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On the formation of onion rings in friction stir welds
Materials Science and Engineering A327 (2002) 246– 251 www.elsevier.com/locate/msea
On the formation of onion rings in friction stir welds K.N. Krishnan * CRC for Welded Structures, Department of Mechanical Engineering, Adelaide Uni6ersity, Adelaide 5005, Australia Received 12 December 2000; received in revised form 7 May 2001
Abstract Onion rings are the most prominent features of most friction stir welds. The origin and the effect of these on properties are not clearly understood. In this paper, an attempt has been made to explain the formation of onion rings. The formation of onion ring is found to be a geometric effect due to the fact that cylindrical sheets of material are extruded during each rotation of the tool and the cutting through the section of the material produces an apparent ‘Onion Rings’. It is postulated that the tool appears to wait for a very short time to produce frictional heat and extrude a cylindrical shaped material around to the retreating side of the joint. The spacing of the markings has been found to be equal to the forward motion of the tool in one rotation. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Onion rings; Friction stir welding; Welding; Aluminium; Extrusion; Friction
1. Introduction One of the first things that strikes anyone looking at the cross-section of a friction stir weld (FSW) is probably the onion rings. A picture of the onion rings in the cross-section of a FSW is shown in Fig. 1. It would be rather difficult to understand that these swirl patterns are in a plane 90° to the rotation plane of the tool. Some explanations for the presence of onion rings have been offered. Biallas et al. [1] explained the formation of onion rings was due to the reflection of the material flow from the cooler walls of the HAZ. The induced circular motion leads to circles that decrease in radii and form the tube system. Threadgill [2] correctly guessed that the onion ring formation was associated with the forward motion of the tool in one revolution. He, however, did not offer any explanation and said that the formation process is unlikely to affect the properties of the weld. Leonard [3] found that longitudinal and plan sections revealed semicircular features that corresponded to the onion rings. He also felt that these rings appear to have no practical significance as the properties of the nugget are generally good. * Tel.: + 61-8-8303-6156; fax: +61-8-8303-4367; www.mecheng. adelaide.edu.au/staff/kris/krishna.html. E-mail address: [email protected] (K.N. Krishnan).
An explanation to the formation of the onion rings that are an integral part of the FSW has been eluding. An attempt has been made to explain the formation of the onion rings, the practical significance, and their likely effect on properties.
2. Experimental details FSW of 6061 and 7075 alloys have been carried out at different welding parameters. Table 1 gives a summary of the welding parameters used. The welds were sectioned in all the three perpendicular planes and macrostructural pictures taken. The spacings between various features were measured.
3. Results and discussion The scheme for the various sections photographed is shown in Fig. 2. The macrostructure of half of a weld showing the onion rings (XZ Plane) is exhibited in Fig. 3. The important aspect of the onion ring structure is that in the centre the size of the light regions are wider and they become narrower as we progress towards the periphery.
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 4 7 4 - 5
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
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The side view (ZY plane) of the FSW is shown in Fig. 4. This shows it consists of regularly spaced curved lines. The spacing of the lines is about 0.29 mm. There is a region on top, which corresponds to what was compacted by the shoulder. The lines in the centre are curved away from the welding direction. The macrophotograph of the weld cut in the middle
Fig. 3. Picture of half of the cross-section (XZ plane) showing onion rings; 400 rpm, 120 mm/min.
Fig. 1. Onion rings in the cross-section of a FSW. Table 1 Various welding parameters trialled in this work
1 2 3 4 5
rpm
Welding speed (mm/min)
400 1440 1440 800 800
120 120 288 120 288
Fig. 2. Scheme for the various pictures presented later in this work.
of the Z-axis consisting of the XY plane is shown in Fig. 5. This picture shows semicircular rings with a spacing of about 0.27 mm. What is also seen in the picture is that the Thermo Mechanically Affected Zone extends further away from the main ‘nugget’ region. The picture of the top surface of the weld is shown in Fig. 6. The spacings of the semicircular rings are about 0.28 mm matching the spacing of the rings in Fig. 5. A schematic picture combining all the pictures shown in Figs. 3–6 is presented in Fig. 7. On top surface, we see the familiar semicircular marks and on the side containing the welding direction are the curved lines that were shown in Fig. 4. The onion rings are on the cross-sectional plane. The features, shown in Fig. 7, can be found if the semicircular marks are essentially considered to be part of a cylinder cut in half. The friction welding process can be thought to be simply extruding one layer of semicylinder in one rotation of the tool. A cross-sectional slice through such a set of semicylinder results in the familiar onion ring structure. This idea is further demonstrated by the use of modelling clay. This model shows clay of two different colours as seen in Fig. 8. The semicircular rings can be seen on the top. The model has been made by pressing different coloured semicylinders together. When a section is taken through the cylinders, the onion ring structure is got as shown in Fig. 9. The presence of semicircles on the top surface shows that the tool seems to wait for a short time during which the rotation of the tool produces heat and the forward motion of the tool extrudes the hot metal. The continuation of this process produces a continuous set of semicircular rings. The spacing between the rings also matches the distance moved by the tool in one rotation. The idea that the FSW is likened to an extrusion process is described by Colligan [4] and Reynolds et al. [5]. During each rotation of the tool a semicylindrical
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K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
Fig. 4. Picture of the section from side (ZY plane) showing lines curving away from the welding direction, 400 rpm, 120 mm/min.
