- Email: [email protected]
Relationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Relationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool M.St. We˛glowskia,n, S. Dymekb a
Institute of Welding, Testing of Materials Weldability and Welded Constructions Department, Błogosławionego Czesława Str. 16–18, 44-100 Gliwice, Poland b AGH-University of Science & Technology, 30-059 Krako´w, 30 Mickiewicza Ave., Poland
art i cle i nfo
ab st rac t
Article history:
In the present study, an investigation has been carried out on the friction stir processing
Received 27 November 2012
(FSP) of an AlSi9Mg cast aluminium alloy. The relationship between the FSP parameters
Accepted 4 January 2013
and the torque action on the tool, temperature of the modified surface and microstructure was investigated. It was found that an increase in rotational speed of the tool causes a
Keywords:
decrease in the torque and an increase in temperature of the processed material.
Friction modified processing
Simultaneously, the results showed that an increase in travelling speed of the tool
Aluminium alloys
increases the torque and decreases the temperature. The metallographic examination of
Modification
the processed surface layer of the material has shown that the microstructure of stir zone can be refined. The penetration depth in which the microstructure was modified by the shoulder action mainly depended on the rotation speed and to a lesser extent on the travelling speed. & 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Friction stir processing (FSP) is based on the friction stir welding (FSW) technique which was invented by The Welding Institute (TWI, Great Britain) in 1991 [9]. The phenomena occurring during FSW, mainly grain refinement, were adopted to the modification of microstructure rather than to material joining. Friction stir processing is also a solid-state process in which a specially designed rotating cylindrical tool, consisting of a pin (or without pin) and a shoulder, is plunged into the material. The tool is then traversed in the desired direction. The contact of the rotating shoulder increases the temperature of the modified surface, the temperature, however, remaining
Fig. 1 – Schematic illustration of the friction stir processing.
n
Corresponding author. Tel.: þ48 32 335 8236; fax: þ48 32 231 4652. E-mail addresses: [email protected] (M.St. We˛glowski), [email protected] (S. Dymek).
1644-9665/$ - see front matter & 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.01.003
Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
2
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
below the melting point of the alloy. The generated heat softens the material which can then be easily stirred by the rotating tool. The material undergoes intense plastic deformation yielding a dynamically recrystallised fine grain structure. Schematic illustration of FSP is shown in Fig. 1. Friction stir processing utilises all of the benefits associated with friction stir welding [6]. Therefore, the friction stir process can be used as a unique process to modify the microstructure in the surface layers of metallic materials. Research in friction stir techniques over the past decade has mainly focused on the properties of welded joints by the FSW process [14]. A large number of experimental and modelling works have been performed in order to characterise the different zones in the joint area, e.g. the heat affected zone (HAZ). Specific for the FSW process is the presence of a nugget and a thermomechanically affected zone (TMAZ), which makes the description of the weld properties complex. The mechanical properties of a FSW joint, i.e. strength and toughness, are related to microstructure of the particular zones and are well described in the literature [2,3,5,7]. The current status of friction stir research has been well summarized by Lohwasser [4]. The friction modified processing involves complex processes such as flow of material, temperature distribution, rotary forces, tool design etc. that are not fully understood. Focusing on friction stir welding, but applicable to frictional modification process, the review papers of Mishra
[5] and Threadgill et al. [10] identify three critical research issues: material flow, tool material and shape, and microstructural stability. Although, intensive investigations of FSW have been performed in recent years [8], there is an urgent need of new developments in the FSP technique. Most of the available results of FSP focus on microstructural characteristics of the processing zone. Some preliminary investigations about the relationship between parameters of friction stir processes and forces, torque and temperature are also available [11–13]. The objective of this work is to estimate the correlation between FSP parameters and torque, temperature and microstructure. One major difference between the present work and previous ones, cited in literature, is the wider range of applied rotational and travelling speeds as well as more precise measurement of torque acting on the tool and processing temperature.
2.
Experimental procedure
Investigations were carried out on a specially designed work stand assembled on a conventional FYF32JU2 vertical milling
Table 1 – Chemical composition of AlSi9Mg aluminium alloy (wt%). Si
Fe
Cu
Mn
Mg
Zn
Ti
Al
9.0–10.0
0.19
0.05
0.10
0.25–0.45
0.07
0.15
Bal.
