Investigation of the Feasibility of the Friction Stir Welding (FSW) Process for Overlapped UNS C27200 and UNS C19400 Alloy Sheets

Abstract

FSW is a process that uses a solid tool, which, through the energy generated by its rotation and advancement, plasticizes the material at the joint interface, enabling material mixing and consolidation. This process provides a new way to join dissimilar materials by uniting them in a plasticized state where both materials are below their melting points. This work aims to optimize the FSW process parameters for overlapping sheets of UNS C19400 and UNS C27200 alloys. Four rotation speeds (850, 950, 1050, and 1150 rpm) were tested at two feed rates (20 and 80 mm/min). The sample produced at 1050 rpm and 20 mm/min was the only one free of defects. Temperature was controlled using thermocouples, and this sample was analyzed through macroscopic and microscopic examinations, as well as a microhardness profile. Among the tested conditions, the parameter set of 1050 rpm and 20 mm/min produced the best weld, with a smooth surface. Increasing the rotational speed raised the process temperature and improved material mixing. Microstructural analysis revealed grain refinement and dynamic recrystallization, along with partial mixing of the Cu and Zn phases. Microhardness values indicated an average reduction of 30% compared to the base metals.

Keywords:
Friction Stir Welding; Micrographic; Microhardness; Welding Temperature

1. Introduction

Copper and its alloys are used in various applications due to their excellent cold workability and corrosion resistance in marine, atmospheric, and chemical environments. Brass is the most well-known copper alloy, with zinc (Zn) as the primary alloying element (Cu-Zn)¹-³. However, conventional welding processes for these alloys pose challenges, including color changes due to oxidation at high temperatures, limited material penetration, loss of strength, and permanent deformations¹,⁴,⁵.

A significant issue encountered during brass welding is the vaporization of zinc and copper constituents. Under the intense heat of TIG welding, where the electrode can attain 4200°C and the workpiece 3200°C, the lower boiling point of zinc (907°C) makes it particularly susceptible to evaporation. This phenomenon leads to the formation of voids or pores within the weld, surface oxidation, and alterations in the material's original color⁶.

The Friction Stir Welding (FSW) process employs a non-consumable tool that is harder than the workpieces. The differentiator of this process is that the temperature reached does not approach the melting point of the joined materials, allowing us to join materials with different properties regardless of their melting point⁷-¹⁰.

While initially developed for aluminum alloys, FSW has demonstrated versatility in joining a wide range of metals. FSW has been intensively researched for copper and brass joints over the past five years, with a focus on process parameters and post-weld material properties¹¹-¹⁴. FSW excels at welding dissimilar materials, which is challenging for conventional welding methods due to differences in the materials' melting points and thermal expansion¹⁵. In recent years, Friction Stir Welding has begun to evolve beyond the research environment and establish itself as a crucial technology across industries such as aerospace, batteries, automotive, marine, and many others¹⁶-²⁰.

Numerous parameters must be optimized to achieve a defect-free weld. These include, but are not limited to, rotational speed, feed rate, tilt angle, plunge depth, and pin shape²¹-²⁴. The appropriate selection of these parameters is crucial to minimizing defects during the process, as such flaws are often associated with inadequate welding conditions¹⁸. Specifically, the tool's rotational and travel speeds significantly influence the mechanical strength of the welded joints, as reflected in the samples' performance during mechanical testing¹⁹. Furthermore, using the proper tool tilt angle is important. These angles typically range from 0 to 4° and are directly related to the compressive force on the material. Adequate compressive force is key to achieving a higher quality joint²⁵,²⁶. Joint strength loss primarily results from material scooping on the advancing side, large hardness differences, insufficient material mixing, or unfavorable microstructure²⁵. These parameters affect the amount of heat generated during the process and, consequently, the weld's quality and properties²⁰.

This work presents a study on the applicability of the Friction Stir Welding (FSW) process for the dissimilar joining of UNS C19400 and UNS C27200, analyzing the microstructure and characteristics of the process and joined materials.

