The wear issue of a polycrystalline boron nitride (PCBN) tools during the friction stir welding of two grades of steel, DH36 and EH46, was studied. Two welding traverse and tool rotational speeds were used when welding the DH36 steel. A low tool speed (200RPM, 100 mm/min) and a high tool speed (550RPM, 400 mm/min) were denoted by W1D and W2D, respectively. Nine welding conditions were applied to the welding of EH46 steel plate including seven plunge/dwell trials (W1E–W7E) and two steady-state trials (W8E and W9E). SEM–EDS and XRD tests were applied in order to reveal the boronitride (BN) particles inside the welded joints, and the percentage (%) of BN was calculated according to the standard quantitative metallographic technique. The findings showed that tool wear increases when the tool rotational speed increases as a result of binder softening which is a function of the peak temperature (exceeds 1250 °C) at the tool/workpiece interface. When considering the EH46 steel trials, it was found that an increase in the tool traverse speed in friction stir welding caused a significant tool wear with 4.4% of BN in the top of the stirred zone of W9E compared to 1.1% volume fraction of BN in W8E which was attributed to the higher thermomechanical action on the PCBN tool surface. Tool wear was also found to increase with an increase in tool plunge depth as a result of the higher contact between the surface of friction stir welding tool and the workpiece.
The physical properties of PCBN compared with some other materials [2]
Property
Units
PCBN
Tungsten carbide
4340 Steel
Coefficient of friction
…
0.10–0.15
0.2
0.78
Coefficient of thermal expansion
10−6/°C
4.6–4.9
4.9–5.1
11.2–14.3
Thermal conductivity
W/mK
100–250
95
48
Compressive strength
N/mm2
2700–3500
6200
690
103 psi
391–507
899
100
Fracture toughness
MPa m2
3.5–6.7
11
100
Hardness
Knoop kg/mm2
2700–3200
…
278
Vickers kg/mm2
2600–3500
1300–1600
280
Tensile strength
N/mm2
…
1100
620
103 psi
…
160
89.9
Transverse
N/mm2
500–800
2200
…
Rupture strength
103 psi
72–115
319
…
Fig. 1
(a) An SEM image of the microstructure of a cross section through a PCBN Q70 (70% cBN, 30%WRe) [6] FSW tool, (b) PCBN tool image
Table 2
PCBN tool geometry provided by TWI
Tool
PCBN shoulder
PCBN probe
Collar
Shank
PCBN tool geometry for welding 6 mm thickness
23.7 mm diameter with spiral convex shape
5.5 mm length, 10 mm base diameter
Spiral tapered with 20 thread per inch (TPI)
23.9 mm inner diameter
37 mm outer diameter
24 mm length
23.9 mm diameter
80.35 mm length
PCBN tool geometry for welding 10–15 mm thickness
38 mm mm diameter with spiral convex shape
12 mm length, 20 mm base diameter
Spiral tapered with 20 thread per inch (TPI)
38.1 mm inner diameter
52 mm outer diameter
30 mm length
38.1 mm diameter 100 mm length
×
Wear resistance in hybrid PCBN FSW tools is considered better than other refractory materials such as tungsten-based tools. W-25%Re FSW tool life of 4 m was reported in welding steel grades and titanium [7]. However, wear issues still exist in PCBN tools when welding high melting alloys especially the high strength alloys such as the EH46 steel grade. An experimental study of a PCBN tool has been carried out by Rai et al. [3] and showed that the main factors which can cause wear are abrasion and diffusion. Softening and recrystallization of the binder after reaching a welding temperature of 1000 °C is also found to be one reason for reducing the tools resistance to wear [3]. Ramalingam and Jacobson [8] reported a decrease in Knoop hardness (HK) of W-25Re from HK 675 (HV 638) to HK 500 (HV 478) when carrying out heating from room temperature to 1225 °C. The hardness was found to decrease dramatically to HK 300 (HV 290) when the temperature increases to 1450 °C. Hooper et al. [9] suggested that the higher thermal conductivity of cBN (100–250 W/m K) can result in defects in the microstructure when the temperature exceeds 1200 K, while with a comparison with cBN-TiC, they found that a protective layer (mainly TiC) is formed on the latter and is associated with the higher temperature as a result of lower thermal conductivity of cBN-TiC. PCBN tool wear can also change the properties of the material being welded. For example tool wear can affect negatively a stainless steel welded joint as boron particles can react with Cr to form borides which in turn can result in a reduction in corrosion resistance [3]. When welding titanium plates, boron can improve the mechanical properties of the joint because of the ability to react with Ti-forming TiB2 which in turn can cause grain refinement and a hardness increase [10]. PCBN tool breakage is more likely to occur during the FSW process due to the lower fracture toughness of a PCBN tool compared to a WC tool, Table 1. Unsuitable welding parameters such as improper plunging or extracting and low welding temperatures were determined as the main factors for tool breakage [5].