Fig. 2 – (a) Side view of the location of thermocouple and (b) calibration curve of tempetature.
Fig. 3 – Schematics of the stages of friction modified processing. Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
Fig. 4 – Influence of travelling speed on torque (a) and temperature (b) of the modified surface.
3
machine. The work stand was provided with the necessary instrumentation such as clamps for rigid fastening of the processed plates, and a tool-cooling system using compressed air. Additionally, the stand was equipped with LOWSTIR and TermSTIR devices. LOWSTIR is the acronym of LOW cost processing unit for friction STIR welding. The LOWSTIR system, designed with advanced modelling techniques, can be used in conjunction with the milling machines, to produce high quality Friction Stir Welded joints. The LOWSTIR system can measure: transverse force, vertical force and spindle torque. The TermSTIR head serves to measure the temperature of the tool and, after suitable recalculations, the temperature of the processed surface. A non-conventional tool, made of high-speed steel (HS6-5-2), was used in the experiments. The diameter of the tool shoulder was 20 mm. The tool did not have a pin. The material subjected to processing was an AlSi9Mg cast aluminium alloy, supplied as a 6 mm thick plate. The chemical composition of the investigated alloy is given in Table 1. The range of travelling speeds applied in experiments was between 112 and 900 mm/min, whereas the rotational speed varied from 112 to 1800 rpm. Other parameters were kept as constant as possible. The influence of tool rotational and travelling speeds on the torque, temperature and the size of the stir zone was investigated. The variations in temperature during the processing were measured by a K-type thermocouple inserted to a place located 1.0 mm away from the tip of the rotating tool shoulder (Fig. 2a). All measured temperatures were then adjusted based on calibration curve (Fig. 2b) to obtain the surface temperature. Calibration curves were calculated for each
Fig. 5 – Influence of rotational speed on torque (a) and temperature (b) of the modified surface.
4
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
travelling speeds applied in experiments. The mean value of the spindle torque was measured by LOWSTIR and calculated from 100 points in the area of the fully stabilized FMP process. The temperature and torque measurements were repeated three times at each FMP parameter setting. The microstructure of the processed material specimens was investigated by a light microscope (LM Leica MEF4M) and a scanning electron microscope (SEM FEI Quanta 200 FEG supplemented by energy dispersive spectrometry (EDS) provided by the EDAX company). For light microscopy the samples were mechanically polished and observed without etching. For SEM the samples were electropolished in a solution of perchloric acid and ethanol (1:5) at 10 V and 12 1C for 1 min and observed in SEM also without etching—the Z-contrast formed by backscattered electrons was utilised for revealing constituent phases. The processed plates were fixed to the stand with the special clamps. The contact surface of processed plates was not cleaned (after milling). The particular stages of the process are schematically shown in Fig. 3. In FSP, the heat is initially generated due to friction between the tool shoulder being brought into contact with the processed material. The tool is then hard pressed on the workpiece and softens the material. When temperature is raised to a proper value, the tool starts to travel. This, in turn, starts the process of microstructure modification through stirring of the material by the rotating tool. Once a desired length of modification is achieved, the process is completed and the tool is removed.
3.
Results and discussion
3.1.
Measurement
It should be emphasised that the signals recorded during FSP are characteristic for the specific tool geometry (dimensions and shape of shoulder and pin (if present)), parameters of the process, parent material, measurement system and experimental setup. The influence of travelling and rotational speeds on spindle torque is shown in Figs. 4a and 5a, respectively. The dependence of torque on the travelling speed (Fig. 4a) at a constant rotational speed was evidenced in the literature [1]. The decrease in torque may be due to two contributing factors. Firstly, for a constant tool rotation speed and decreasing travelling speed, the volume of material being deformed on each revolution decreases, hence the heat is generated in a smaller volume—this in turn may lead to slightly higher temperatures (Fig. 4b) and lower flow stress. Secondly, lower travelling speeds will reduce the convective cooling, resulting from slower movement into the relatively cooler material in the front of the tool. The spindle torque strongly depends on the rotational speed of the FSP tool (Fig. 5a). This is because the rotational speed stimulates the temperature in the FSP area (modified material) and thus the friction coefficient decreases when temperature increases (Fig. 5b). It should be noted that during experiments the down force was almost constant and undergo only negligible fluctuations.