Fig. 5. Picture showing semicircular features from a section of XY plane cut half way from the Z axis; 400 rpm, 120 mm/min.
Fig. 6. Picture of the top surface of a FSW, 400 rpm, 120 mm/min.
portion of the material is pushed to the back of the tool and around to the retreating side of the tool. This means that there is very little material mixing of the material. This idea has been supported by Colligan [4] in that he found that the tracer steel balls in his experiments moves around the pin to the retreating side and also rose to the top. There were more amounts of ‘stirring’ near the top. Similar ideas have been put forth by Reynolds et al. [5]. They state that this process is likened to an extrusion process most certainly for the ‘cold welds’ but only varies in degree for ‘hot welds’. Larsson et al. [3] measured composition profiles in dissimilar alloy FSW and found that though there was intimate bonding between the alloys, there was only limited chemical mixing. All these results lead to the idea that FSW in most cases is an extrusion process. A different explanation to the formation of onion rings had been put forth by Biallas et al. [1]. They say that when the material flows around the pin within the plane of the sheet, it is reflected approximately at the imaginary walls of the groove that would be formed in the case of regularly milling the metal. The induced circular movement leads to circles that decrease in radii and form the tube system. This would mean that there should be thorough mixing in the ‘nugget’ region. This is not possible in that if such a ‘reflection’ were to take place, the material behind the tool must be in a fluid state. This is quite contrary to what has been generally observed and, as explained previously, there is very little mixing and the material is very viscous and the process is simply an extrusion process. A FSW was prepared with a rusted backing plate. The weld failed in bend test and the picture of the fracture surface is shown in Fig. 10. It can be seen from this picture the curved line features, similar to what was shown in Fig. 4. What can also be seen is that the rust has been actually stirred up into the joint at the interface between the curved lines. Colligan [4] also found that the material flow was upward. So clearly the FSW consists of semicylinders extruded by the tool. Next, we have to consider if the onion rings have any practical significance. According to the above discussion the FSW is an extrusion process. During each rotation of the tool the metal in front of the tool is heated by friction and the forward motion extrudes the metal to the back of the tool. If the material is to be extruded to the back of the tool, then there should be a cavity at the back. If there is a cavity, most certainly air occupies the space and oxidation of the clean extruded metal must take place. Short time contact with the air is likely to produce only slight oxidation and this oxidised material is most likely to be at on the surface of each semicylinder. Larsson et al. [3] actually found oxidised particles in their investigation. If one were to look at the photographs presented in their paper, one
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
249
can see such oxide particles being distributed along an arc. This is most likely to be along the onion rings and which will be on the surface of the semicylindrically shaped extruded material. Because these oxide particles are probably not oriented favourably to the stress axis, they have not given any cause for concern. Time may come when there would be a need to use a shielding gas for removal of the oxidation.
4. Size of the rings and the welding parameters
Fig. 7. Schematic of the various microstructural features in a FSW.
Fig. 8. Clay model showing that semicylinders are pressed together to model the microstructural features in a FSW.
Fig. 9. A section through the semicylinders in Fig. 8 produces the onion rings.
The size of the semicircular features (XY plane in Fig. 2) and the curved lines (ZY plane in Fig. 2), found to be related to the forward motion of the tool in one rotation, was discussed in previous sections. Various welds with different welding parameters were produced, as shown in Fig. 11, and the spacing between the semicircular features was measured and there was a close match between measured and calculated values. A graph illustrating this point is presented in Fig. 12. What can be seen from this picture is that for various combinations of the welding parameters the spacing of the semicircular features can be equal. At low rpm values the differences in the spacing can be quite large while at high rpm the spacings differ very little. Biallas et al. [1] found that when they increased the tool speed the onion rings vanished. The ratio of welding speed to rpm was kept constant. It is just possible that a very high increase in rpm resulted in very hot metal, the mechanism of weld formation no longer stop – produce frictional heating – extrude. That is the reason why semicircular rings were probably no longer visible. Very high temperatures would also reduce friction and the weld is likely to contain a lot of flash as well.
