figure 8. Stress–strain graphs of AA5754 BM and FSW joint.
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Figure 9. Hardness measurement [HV] of the AA5754 FSW joint—dashed line shows the tool penetration.
Figure 9. Hardness measurement [HV] of the AA5754 FSW joint—dashed line shows the tool penetration.
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Table 1. Composition and mechanical properties of AA5754 H22.
Table 1. Composition and mechanical properties of AA5754 H22.
MaterialElementsRp0.2Rm
wt.%MPaMPa
AA5754 Si Fe Cu Mn Mg Cr Zn TiAl164232
0.40.40.10.52.6–3.60.30.20.15Bal.
Table 2. Properties of SN-PU ceramic [20], SN-GP ceramic [21], and H13 tool steel [22,23].
Table 2. Properties of SN-PU ceramic [20], SN-GP ceramic [21], and H13 tool steel [22,23].
PropertySN-PUSN-GPH13
composition97 wt.% Si3N4
bal. RE2O3/Al2O3		85–92 wt.% Si3N4
bal. RE2O3/Al2O3		DIN EN ISO 4957
bulk density [g/cm3]3.18–3.223.18–3.307.74
residual porosity [%]0.5< 1-
including open
porosity [%]		00-
hardness [GPa]15.314.51.7
(50 HRC)
Young’s modulus [GPa]310–320290210
strength [MPa]3000
(compressive)		3000
(compressive)		1800
(tensile)
thermal expansion
coefficient [K−1]		1.2 × 10−61.4 × 10−610.8 × 10−6
Table 3. EDS analysis and Kanaya–Okayama range of elements of the FSW joint at 25 keV.
Table 3. EDS analysis and Kanaya–Okayama range of elements of the FSW joint at 25 keV.
MethodElement [wt.%]
MgAlSiMnFe
EDS13.4996.51---
EDS22.6880,662.842.7911.04
element [µm]
MgAlSiMnFe
Kanaya–Okayama range9.156.096.852.522.34
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Sprigode, T.; Gester, A.; Wagner, G.; Degenhardt, U. Realization of Friction Stir Welding of Aluminum Alloy AA5754 Using a Ceramic Tool. Metals 2024, 14, 1089. https://doi.org/10.3390/met14091089

AMA Style

Sprigode T, Gester A, Wagner G, Degenhardt U. Realization of Friction Stir Welding of Aluminum Alloy AA5754 Using a Ceramic Tool. Metals. 2024; 14(9):1089. https://doi.org/10.3390/met14091089

Chicago/Turabian Style

Sprigode, Toni, Andreas Gester, Guntram Wagner, and Ulrich Degenhardt. 2024. "Realization of Friction Stir Welding of Aluminum Alloy AA5754 Using a Ceramic Tool" Metals 14, no. 9: 1089. https://doi.org/10.3390/met14091089

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