Anzeige
Previous work on PCBN tool wear during the friction stir welding of steel did not investigate the possibility of W-Re binder softening as a result of high temperature which may reach the local melting of steel alloys during the process, especially when the tool rotational speed reaches specific limits. Also limited work has been carried out to investigate the effect of tool traverse speed on the wear rate and the consequences on the physical properties of the welded joints. The current work focuses on PCBN tool wear which occurs during the FSW process of two steel grades, DH36 and EH46. The FSW tool wear has been studied by calculating the volume fraction of BN (originally from the PCBN FSW tool) existed in the thermomechanical affected zone (TMAZ) microstructure. The aims are to identify the causes of PCBN tool wear and to highlight the different suitable welding parameters that can increase the tool longevity.
Experimental Method
Chemical Composition of Parent Metal and Welding Conditions
Tables 3 and 4 show the as-received chemical composition (wt.%) of the 6-mm-thick DH36 and 14-mm-thick EH46 steel plates. The welded samples have been produced at TWI/Yorkshire using their PowerStir™ FSW machine, employing a Q70 PCBN FSW tool. The welding conditions used for the production of samples of DH36 steel (W1D and W2D) are shown in Table 5. These were chosen to examine the effect of tool rotational and traverse speeds on tool wear. In order to examine tool traverse speeds above 200 mm/min, it is necessary to simultaneously increase the tool rotational speed. The welding conditions for the production of samples of EH46 steel (W1E–W7E) which examined the effect of rotational speed and plunge depth on tool wear are shown in Table 6. The welding process parameters used to examine the effect of tool traverse speed on tool wear (W8E and W9E) for EH46 steel steady state are shown in Table 7.
Table 3
Chemical composition of as-received DH36 steel grade (wt.%)
C
Si
Mn
P
S
Al
N
Nb
V
Ti
Cu
Cr
Ni
Mo
0.16
0.15
1.2
0.01
0.005
0.043
0.02
0.02
0.002
0.001
0.029
0.015
0.014
0.002
Table 4
Chemical composition of as-received EH46 steel grade (wt.%)
C
Si
Mn
P
S
Al
N
Nb
V
Ti
0.20
0.55
1.7
0.03
0.03
0.015
0.02
0.03
0.1
0.02
Table 5
Welding conditions for FSW DH36 steel samples W1D and W2D
Examining the effect of traverse speed on tool wear
Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) Examination
SEM and EDS were carried out on polished (until 1 μm) and etched (2% nital) FSW samples using the FEI Nova Nano SEM. The examination included the surface of the stirred zone (SZ) and thermomechanical affected zone (TMAZ) which are the regions which experienced both thermal and mechanical effects during the FSW process under steady-state conditions and at the plunge period. The SEM produced high-quality and high-resolution images of microconstituents using the secondary electron (SE) imaging mode with an accelerating voltage of between 10 and 20 kV. The working distance (WD) used was 5 mm, but in some cases, it was altered (decreased or increased) to enhance the contrast at higher magnification. EDS–SEM examination was mainly used to detect the BN particle inside the TMAZ. Spot analysis (point and ID) has been employed to elementally identify small second-phase particles in the scanned SEM images. Additionally, EDS mapping was used to analyze the whole scanned SEM area.