3.2.
Fig. 6 – Typical cross-section of the processed surface layer; light microscope.
Metallography examination
The typical LM microstructure of the processed material is shown in Fig. 6. Two well defined regions (separated by a white line in Fig. 6) could be easily distinguished: the parent material (lower area) and the FSP zone (upper area). In the parent material coarse acicular Si particles were distributed along the primary aluminium dendrite boundaries while the microstructure in the processed zone was refined by the tool action. Furthermore, the parent material exhibited numerous pores. The SEM BSE examination revealed characteristic dendrites in the base material (Fig. 7). Fig. 8 shows a region closer to the surface where the microstructure changes in a continuous way from that typical for the parent material to the refined one
Fig. 7 – Microstructure SEM of as-cast AlSi9Mg alloy. Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
5
Fig. 8 – Typical cross-section of the processed surface layer; SEM.
adjacent to the surface. FSP resulted in a significant refinement of large Si particles and subsequent uniform distribution in the aluminium matrix. Furthermore, porosity in the as-cast AlSi9Mg was nearly eliminated by FSP. So, as evident from the present study a dramatic change in microstructure occurs, even when a tool without a pin is used in the FSP (Fig. 8). The metallographic examination allowed for evaluating the thickness of the layer under the surface where the microstructure was refined due to the FSP process (penetration depth). The dependence of the penetration depth on the travelling and rotational speeds is illustrated in Fig. 9. The results shown in Fig. 9 indicate that the rotational speed strongly affected the penetration depth of the modified zone. The decrease in penetration following an increase in rotation speed is due to an increase in temperature within the stir zone. The increase in temperature of the modified material lowers its strength and thus the processed alloy carries lower load. In the same time the increase of rotational speed caused increase the friction linear speed, and that friction adhesions become lower. For these two reasons the volume and mass of modified material become lower at increase of rotational speed and temperature. Therefore the penetration depth of modified zone is lower.
4.
Conclusions
Based on the experimental results—measurement and metallographic examination—the following conclusions may be formulated: the increase of the rotational speed decreases the spindle torque acting on the tool; simultaneously, the temperature of the modified surface decreases; the increase of the travelling speed does not influence the spindle torque acting on the tool in a substantial way— only a slight increase was observed; simultaneously it decreases the surface temperature of the modified area; the increase of the rotational speed decreases the penetration depth; the increase of the travelling speed has a negligible effect on the penetration depth compared to the influence of the rotational speed;
Fig. 9 – Influence of travelling speed (a) and rotational speed (b) of penetration depth.
the friction stir processing applied to the alloy surface refines the microstructure in the near-surface layer; and the FMP process of an cast AlSi9Mg aluminium alloy reduces casting porosity.
Acknowledgements This work was financially supported by the Polish Ministry of Science and Higher Education. The research was performed
Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
6
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
within the framework of the statutory activity of the Instytut Spawalnictwa and was partly supported by the Grant no. N507 295439 realized in the AGH University of Science and Technology Faculty of Metals Engineering and Industrial Computer Science.
references
[1] A. Arora, R. Nandan, A.P. Reynolds, T. DebRoy, Torque, power requirement and stir zone geometry in friction stir welding through modelling and experiments, Scripta Materialia 60 (2009) 13–16. [2] C. Hamilton, S. Dymek, I. Kalemba, M. Blicharski, Friction stir welding of aluminium 7136-T76511 extrusions, Science and Technology of Welding and Joining 13 (8) (2008) 714–720. [3] I. Kalemba, S. Dymek, C. Hamilton, M. Blicharski, Microstructure evaluation in friction stir welded aluminium alloys, Archives of Metallurgy and Materials 54 (1) (2009) 75–82. [4] D. Lohwasser, Z. Chen, Friction stir welding: From basics to applications, Woodhead Publishing, Cambridge, 2009. [5] R.S. Mishra, Friction stir welding and processing, Materials Science and Engineering: R: Reports 50 (2005) 1–78. [6] R.S. Mishra, M.W. Mahoney, Friction Stir Welding and Friction Stir Processing, ASM International, Materials Park (Ohio), 2007.