Fig. 10. Curved lines can be seen on the fracture surface of a FSW.
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K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
Fig. 11. A good match between calculated and measured values for different welding parameters. a) 1440 rpm, 120 mm/min; spacing, mm = 0.079 (measured), 0.08 (calculated). b) 1440 rpm, 288 mm/min; spacing, mm =0.188 (measured), 0.2 (calculated). c) 800 rpm, 120 mm/min; spacing, mm= 0.143 (measured), 0.15 (calculated). d) 800 rpm, 288 mm/min; spacing, mm= 0.33 (measured), 0.36 (calculated).
Fig. 12. Relationship between semicircular features and the rpm for different welding speeds.
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
5. Conclusions (1) The appearance of onion rings has been attributed to a geometrical effect in that a section through a stack of semicylinders would appear like onion rings with ring spacing being wider at the centre and narrower towards the edge. (2) The formation of the onion rings is due to the process of friction heating due to the rotation of the tool and the forward movement extrudes the metal around to the retreating side of the tool. (3) The spacing of the rings is equal to the forward movement of the tool in one rotation.
Acknowledgements The work reported herein was undertaken as part of a Research Project 99.80 of the Cooperative Research Centre for Welded Structures. The University of Ade-
251
laide is a core partner of the CRC-WS. The Cooperative Research Centre for Welded Structures was established in July 1999 and is supported under the Australian Government’s Cooperative Research Centres Program.
References [1] G. Biallas, et al., Mechanical Properties and corrosion behaviour of friction stir welded 2024-T6, First International Conference on Friction Stir Welds, Thousand Oaks, CA, 1999. [2] P.L. Threadgill, Friction Stir Welding – State of the art, TWI report-678/1999. [3] H. Larsson, et al., Joining of dissimilar Al-alloys by Friction Stir Welding, Second International Conference on Friction Stir Welds, Gothenburg, Sweden, 2000. [4] K. Colligan, Material Flow Behaviour During Friction Stir Welding of Aluminium, unpublished data. [5] A.P. Reynolds, et al., Visualisation of material flow in an autogenous friction stir weld, First International Conference on Friction Stir Welds, Thousand Oaks, CA, 1999.
.
On the formation of onion rings in friction stir welds K.N. Krishnan * CRC for Welded Structures, Department of Mechanical Engineering, Adelaide Uni6ersity, Adelaide 5005, Australia Received 12 December 2000; received in revised form 7 May 2001
Abstract Onion rings are the most prominent features of most friction stir welds. The origin and the effect of these on properties are not clearly understood. In this paper, an attempt has been made to explain the formation of onion rings. The formation of onion ring is found to be a geometric effect due to the fact that cylindrical sheets of material are extruded during each rotation of the tool and the cutting through the section of the material produces an apparent ‘Onion Rings’. It is postulated that the tool appears to wait for a very short time to produce frictional heat and extrude a cylindrical shaped material around to the retreating side of the joint. The spacing of the markings has been found to be equal to the forward motion of the tool in one rotation. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Onion rings; Friction stir welding; Welding; Aluminium; Extrusion; Friction
1. Introduction One of the first things that strikes anyone looking at the cross-section of a friction stir weld (FSW) is probably the onion rings. A picture of the onion rings in the cross-section of a FSW is shown in Fig. 1. It would be rather difficult to understand that these swirl patterns are in a plane 90° to the rotation plane of the tool. Some explanations for the presence of onion rings have been offered. Biallas et al. [1] explained the formation of onion rings was due to the reflection of the material flow from the cooler walls of the HAZ. The induced circular motion leads to circles that decrease in radii and form the tube system. Threadgill [2] correctly guessed that the onion ring formation was associated with the forward motion of the tool in one revolution. He, however, did not offer any explanation and said that the formation process is unlikely to affect the properties of the weld. Leonard [3] found that longitudinal and plan sections revealed semicircular features that corresponded to the onion rings. He also felt that these rings appear to have no practical significance as the properties of the nugget are generally good. * Tel.: + 61-8-8303-6156; fax: +61-8-8303-4367; www.mecheng. adelaide.edu.au/staff/kris/krishna.html. E-mail address: [email protected] (K.N. Krishnan).
An explanation to the formation of the onion rings that are an integral part of the FSW has been eluding. An attempt has been made to explain the formation of the onion rings, the practical significance, and their likely effect on properties.