X-ray Diffraction
X-ray diffraction (Cu X-ray tube) using the Empyrean Philips XRD has been carried out on the FSW samples for the following reasons:
To reveal and characterize the as-received sample phases in order to allow for the detection of any phase changes in the steel following FSWing.
To detect other additional phases, elements, and/or boronitride (BN) particles which may appear in the welded joints.
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Infinite Focus Microscopy (IFM)
The infinite focus microscopy (IFM) (Alicona) has been employed to create accurate optical light microscopy images of the welded joint. The IFM is a device based on optical 3-dimensional measurements which has the ability to vary the focus in order to obtain a 3D vertical scanned image of the surface. The scanned area of interest can be transferred into a 3D image by the aid of Lyceum software; thus, the surface area can be calculated accurately.
Calculating the Percentage (%) of BN in the Welded Joints
The % of BN particles originating from the PCBN FSW tool in the TMAZ of the welded joints were calculated using the SEM. A 1 mm2 area between the shoulder and the probe side (W1E–W7E) and a square mm (1 mm2) from the middle top of the SZ of sample W8E and W9E were scanned at a maximum magnification of 10,000×. The area fraction of BN particles and TiN precipitates has been measured manually using the square grid method. The number of intersections of the grid falling in the BN particles is counted and compared with the total number of points laid down [11].
Result
Effect of Tool Rotation and Traverse Speed on Tool Wear in FSW of DH36 Steel: Samples W1D and W2D
The existence of BN particles originating from the PCBN FSW tool was investigated in the FSW joints of grade DH36 steel which had been produced at high tool speeds in order to attempt to assess tool wear. SEM–EDS examination of FSW joint W2D (DH36, 550RPM, 400 mm/min) in Figs. 2 and 3 shows different sizes of BN particles in the SZ. Figure 4a and b shows the XRD scan results from samples W1D and W2D, respectively. Note that peaks of ferrite are shown in both welds and BN peaks are seen in sample W2D. The low tool speed welded joint of DH36 W1D (200RPM, 100 mm/min) did not show a significant presence of BN particles in the microstructure.
Fig. 2
Ten-micrometer BN particles in SZ of FSW DH36 joint (W2D 550RPM, 400 mm/min)
Fig. 3
SEM image of the top of SZ of DH36 W2D shows the existence of BN
Fig. 4
XRD scan of FSW of DH36 steel (a) W1D (200RPM, 100 mm/min) microstructure consists of BCC ferrite after the FSW process. (b) W2D (550RPM, 400 mm/min), microstructure is mainly ferritic BCC phase; BN peaks are also present (Co-X-ray tube)
×
×
×
Effect of Tool Rotational Speed and Actual Plunge Depth on Tool Wear in FSW EH46 Steel: Samples W1E–W7E
The presence of BN particles has been investigated in the microstructure of FSW samples of EH46 weld joints during the plunge/dwell period in the shoulder-probe region. The shoulder-probe region is believed to experience the highest material flowing due to the thermomechanical combination of both the shoulder and the probe tool parts. Figures 5, 6, 7, 8, 9, 10, and 11 are SEM images (low and high magnification) of samples W1E–W7E (plunge/dwell cases of EH46 grade steel), respectively, which show different sizes and volume fraction of BN particles. Figure 12a and b shows the BN particles at the probe side bottom (region-2 bottom) of samples W2E and W6E (EH46 plunge/dwell period), respectively. Figure 13 shows the XRD scans taken of samples W1E–W7E; peaks of ferrite and BN can be recognized. Table 8 shows the IFM measurements of plunge depth and areas of affected zones of samples W1E–W7E (EH46 steel). Table 9 shows the calculated percentage (%) area fraction of BN particles in a 1 mm2 scanned microstructure of the shoulder-probe region [11].