[7] K. Mroczka, J. Dutkiewicz, A. Pietras, Microstructure of friction stir welded joints of 2017A aluminium alloy sheets, Journal of Microscopy 237 (2010) 521–525. [8] L. Murr, A review of FSW research on dissimilar metal and alloy systems, Journal of Materials Engineering and Performance 19 (2010) 1071–1089. [9] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Much, P. Templesmith, C.J. Dawes, Friction Stir Butt Welding, GB Patent Application no, 9125978.8, 1991. [10] P.L. Threadgill, A.J. Leonard, H.R. Shercliff, P.J. Withers, Friction stir welding of aluminium alloys, International Materials Reviews 54 (2) (2009) 49–93. [11] P. Uliasz, T. Knych, A. Mamala, New method of manufacturing the gradient structure materials on the industrial scale and their application, Archives of Metallurgy and Materials 54 (3) (2009) 711–721. [12] M.St. We˛glowski, A. Pietras, Friction stir processing—analysis of the process, Archives of Metallurgy and Materials 56 (3) (2011) 779–788. [13] M.St. We˛glowski, S. Dymek, Microstructural modification of cast aluminium alloy AlSi9Mg via friction modified processing, Archives of Metallurgy and Materials 57 (1) (2012) 71–78. [14] D. Zhang, F. Long, X. Chen, X. Wen, H. Luo, A review on research status of friction stir welding technology, Advanced Materials Research 335–336 (2011) 379–382.
Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Relationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool M.St. We˛glowskia,n, S. Dymekb a
Institute of Welding, Testing of Materials Weldability and Welded Constructions Department, Błogosławionego Czesława Str. 16–18, 44-100 Gliwice, Poland b AGH-University of Science & Technology, 30-059 Krako´w, 30 Mickiewicza Ave., Poland
art i cle i nfo
ab st rac t
Article history:
In the present study, an investigation has been carried out on the friction stir processing
Received 27 November 2012
(FSP) of an AlSi9Mg cast aluminium alloy. The relationship between the FSP parameters
Accepted 4 January 2013
and the torque action on the tool, temperature of the modified surface and microstructure was investigated. It was found that an increase in rotational speed of the tool causes a
Keywords:
decrease in the torque and an increase in temperature of the processed material.
Friction modified processing
Simultaneously, the results showed that an increase in travelling speed of the tool
Aluminium alloys
increases the torque and decreases the temperature. The metallographic examination of
Modification
the processed surface layer of the material has shown that the microstructure of stir zone can be refined. The penetration depth in which the microstructure was modified by the shoulder action mainly depended on the rotation speed and to a lesser extent on the travelling speed. & 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Friction stir processing (FSP) is based on the friction stir welding (FSW) technique which was invented by The Welding Institute (TWI, Great Britain) in 1991 [9]. The phenomena occurring during FSW, mainly grain refinement, were adopted to the modification of microstructure rather than to material joining. Friction stir processing is also a solid-state process in which a specially designed rotating cylindrical tool, consisting of a pin (or without pin) and a shoulder, is plunged into the material. The tool is then traversed in the desired direction. The contact of the rotating shoulder increases the temperature of the modified surface, the temperature, however, remaining
Fig. 1 – Schematic illustration of the friction stir processing.
n
Corresponding author. Tel.: þ48 32 335 8236; fax: þ48 32 231 4652. E-mail addresses: [email protected] (M.St. We˛glowski), [email protected] (S. Dymek).
