The friction stir welding process (friction stir welding/processing, FSW/FSP) has wear problems related to stirring tools. In this study, the plasma transfer arc (PTA) method was used with stellite 1 powders (Co-based) to coat on the head of a SKD61 stirring tool (SKD61-ST1) in order to investigate the wear performance and phase transformation of SKD61-ST1 after FSW. Under the same experimental parameters, the wear data were compared with the high-speed steel SKH51 (tempering material SKH51-T and annealed material SKH51-A) and tungsten-carbide cobalt (TCC). Results showed the PTA coating was a
Temperature of friction stir welding (FSW) occurs under the melting point of metal and thus can prevent typical solidification defects of traditional joining methods [
In recent years, FSW has been widely used in Al alloys, but the stirring rods have wear problems and reduce the life of the stirring rods [
The SKD61 is a tool steel with good thermal-crack resistance, and it is cheaper than SKH51 [
For above reasons, this study used PTA process to coat stellite 1 powders on SKD61 alloy steel to form the stirring tool (SKD61-ST1). The coating structure of the SKD61-ST1 tool was investigated and also used in FSW for a 5083 aluminum alloy substrate to discuss the wear characteristics. Notably, the stirring wear rates of SKH51 high-speed steels (annealed SKH51-A and tempering SKH51-T) and TCC were compared with SKD61-ST1 under the same experimental parameters. In addition, the wear mechanism (wear induced phase transformation) of SKD61-ST1 was investigated. The phase transformation of the stirring tool can confirm the wear fracture characteristics, and the correlation data may serve as a reference for the application of a FSW system.
PTA coated stellite 1 powders were used on the surface of SKD61 mold steel to form an SKD61-ST1 stirring rod. SKH51 high-speed steel (SKH51-A) was annealed, underwent oil quenching, and was then tempered to form an SKH51 high-speed steel tempering material (SKH51-T: oil quenching at 1230°C for 4 minutes; tempering at 560°C for 2 hours). This experiment involved the use of four stirring rods: (1) SKD61-ST1, (2) SKH51-A, (3) SKH51-T, and (4) TCC. FSW performed four rods in a 5083 aluminum alloy substrate in order to acquire the wear data for the stirring rods under the same experimental parameters. After the FSW, the pin edge of the stirring rod was observed, and the hardness of the head of the stirring rod was measured. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe X-ray microanalyzer (EPMA), and X-ray diffraction (thin film XRD) were used to investigate the wear characteristics and wear induced phase transformation.
Stellite 1 powders were coated on the head of the SKD61 stirring rod, the thickness of the stellite 1 overlayer was 3~4 mm. To polish the stellite 1 overlayer, a pin with a height of 1 mm and a diameter of 5 mm was formed on the surface of the shoulder with a diameter of 19.3 mm. A schematic diagram of the SKD61-ST1 stirring rod is shown in Figure
Chemical composition of the SKD61 mold steel (PTA coated substrate) (wt.%).
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0.35 | 0.80 | 0.40 | 0.02 | 0.02 | 4.50 | 1.20 | 1.00 | Bal. |
Chemical composition of the stellite 1 cobalt-based alloy (PTA coating powder) (wt%).
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32.00 | 2.50 | 11.50 | 2.00 | Bal. |
PTA process parameters.
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1200 g/hr |
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4 l/min |
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3 l/min |
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24 l/min |
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90A |
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36V |
Stellite overlayer processing to the pin and shoulder.
OM metallurgical phase of the SKD61 annealing material.
The SKD61-ST1, SKH51-A, SKH51-T, and TCC stirring rods were the same size (Figure
The friction stir process (FSP) parameters.
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1256 RPM |
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4 mm/s |
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1.5° |
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43.4 MPa |
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Annealed 5083 Al alloy |
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150 × 46 × 13 mm3 |
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5 m |
Chemical composition of 5083 aluminum alloy (wt.%).
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0.40 | 0.40 | 0.10 | 0.45 | 4.50 | 0.25 | 0.15 | 0.05 | Bal. |
Schematic diagram of the friction stir process.
Microstructure of 5083 aluminum alloy annealed materials.
Before and after the FSW, the weight of stirring rod was measured to obtain the wear rate. After the FSW, the head of the stirring was adhered to the 5083 aluminum alloy, so the stirring head was put into an aqueous solution of 50 wt.% NaOH to remove the 5083 aluminum alloy and then the weight loss was measured.