Fig. 5
SEM images of EH46 (W1E) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 µm. (a) Low magnification (region under tool shoulder) and (b) high magnification
Fig. 6
SEM images of EH46 (W2E) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 µm. (a) Low magnification (region under tool shoulder) and (b) high magnification
Fig. 7
SEM images of EH46 (W3E) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 µm. (a) Low magnification (region under tool shoulder) and (b) high magnification
Fig. 8
SEM images of EH46 (W4E) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 µm. (a) Low magnification (region under tool shoulder) and (b) high magnification
Fig. 9
SEM images of EH46 (W5E) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 µm. (a) Low magnification (region under tool shoulder) and (b) high magnification
Fig. 10
SEM images of EH46 (W6E) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 µm. (a) Low magnification (region under tool shoulder) and (b) high magnification
Fig. 11
SEM images of EH46 (W7E) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 µm. (a) Low magnification (region under tool shoulder) and (b) high magnification
Fig. 12
EH46 at plunge/dwell period, probe side bottom (region-2 bottom), (a) W2E and (b) W6E
Fig. 13
XRD scan of FSW of grade EH46 steel (plunge cases from W1E–W7E at region 1 under the shoulder). The microstructure is mainly ferrite phase. BN peaks are presented
Table 8
The welding conditions and resulting IFM measurements of the plunge depth and areas of the various affected zones for samples W1E–W7E EH46 steel
Weld no.
Tool rotational speed ω (RPM) at maximum plunge depth
Max. plunge force (Fz) KN
Max. traverse force (Fx) KN
Max. torque (M) N m
Plunge depth (Z) mm (IFM)
Time (t) sec at (dwelling) plunge period
Total TMAZ (region 1 + 2) area mm2 (IFM)
(Region 3) IHAZ mm2 (IFM)
(Region 4) OHAZ mm2 (IFM)
W1E
200
157
17
498
11.05
6
47.46
64.7
82
W2E
200
127
17
471
11.43
8
67.5
78.5
102
W3E
120
116
21
598
11.56
7
58
69.6
112.6
W4E
120
126
20
549
11.47
6
66
64.25
118
W5E
120
115
17
532
11.47
7
55
93.5
120.4
W6E
120
105
18
583
11.78
7
68
99.5
143.8
W7E
120
119
20
548
11.57
7
57.2
91
120
Table 9
BN percentage (%) in EH46 plunge period at shoulder/probe side region, the scanned area is 1 mm2
Weld no.
W1E
W2E
W3E
W4E
W5E
W6E
W7E
BN%
1.4
2.8
0.65
0.7
0.9
3.3
1.2
×
×
×
×
×
×
×
×
×
Tool Wear in FSW EH46 Steel for Samples W8E and W9E FSW Under Steady-State Conditions
PCBN tool wear in samples W8E and W9E (EH46) steady state has also studied by revealing the BN particles in the microstructure in order to understand the effect of tool rotational/traverse speeds on the wear issue. Figure 14 is a high magnified SEM image showing a 13-µm BN particle found in the SZ of W8E; the binder of W-Re is also shown (the darker phase). Figures 15 and 16 show the SEM–EDS scanning (point and ID technique) of BN of FSW W8E at the top surface of the SZ and at the probe end, respectively. Figures 17 and 18 show the SEM–EDS of BN in the SZ of FSW W9E at the top surface of the SZ and at the probe end, respectively.
Fig. 14
Large BN particle in the top of SZ of FSW EH46 (W8E)
Fig. 15
BN particles with different sizes at the top of SZ of FSW EH46 (W8E)
Fig. 16
0.5-µm BN particle at the probe end of FSW EH46 (W8E)
Fig. 17
Top surface of the SZ (steady state) showing evidence of different sizes of BN particle (EH46 W9E)
Fig. 18
Evidence of BN particle in SZ of the probe end region of FSW EH46 (W9E)
×
×
×
×
×
SEM images of the top surface of SZ and in the probe end of W8E and W9E are shown in Figs. 19, 20, 21, and 22, respectively. Figure 23 is an SEM image with high magnification of W9E at the probe end which shows BN particles. Table 10 shows the calculated percentage (%) [11] of BN in a 1 mm2 scanned microstructure at the middle top of the SZ and at the probe end of samples W8E and W9E. Figure 24 shows the XRD of samples W8E and W9E, respectively.