1644-9665/$ - see front matter & 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.01.003
Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
2
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
below the melting point of the alloy. The generated heat softens the material which can then be easily stirred by the rotating tool. The material undergoes intense plastic deformation yielding a dynamically recrystallised fine grain structure. Schematic illustration of FSP is shown in Fig. 1. Friction stir processing utilises all of the benefits associated with friction stir welding [6]. Therefore, the friction stir process can be used as a unique process to modify the microstructure in the surface layers of metallic materials. Research in friction stir techniques over the past decade has mainly focused on the properties of welded joints by the FSW process [14]. A large number of experimental and modelling works have been performed in order to characterise the different zones in the joint area, e.g. the heat affected zone (HAZ). Specific for the FSW process is the presence of a nugget and a thermomechanically affected zone (TMAZ), which makes the description of the weld properties complex. The mechanical properties of a FSW joint, i.e. strength and toughness, are related to microstructure of the particular zones and are well described in the literature [2,3,5,7]. The current status of friction stir research has been well summarized by Lohwasser [4]. The friction modified processing involves complex processes such as flow of material, temperature distribution, rotary forces, tool design etc. that are not fully understood. Focusing on friction stir welding, but applicable to frictional modification process, the review papers of Mishra
[5] and Threadgill et al. [10] identify three critical research issues: material flow, tool material and shape, and microstructural stability. Although, intensive investigations of FSW have been performed in recent years [8], there is an urgent need of new developments in the FSP technique. Most of the available results of FSP focus on microstructural characteristics of the processing zone. Some preliminary investigations about the relationship between parameters of friction stir processes and forces, torque and temperature are also available [11–13]. The objective of this work is to estimate the correlation between FSP parameters and torque, temperature and microstructure. One major difference between the present work and previous ones, cited in literature, is the wider range of applied rotational and travelling speeds as well as more precise measurement of torque acting on the tool and processing temperature.
2.
Experimental procedure
Investigations were carried out on a specially designed work stand assembled on a conventional FYF32JU2 vertical milling
Table 1 – Chemical composition of AlSi9Mg aluminium alloy (wt%). Si
Fe
Cu
Mn
Mg
Zn
Ti
Al
9.0–10.0
0.19
0.05
0.10
0.25–0.45
0.07
0.15
Bal.
Fig. 2 – (a) Side view of the location of thermocouple and (b) calibration curve of tempetature.
Fig. 3 – Schematics of the stages of friction modified processing. Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
Fig. 4 – Influence of travelling speed on torque (a) and temperature (b) of the modified surface.
3
machine. The work stand was provided with the necessary instrumentation such as clamps for rigid fastening of the processed plates, and a tool-cooling system using compressed air. Additionally, the stand was equipped with LOWSTIR and TermSTIR devices. LOWSTIR is the acronym of LOW cost processing unit for friction STIR welding. The LOWSTIR system, designed with advanced modelling techniques, can be used in conjunction with the milling machines, to produce high quality Friction Stir Welded joints. The LOWSTIR system can measure: transverse force, vertical force and spindle torque. The TermSTIR head serves to measure the temperature of the tool and, after suitable recalculations, the temperature of the processed surface. A non-conventional tool, made of high-speed steel (HS6-5-2), was used in the experiments. The diameter of the tool shoulder was 20 mm. The tool did not have a pin. The material subjected to processing was an AlSi9Mg cast aluminium alloy, supplied as a 6 mm thick plate. The chemical composition of the investigated alloy is given in Table 1. The range of travelling speeds applied in experiments was between 112 and 900 mm/min, whereas the rotational speed varied from 112 to 1800 rpm. Other parameters were kept as constant as possible. The influence of tool rotational and travelling speeds on the torque, temperature and the size of the stir zone was investigated. The variations in temperature during the processing were measured by a K-type thermocouple inserted to a place located 1.0 mm away from the tip of the rotating tool shoulder (Fig. 2a). All measured temperatures were then adjusted based on calibration curve (Fig. 2b) to obtain the surface temperature. Calibration curves were calculated for each
Fig. 5 – Influence of rotational speed on torque (a) and temperature (b) of the modified surface.
4
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
travelling speeds applied in experiments. The mean value of the spindle torque was measured by LOWSTIR and calculated from 100 points in the area of the fully stabilized FMP process. The temperature and torque measurements were repeated three times at each FMP parameter setting. The microstructure of the processed material specimens was investigated by a light microscope (LM Leica MEF4M) and a scanning electron microscope (SEM FEI Quanta 200 FEG supplemented by energy dispersive spectrometry (EDS) provided by the EDAX company). For light microscopy the samples were mechanically polished and observed without etching. For SEM the samples were electropolished in a solution of perchloric acid and ethanol (1:5) at 10 V and 12 1C for 1 min and observed in SEM also without etching—the Z-contrast formed by backscattered electrons was utilised for revealing constituent phases. The processed plates were fixed to the stand with the special clamps. The contact surface of processed plates was not cleaned (after milling). The particular stages of the process are schematically shown in Fig. 3. In FSP, the heat is initially generated due to friction between the tool shoulder being brought into contact with the processed material. The tool is then hard pressed on the workpiece and softens the material. When temperature is raised to a proper value, the tool starts to travel. This, in turn, starts the process of microstructure modification through stirring of the material by the rotating tool. Once a desired length of modification is achieved, the process is completed and the tool is removed.