2. Experimental details FSW of 6061 and 7075 alloys have been carried out at different welding parameters. Table 1 gives a summary of the welding parameters used. The welds were sectioned in all the three perpendicular planes and macrostructural pictures taken. The spacings between various features were measured.
3. Results and discussion The scheme for the various sections photographed is shown in Fig. 2. The macrostructure of half of a weld showing the onion rings (XZ Plane) is exhibited in Fig. 3. The important aspect of the onion ring structure is that in the centre the size of the light regions are wider and they become narrower as we progress towards the periphery.
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 4 7 4 - 5
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
247
The side view (ZY plane) of the FSW is shown in Fig. 4. This shows it consists of regularly spaced curved lines. The spacing of the lines is about 0.29 mm. There is a region on top, which corresponds to what was compacted by the shoulder. The lines in the centre are curved away from the welding direction. The macrophotograph of the weld cut in the middle
Fig. 3. Picture of half of the cross-section (XZ plane) showing onion rings; 400 rpm, 120 mm/min.
Fig. 1. Onion rings in the cross-section of a FSW. Table 1 Various welding parameters trialled in this work
1 2 3 4 5
rpm
Welding speed (mm/min)
400 1440 1440 800 800
120 120 288 120 288
Fig. 2. Scheme for the various pictures presented later in this work.
of the Z-axis consisting of the XY plane is shown in Fig. 5. This picture shows semicircular rings with a spacing of about 0.27 mm. What is also seen in the picture is that the Thermo Mechanically Affected Zone extends further away from the main ‘nugget’ region. The picture of the top surface of the weld is shown in Fig. 6. The spacings of the semicircular rings are about 0.28 mm matching the spacing of the rings in Fig. 5. A schematic picture combining all the pictures shown in Figs. 3–6 is presented in Fig. 7. On top surface, we see the familiar semicircular marks and on the side containing the welding direction are the curved lines that were shown in Fig. 4. The onion rings are on the cross-sectional plane. The features, shown in Fig. 7, can be found if the semicircular marks are essentially considered to be part of a cylinder cut in half. The friction welding process can be thought to be simply extruding one layer of semicylinder in one rotation of the tool. A cross-sectional slice through such a set of semicylinder results in the familiar onion ring structure. This idea is further demonstrated by the use of modelling clay. This model shows clay of two different colours as seen in Fig. 8. The semicircular rings can be seen on the top. The model has been made by pressing different coloured semicylinders together. When a section is taken through the cylinders, the onion ring structure is got as shown in Fig. 9. The presence of semicircles on the top surface shows that the tool seems to wait for a short time during which the rotation of the tool produces heat and the forward motion of the tool extrudes the hot metal. The continuation of this process produces a continuous set of semicircular rings. The spacing between the rings also matches the distance moved by the tool in one rotation. The idea that the FSW is likened to an extrusion process is described by Colligan [4] and Reynolds et al. [5]. During each rotation of the tool a semicylindrical
248
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
Fig. 4. Picture of the section from side (ZY plane) showing lines curving away from the welding direction, 400 rpm, 120 mm/min.
Fig. 5. Picture showing semicircular features from a section of XY plane cut half way from the Z axis; 400 rpm, 120 mm/min.
Fig. 6. Picture of the top surface of a FSW, 400 rpm, 120 mm/min.
portion of the material is pushed to the back of the tool and around to the retreating side of the tool. This means that there is very little material mixing of the material. This idea has been supported by Colligan [4] in that he found that the tracer steel balls in his experiments moves around the pin to the retreating side and also rose to the top. There were more amounts of ‘stirring’ near the top. Similar ideas have been put forth by Reynolds et al. [5]. They state that this process is likened to an extrusion process most certainly for the ‘cold welds’ but only varies in degree for ‘hot welds’. Larsson et al. [3] measured composition profiles in dissimilar alloy FSW and found that though there was intimate bonding between the alloys, there was only limited chemical mixing. All these results lead to the idea that FSW in most cases is an extrusion process. A different explanation to the formation of onion rings had been put forth by Biallas et al. [1]. They say that when the material flows around the pin within the plane of the sheet, it is reflected approximately at the imaginary walls of the groove that would be formed in the case of regularly milling the metal. The induced circular movement leads to circles that decrease in radii and form the tube system. This would mean that there should be thorough mixing in the ‘nugget’ region. This is not possible in that if such a ‘reflection’ were to take place, the material behind the tool must be in a fluid state. This is quite contrary to what has been generally observed and, as explained previously, there is very little mixing and the material is very viscous and the process is simply an extrusion process. A FSW was prepared with a rusted backing plate. The weld failed in bend test and the picture of the fracture surface is shown in Fig. 10. It can be seen from this picture the curved line features, similar to what was shown in Fig. 4. What can also be seen is that the rust has been actually stirred up into the joint at the interface between the curved lines. Colligan [4] also found that the material flow was upward. So clearly the FSW consists of semicylinders extruded by the tool. Next, we have to consider if the onion rings have any practical significance. According to the above discussion the FSW is an extrusion process. During each rotation of the tool the metal in front of the tool is heated by friction and the forward motion extrudes the metal to the back of the tool. If the material is to be extruded to the back of the tool, then there should be a cavity at the back. If there is a cavity, most certainly air occupies the space and oxidation of the clean extruded metal must take place. Short time contact with the air is likely to produce only slight oxidation and this oxidised material is most likely to be at on the surface of each semicylinder. Larsson et al. [3] actually found oxidised particles in their investigation. If one were to look at the photographs presented in their paper, one
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
249
can see such oxide particles being distributed along an arc. This is most likely to be along the onion rings and which will be on the surface of the semicylindrically shaped extruded material. Because these oxide particles are probably not oriented favourably to the stress axis, they have not given any cause for concern. Time may come when there would be a need to use a shielding gas for removal of the oxidation.