Fig. 19
Top center of SZ of EH46 W8E (steady state), showing evidence of BN particles, (a) low magnification and (b) high magnification (etched in 2% nital)
Fig. 20
EH46 W8E (steady state). (a) The middle of SZ (no BN particles), microstructure is mainly acicular ferrite, (b) probe end SZ (BN particles are present), microstructure is mainly granular ferrite and some short plated cementite
Fig. 21
Top middle center of SZ of EH46 W9E (steady state) showing evidence of BN particles, (a) low magnification (un-etched) and (b) high magnification (etched)
Fig. 22
EH46 W9E (steady state). (a) The middle of SZ (no BN particles), microstructure is mainly acicular ferrite, (b) probe end (BN particles are evident), the microstructure is mainly granular ferrite and cementite
Fig. 23
High-magnification SEM image of sample W9E (EH46) (steady state) at the probe end showing BN particles
Table 10
BN percentage (%) in EH46 steady-state of samples W8E and W9E at the middle top of SZ and at the probe end, the scanned area is 1 mm2
Weld no. and region
W8E top SZ
W8E probe end
W9E top SZ
W9E probe end
BN%
1.1
1.3
4.4
2.8
Fig. 24
XRD scan of FSW of EH46 grade (comparison between W8E and W9E at the top of SZ, W9E shows a stronger peak of BN than W8E, Cu X-ray-tube)
×
×
×
×
×
×
Discussion
Tool Wear in FSW DH36 at High Tool Rotational/Traverse Speeds
SEM–EDS of FSW DH36 joint W2D (550RPM, 400 mm/min) in Figs. 2 and 3 shows different sizes of BN particles in the SZ. Tool wear is expected to be a result of the parent material resistance to the thermomechanical process that is friction stir welding. Although the tool torque for sample W1D (Table 5) was higher than that of W2D, it did not show significant evidence of BN particles in the microstructure [as revealed by X-ray diffraction, Fig. 4a and b]. The W-Re binder softening [8] as a result of temperature increase coming from the increase in tool rotational speed is most likely to be the reason for the higher % of BN and therefore the higher PCBN tool wear in sample W2D rather than in W1D. Also the higher traverse speed of the FSW tool in W2D accompanied with higher-temperature generation can exacerbate the tool wear, leading to a disastrous damage of the FSW tool. The top surface of the SZ as shown in Fig. 3 showed the highest presence of BN particles. These results are in agreement with the published work [12] in which it was found from modeling of the FSW of DH36 that the maximum temperature can approach the melting point of DH36 steel (1450 °C) when applying tool speeds of 550RPM/400 mm/min. W-25Re hardness was found in previous work to reduce by about 50% when the temperature increases from room temperature to 1450 °C [8].
Tool Wear in FSW EH46 W1–W7 Plunge/Dwell Cases
Figures 5, 6, 7, 8, 9, 10, and 11 are SEM images (low and high magnification) of samples W1E–W7E plunge/dwell cases, respectively. The images are taken from the shoulder-probe region and show the different sizes and amounts of BN particles present. BN particles sizes were detected between 0.5 and 13 µm, and the percentage (%) of BN was calculated [11] and is reported in Tables 9 and 10. Depending on the welding conditions and the calculated results of TMAZ size and plunge depth mentioned in Table 8, the calculated percentage (%) of BN particles have varied as shown in Table 9. Sample W6E showed the maximum percentage (%) of BN in the shoulder-probe region as a result of maximum plunge depth and TMAZ size. W3E, W4E, W5E, and W7E have shown the lowest values of BN particles which can be attributed to the low tool rotational speed and also low plunge depth. W2E also showed a higher % of BN in the shoulder-probe region compared to W1E which may be a result of the higher plunge depth despite the fact that the same tool rotational speeds were applied. The %BN in sample W2E was also higher than W3E, W4E, W5E, and W7E which have almost the same plunge depth as W2E. This finding can be attributed to the higher tool rotational speed of W2E which in turn can cause an increase in the temperature at the tool/workpiece contact region, and thus, greater softening of the W-Re binder can be expected. Figure 12a and b is SEM images of the probe side bottom of sample W2E and W6E, respectively, which show significant % of BN particles, particularly in W6E and less in W2E. The higher plunge depth in W6E may be the reason for this increase in tool wear. The XRD result (Fig. 13) shows ferrite and BN peaks, but no phase change in the microstructure or no recrystallization of the cubic BN has occurred.