After the FSW, the pin and shoulder of the SKH51-A, SKH51-T, and SKD61-ST1 stirring rods were cut to observe the worn subsurface using SEM, EDS, and EPMA. In addition, the coverage area, hardness, and thickness of wear inducing a new phase on the pin and shoulder of the stirring rods were measured, for which the measurement data was the average of 10 values. The wear inducing phases by FSW on the pin surface were identified using thin film XRD. Notably, the wear inducing phases was removed and then detected the structure of the pin to confirm the wear phase transformation.
Using a PTA coated stellite 1 cobalt-based superalloy on SKD61 steel, the cross-section of the coated specimen was observed, as shown in Figure
The PTA coated stellite 1 SKD61 substrate: OM metallographic transverse cross-section coated specimen.
From the literature [
The PTA coated stellite 1 SKD61 in the SEM/EDS analyses.
PTA coating stellite 1 measured in the SKD61 for microhardness.
The stirring rod matrix was an annealed SKD61 steel, for which the matrix was ferrite, and M6C carbides were dispersed in the matrix [
The solidification structure of the stellite 1 overlayer: (a) OM images; (b) SEM images.
X-ray diffraction pattern of the stellite 1 overlayer.
90A current of the stellite 1 alloy on the SKD61 tool steel, carbide overlayer: (a) low-magnification; (b) high-magnification SEM microscopic observations.
SKD61-ST1, SKH51-A, SKH51-T, and TCC stirring rods walked 5 meters under the same FSW process conditions after removal of the 5083 aluminum alloy adhering to the stirring rods in order to measure the wear rate of the four stirring rods. Figure
The wear resistance of the SKD61-ST1, annealed SKH51 (SKH51-A), tempered SKH51 (SKH51-T), and tungsten-carbide cobalt (TCC) stirring tool after FSW walking five meters and measurement of the amount of wear.
After the FSW, a cross-section of the stirring rod was taken at the head prior to removal of the SKD61-ST1 aluminum alloy to observe and compare the hardness of the stirring heads before and after the FSW. The original hardness of the stirring head was 56 HRC, and after FSW, it was 54 HRC, so there was no significant difference. The observations of the high temperature wear on the stirring rod shoulder and pin are shown in Figure
SKD61-ST1 stirring tool after FSW running five meters; observation of the pin edge.
Symbol E shows the left of interface between the induced new microstructure and stellite 1. Symbol D shows the new microstructure on the pin of the stirring rod after FSW associated with the interaction with the 5083 aluminum alloy, which contained dark (Dd) + white (Dw) mixed structures. C shows the 5083 aluminum alloy adhering on the surface of the stirring pin. The semiquantitative data on the surface of the stirring rod using EDX are shown in Figure
SKD61-ST1 after FSW, SEM-EDS semiquantitative elemental analysis of the pin surface (data for the average of five measurements).
EPMA mapping was used to detect the element distribution of Figure
SKD61-ST1 after FSW, EPMA mapping analysis of the pin surface.
To understand the phase transfer layer in Part II, in this experiment, NaOH solution was used to remove the residual aluminum alloy on the rod-head (Part III) to expose and obtain the phase transfer layer in Part II (SKD61-ST1-FSP). No. 2000 sandpaper was used to grind the SKD61-ST1-FSW specimen to remove the transfer layer of Part II and obtained the rod-head (Part I). This specimen was called SKD61-ST1-FSW-MOVE. The original specimen without the FSW process was called SKD61-ST1. These three specimens were detected using thin film XRD, for which the results are shown in Figure
Thin film XRD before and after FSW of SKD61-ST1 spares the rod.
Overlayer of stellite 1 was a solid structure containing M7C3 and M23C6 carbides. The dilution effect between the SKD61 and the stellite 1 overlayer was not obvious. The overlayer solidification was a The stellite 1 stirring rod (SKD61-ST1) had better wear resistance, and its wear rate was lower than that of the SKH51 high-speed steel stirring rod. The wear rate of the SKH51-T stirring rod was much lower than that of the SKH51-A stirring rod. During the FSW, the stellite 1 stirring rod incurred sliding with the 5083 aluminum alloy, and the high temperature created a wear induced phase transfer layer on the pin and shoulder surface of the stirring rods. This transfer layer (Al9Co2) peeled and caused mass loss of the stirring rod.
The OM image, SEM/EDS analysis, microhardness, XRD, and EPMA mapping analysis datas used to support the findings of this study are included within the article.
The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.
The authors declare no conflicts of interest.
The authors are grateful to MOST Instrument Center of National Cheng Kung University (NCKU) and MOST 106-2811-E-006-033 for financial support for this research. This research was funded by the Ministry of Science and Technology (MOST).