2. Materials and Methods

The materials used in this experiment were alloys C194 (copper-iron) and C272 (brass). Spectrometry analysis was performed to determine the chemical composition by weight percentage, and the results are presented in Table 1.

Table 1
Chemical composition of C194 and C272.

C 194	wt. %	C 272	wt. %
Cu	97.0 min	Cu	62.0 – 65.0
Fe	2.1-2.6	Fe	<0.070
P	0.015-0.15
Pb	0.03 max	Pb	<0.070
Zn	0.05-0.20	Zn	34.8-38.0

The test tool was fabricated from H13 tool steel, hardened, and tempered to approximately 56 HRC. The tool dimensions are shown in Figure 1.

Figure 1
FSW tool dimensions and machined tool.

Table 2 presents the input variables and the process output conditions.

Table 2
FSW process input and output conditions.

Inputs	Outputs
Rotational Speed	Visual inspection result (approved/rejected)
(850, 950, 1050, and 1150 rpm)
Feed rate (20 and 80 mm/min)	Macrographs and Micrographs
Tilt angle (3°)	Microhardness profile
Shoulder penetration (0.3 mm)	Correlation analysis between inputs and results

A Vecker 3-axis machining center, model MV-760 ECO, equipped with a Siemens D828 CNC module, was used to run tests and control FSW process parameters. An FSW experimental setup was also fabricated to ensure a 3° of tool tilt angle to the welded materials during the tests. Type K thermocouples located on the test fixture, placed below the weld nugget as illustrated in Figure 2, were used to measure temperature. Temperature values were monitored using an Arduino Uno system with 4 MAX6675 temperature acquisition modules. Data acquisition was performed using CoolTerm software.

Figure 2
Experimental setup and thermocouples positions.

The experiment employed four rotational speeds (850, 950, 1050 and 1150 rpm) and two feeding rates (20 and 80 mm/min), a fixed tool shoulder penetration of 0.3 mm, and a constant tilt angle of 3 degrees. Figure 3 presents the assembly setup of the specimens and their key features.

Figure 3
Dimensions and detailed view of the assembly of the specimens.

To reveal the microstructure, the material was sanded with 200, 400, 600, and 1200 grit papers, and then polished with 1-micron alumina (Al₂O₃). For microstructural analysis, the chemical etching was performed using a 50/50 mixture of HNO₃ (nitric acid) and H₂O for approximately 7 seconds. To evaluate the process, macro and microstructural characterization of the material was performed using a Zeiss Axio Vert A1 microscope with a Zeiss Smartzoom 5 camera.

A microhardness profile was also analyzed using a Buehler Microhardness Tester on the Vickers HV 0.1 kgf scale. Microhardness measurements were performed along four lines, two below and two above the contact interface of the sheets, with a spacing of 0.5 mm between them. Along each line, 13 measurements were taken, starting at the center, with seven toward the advancing side and seven toward the retreating side, with a spacing of 0.5 mm between measurements.

3. Results

Figure 4 displays the results for all test setups that were not visually approved. This is due to the increased energy generated by the tool's rotation. Rotation directly influences defect formation, and a lack of rotation can lead to surface voids due to difficulties in material flow and mixing¹². The authors also noted that excessive rotation can increase flow, thereby enabling the formation of voids in the joint region.

Figure 4
Analyses of the samples obtained by rotation and feed rate variation.

The tests conducted at 850 rpm produced two distinct results. The first sample, welded at a feed rate of 20 mm/min, achieved a defect-free surface; however, its nugget zone lacked mixing and exhibited a significant tunnel defect on the brass side. In contrast, the sample produced at an 80 mm/min feed rate exhibited a surface lack-of-fill defect. For the 950 rpm and 20 mm/min condition, a defect-free surface was achieved; however, the presence of two tunnel defects compromised the nugget zone, and significant flash was observed on both sides of the weld. Conversely, the 80 mm/min condition failed to produce a defect-free surface. The 1050 rpm and 20 mm/min condition was the only set of parameters that achieved a defect-free surface and a welded zone in the process. The condition with a feed rate of 80 mm/min produced a weld bead with intermittent surface defects. The condition with a feed rate of 20 mm/min yielded a weld bead with a continuous surface tunnel. At a feed rate of 80 mm/min, the condition led to a pin fracture due to increased heat input from the tool.