Tool Wear in FSW EH46 W8E and W9E FSW Steady State
Figure 14 is an SEM image with high magnification which shows a 13-µm BN particle in the SZ of FSW of W8E joint; the binder of W-Re was also detected by SEM–EDS and is also shown. Softening of the binder accompanied by mechanical action (tool rotational/traverse speeds) may be the reason for the separation of the BN particles from the PCBN FSW tool which is then followed by those released particles becoming attached and entrapped in the SZ microstructure of the workpiece during the FSW process. Figures 15 and 16 show the SEM–EDS scanning (spot analysis) of BN in a FSW of sample EH46 W8E at the top surface of the SZ and at the probe end, respectively. Figures 17 and 18 show the SEM–EDS of BN in the SZ of FSW EH46 sample W9E at the top surface of the SZ and at the probe end, respectively. More BN particles were shown in sample W9E at the top and bottom of the SZ rather than in sample W8E which is the result of increasing the tool traverse speed from 50 to 100 mm/min. More SEM images of the top surface of SZ and in the probe end of samples W8E and W9E are shown in Figs. 20, 21, and 22, respectively. Figure 21a shows a significant amount of BN particles at the top surface of the SZ of sample W9E until 250 µm depth, whereas Fig. 21b is a higher-magnification SEM image of the SZ which shows BN particles surrounded by W-Re binder (the darker color). The middle of the SZ of samples W8E and W9E did not show a significant presence of BN as shown in Figs. 20a and 22a, respectively, whereas BN at the probe end of both welds is clearly evident in Figs. 20b and 22b. The significant existence of BN in the top and bottom of the SZ but less in the middle of the SZ can be attributed to the fact that tool edge (top and bottom) is the most vulnerable locations with regard to wear as a result of experiencing higher temperatures or higher shear stress. Al-Moussawi et al. [12] showed by simulation that the tool shoulder periphery has experienced the maximum peak temperature on the advancing-trailing side and the maximum shear stress was on the leading-retreating side. They also showed that at higher traverse speeds, the maximum value of shear stress was at the shoulder periphery and probe end. This can be seen in Fig. 23 where BN particles are found at the probe end of sample W9E as a result of higher traverse speed. Table 10 shows the calculated percentage (%) of BN in a 1 mm2 at the middle top of the SZ and at the probe end of samples W8E and W9E. Wear of the FSW tool at the tool shoulder periphery in W9E is about 3 times that in sample W8E, whereas, at the probe end it is approximately double. This finding is supported by the XRD analysis shown in Fig. 24 taken from the middle top of the SZ which shows that the peak associated with BN in sample W9E is stronger than in sample W8E.
Conclusion
From the work carried out the following can be concluded:
PCBN FSW tool wear has been found to increase with an increasing tool rotational speed as a result of W-Re binder softening. The top of the SZ and the weld root regions have showed the maximum presence of BN particles which indicates that the shoulder and probe end are the most affected tool parts for wear as a result of the thermomechanical effect.
Increasing the plunge depth is associated with an increase in tool wear as a result of the increase in the surface contact area which in turn raises the temperature in the tool/workpiece contact region.
Increasing the tool traverse speed has resulted in an increase in tool wear especially at the tool shoulder periphery. The increase in the value of shear stress on the tool surface was the main reason of this wear.
The current study represents a step change in understanding the PCBN tool wear during the FSW process. Tool wear can be reduced by choosing the suitable combination of tool rotational/traverse speeds and plunge depth.
Acknowledgments
The authors would like to thank the Ministry Of Higher Education, Iraq, for funding this project. Thanks are also due to The Welding Institute (TWI) for providing the friction welded samples and processing data.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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