4. Size of the rings and the welding parameters
Fig. 7. Schematic of the various microstructural features in a FSW.
Fig. 8. Clay model showing that semicylinders are pressed together to model the microstructural features in a FSW.
Fig. 9. A section through the semicylinders in Fig. 8 produces the onion rings.
The size of the semicircular features (XY plane in Fig. 2) and the curved lines (ZY plane in Fig. 2), found to be related to the forward motion of the tool in one rotation, was discussed in previous sections. Various welds with different welding parameters were produced, as shown in Fig. 11, and the spacing between the semicircular features was measured and there was a close match between measured and calculated values. A graph illustrating this point is presented in Fig. 12. What can be seen from this picture is that for various combinations of the welding parameters the spacing of the semicircular features can be equal. At low rpm values the differences in the spacing can be quite large while at high rpm the spacings differ very little. Biallas et al. [1] found that when they increased the tool speed the onion rings vanished. The ratio of welding speed to rpm was kept constant. It is just possible that a very high increase in rpm resulted in very hot metal, the mechanism of weld formation no longer stop – produce frictional heating – extrude. That is the reason why semicircular rings were probably no longer visible. Very high temperatures would also reduce friction and the weld is likely to contain a lot of flash as well.
Fig. 10. Curved lines can be seen on the fracture surface of a FSW.
250
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
Fig. 11. A good match between calculated and measured values for different welding parameters. a) 1440 rpm, 120 mm/min; spacing, mm = 0.079 (measured), 0.08 (calculated). b) 1440 rpm, 288 mm/min; spacing, mm =0.188 (measured), 0.2 (calculated). c) 800 rpm, 120 mm/min; spacing, mm= 0.143 (measured), 0.15 (calculated). d) 800 rpm, 288 mm/min; spacing, mm= 0.33 (measured), 0.36 (calculated).
Fig. 12. Relationship between semicircular features and the rpm for different welding speeds.
K.N. Krishnan / Materials Science and Engineering A327 (2002) 246–251
5. Conclusions (1) The appearance of onion rings has been attributed to a geometrical effect in that a section through a stack of semicylinders would appear like onion rings with ring spacing being wider at the centre and narrower towards the edge. (2) The formation of the onion rings is due to the process of friction heating due to the rotation of the tool and the forward movement extrudes the metal around to the retreating side of the tool. (3) The spacing of the rings is equal to the forward movement of the tool in one rotation.
Acknowledgements The work reported herein was undertaken as part of a Research Project 99.80 of the Cooperative Research Centre for Welded Structures. The University of Ade-
251
laide is a core partner of the CRC-WS. The Cooperative Research Centre for Welded Structures was established in July 1999 and is supported under the Australian Government’s Cooperative Research Centres Program.
References [1] G. Biallas, et al., Mechanical Properties and corrosion behaviour of friction stir welded 2024-T6, First International Conference on Friction Stir Welds, Thousand Oaks, CA, 1999. [2] P.L. Threadgill, Friction Stir Welding – State of the art, TWI report-678/1999. [3] H. Larsson, et al., Joining of dissimilar Al-alloys by Friction Stir Welding, Second International Conference on Friction Stir Welds, Gothenburg, Sweden, 2000. [4] K. Colligan, Material Flow Behaviour During Friction Stir Welding of Aluminium, unpublished data. [5] A.P. Reynolds, et al., Visualisation of material flow in an autogenous friction stir weld, First International Conference on Friction Stir Welds, Thousand Oaks, CA, 1999.
.