Table 3 presents the maximum temperatures reached during the FSW process under the tested conditions. It can be observed that the temperature decreases as the feed rate increases. The tool shoulder is the primary source of heat generation in the process, and this, coupled with the tool's linear travel²⁷,²⁸, means that when the tool moves faster, less heat is generated in some areas. The temperature consequently drops, this is attributed to the greater heat input from the tool shoulder, which rotates in the same region for a longer duration at lower advancing speeds.

Table 3
Maximum temperatures on the FSW process.

	Tmax	% ΔT
S850 F20	334.25	20.72
S850 F80	265.00
S950 F20	361.75	24.73
S950 F80	272.29
S1050 F20	458.00	33.95
S1050 F80	302.50
S1150 F20	467.20	30.86
S1150 F80	323.00

Analyzing the maximum temperature values, it can be observed that the 1150 rpm condition reached the highest values in all thermocouples. Analyzing the data, it is evident that all thermocouples' values decrease as the tool moves. Thus, it can be observed that processes that initiated FSW welding at temperatures below 410 °C did not yield a visually approved test result. Therefore, characterization steps were performed only on the 1050 rpm sample. The percentage increase between tests conducted under constant rotational speed and varying traverse speeds is presented as % ΔT.

3.1. Condition approved upon visual inspection

The only successful test was performed at 1050 rpm with a feed rate of 20 mm/min. Figure 5 shows the image obtained after polishing and etching the 1050 rpm sample. A transition in grain size is observed, indicating a difference in homogeneity between the base material and the regions affected by the process. The retreating side (RS) exhibits significantly larger grains and larger copper particles than the advancing side (AS). In copper-aluminum research, large particles have also been observed in the retraction plane within the thermo-mechanically affected zone (TMAZ)²⁰. Additionally, the brass grain size decreases as it approaches the stir zone where the tool pin was inserted. Hook structures were observed in the interface between the materials. A hook formation was also observed in copper-aluminum joining experiments, with the notable similarity of the copper structure being extended into the aluminum alloy region²¹. It is also worth mentioning that a tunnel defect appeared after the material preparation for micrographic analysis. It was also observed in the FSW process of overlapping copper sheets.

Figure 5
Macrograph of the 1050 rpm sample.

3.2. Microstructure observations

Figure 6 depicts the 50x magnifications obtained via macrography. It allows for a more detailed observation of defects such as kissing bonds, hooks, and voids. Figure 5a highlights the microstructural difference between the base metal and the thermo-mechanically affected zone (TMAZ), revealing a change in the grain sizes of the brass at the top of the weld. Figure 5b shows a hook defect, which typically occurs on the retreating side and directly affects the material's physical properties⁹. Figures 55d illustrate a kissing bond defect resulting from inefficient material deformation due to inadequate tool pressure and low material stirring²⁹.

Figure 6
Defects presented under conditions of 1200rpm and F20 mm/min with 5x magnification.

Figure 7 shows 10x magnifications of Figures 5b, 5c, 5d. These magnifications reveal variations in grain size, with less uniformity observed closer to the weld center. Furthermore, the hook defect is caused by smaller grains consistently found surrounding the large Cu particle. It can be observed that the grain size decreases as the stir zone is approached. This fact has also been noted in the FSW process between aluminum and copper³⁰.

Figure 7
Micrographs of 50x magnified regions.

Figure 8 presents the microstructure of the nugget (mixing zone), showing a gradual transition from left to right, which reflects the material flow and mixing during the process. The transition region between the two alloys is where the mechanical mixing of the materials occurs. This region experiences both diffusion and physical mixing, while still preserving some compositional and contrast differences.

Figure 8
Microstructure of the Cu–Brass FSW nugget.

Deformation twins (highlighted in the micrograph) can be observed. Their presence indicates intense deformation, characteristic of the mechanical action of the FSW tool pin, and demonstrates that twinning was an active mechanism for accommodating plastic strain³¹. Furthermore, the larger grain size in this region suggests that grain refinement was less pronounced in the brass compared to the copper.

The observed twins can be classified as deformation twins, formed by the mechanical action of the FSW process. They are in regions adjacent to the nugget (the stir zone), where deformation is most intense. In the nugget zone, dynamic recrystallization of the material can be observed, with the brass predominantly in the α-phase. Under the approved welding condition, the maximum temperature reached 458°C. Sun et al.³¹ reported dynamic recrystallization in the nugget due to high temperatures and intense agitation, noting that brass typically recrystallizes into the α-phase around 300°C.

3.3. Microhardness

Figure 9 presents the microhardness profiles measured in the test conducted at 1050 rpm and 20 mm/min. It can be observed that the microhardness values decrease as the weld nucleus is approached. It is also noticeable that this trend of lower values near the nucleus is mitigated in lines L1 and L2, regions where the tool pin does not act directly. On the other hand, this decrease in microhardness is more pronounced in lines L3 and L4, which are in the more turbulent zone of the mixture. The values obtained consistently declined compared to the base materials.

Figure 9
Microhardness profiles.

The microhardness values indicate a 30% reduction compared to the base metals. A paper noted that the variation is influenced by grain size, which is affected by workpiece heating and friction²⁰. This reduction in hardness can be attributed to factors other than grain size, which typically increases hardness. Key influencing factors include microporosity, tunneling defects, and a lower dislocation density. For the brass, the temperature range is 450–650°C. Consequently, the system's internal energy facilitated recovery and recrystallization in specific microstructural regions, thereby reducing dislocation density³².

4. Conclusions

Based on the experimental results of the Friction Stir Welding (FSW) process used on the overlapped joint between UNS C19400 (Cu–Fe) and UNS C27200 (brass) alloys, the following conclusions can be drawn.

• Among the tested combinations, only the condition of 1050 rpm and 20 mm/min produced a visually defect-free weld. However, metallographic analysis revealed an internal tunnel defect within the nugget zone, indicating that although the surface appeared continuous, complete internal material consolidation was not achieved. This finding highlights the high sensitivity of the FSW process to thermomechanical conditions and material flow behavior.

• A maximum temperature of 458 °C, reached 1050 rpm and a feed rate of 20 mm/min, was sufficient to promote effective mixing between copper and brass without melting either material. A clear link was observed between increased rotational speed and higher process temperatures, while higher travel speeds reduced heat input and hampered material mixing. Welds produced at temperatures below 410 °C exhibited surface defects.

• Microstructural analysis revealed grain refinement within the stir zone due to dynamic recrystallization from severe plastic deformation. The interface showed partial interpenetration of the Cu-rich and Zn-rich phases, suggesting mechanical mixing with limited interdiffusion. The presence of deformation twins confirms high shear strain and the activation of plastic accommodation mechanisms typical of the FSW process.

• The microhardness profile showed an average reduction of 30% compared to the base metals, especially in the central nugget region. This drop is due to thermal recrystallization, lower dislocation density, and the presence of microvoids and internal flaws. The thermomechanically affected zones (TMAZ) displayed intermediate hardness levels, while the unaltered regions kept hardness close to that of the original base metals.

5. Data Availability

The datasets generated and analyzed during the current study that support the findings are available from the corresponding author upon reasonable request.

6. Acknowledgments

To the São Paulo Research Foundation (FAPESP) and the São Paulo Federal Institute of Education, Science and Technology, Brazil, for the infrastructure provided for the experiments for the development of the research. Termomecanica São Paulo S.A. for the opportunity to carry out this work. To the Center for Research and Innovation in Materials and Structures (CEPIMATE) for the technical support.

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Edited by

• Associate Editor:
  Ana Sofia de Oliveira.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Publication Dates

• Publication in this collection
  28 Aug 2026
• Date of issue
  2026

History

• Received
  05 Dec 2025
• Reviewed
  27 Mar 2026
• Accepted
  02 July 2026