3.
Results and discussion
3.1.
Measurement
It should be emphasised that the signals recorded during FSP are characteristic for the specific tool geometry (dimensions and shape of shoulder and pin (if present)), parameters of the process, parent material, measurement system and experimental setup. The influence of travelling and rotational speeds on spindle torque is shown in Figs. 4a and 5a, respectively. The dependence of torque on the travelling speed (Fig. 4a) at a constant rotational speed was evidenced in the literature [1]. The decrease in torque may be due to two contributing factors. Firstly, for a constant tool rotation speed and decreasing travelling speed, the volume of material being deformed on each revolution decreases, hence the heat is generated in a smaller volume—this in turn may lead to slightly higher temperatures (Fig. 4b) and lower flow stress. Secondly, lower travelling speeds will reduce the convective cooling, resulting from slower movement into the relatively cooler material in the front of the tool. The spindle torque strongly depends on the rotational speed of the FSP tool (Fig. 5a). This is because the rotational speed stimulates the temperature in the FSP area (modified material) and thus the friction coefficient decreases when temperature increases (Fig. 5b). It should be noted that during experiments the down force was almost constant and undergo only negligible fluctuations.
3.2.
Fig. 6 – Typical cross-section of the processed surface layer; light microscope.
Metallography examination
The typical LM microstructure of the processed material is shown in Fig. 6. Two well defined regions (separated by a white line in Fig. 6) could be easily distinguished: the parent material (lower area) and the FSP zone (upper area). In the parent material coarse acicular Si particles were distributed along the primary aluminium dendrite boundaries while the microstructure in the processed zone was refined by the tool action. Furthermore, the parent material exhibited numerous pores. The SEM BSE examination revealed characteristic dendrites in the base material (Fig. 7). Fig. 8 shows a region closer to the surface where the microstructure changes in a continuous way from that typical for the parent material to the refined one
Fig. 7 – Microstructure SEM of as-cast AlSi9Mg alloy. Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
5
Fig. 8 – Typical cross-section of the processed surface layer; SEM.
adjacent to the surface. FSP resulted in a significant refinement of large Si particles and subsequent uniform distribution in the aluminium matrix. Furthermore, porosity in the as-cast AlSi9Mg was nearly eliminated by FSP. So, as evident from the present study a dramatic change in microstructure occurs, even when a tool without a pin is used in the FSP (Fig. 8). The metallographic examination allowed for evaluating the thickness of the layer under the surface where the microstructure was refined due to the FSP process (penetration depth). The dependence of the penetration depth on the travelling and rotational speeds is illustrated in Fig. 9. The results shown in Fig. 9 indicate that the rotational speed strongly affected the penetration depth of the modified zone. The decrease in penetration following an increase in rotation speed is due to an increase in temperature within the stir zone. The increase in temperature of the modified material lowers its strength and thus the processed alloy carries lower load. In the same time the increase of rotational speed caused increase the friction linear speed, and that friction adhesions become lower. For these two reasons the volume and mass of modified material become lower at increase of rotational speed and temperature. Therefore the penetration depth of modified zone is lower.
4.
Conclusions
Based on the experimental results—measurement and metallographic examination—the following conclusions may be formulated: the increase of the rotational speed decreases the spindle torque acting on the tool; simultaneously, the temperature of the modified surface decreases; the increase of the travelling speed does not influence the spindle torque acting on the tool in a substantial way— only a slight increase was observed; simultaneously it decreases the surface temperature of the modified area; the increase of the rotational speed decreases the penetration depth; the increase of the travelling speed has a negligible effect on the penetration depth compared to the influence of the rotational speed;
Fig. 9 – Influence of travelling speed (a) and rotational speed (b) of penetration depth.
the friction stir processing applied to the alloy surface refines the microstructure in the near-surface layer; and the FMP process of an cast AlSi9Mg aluminium alloy reduces casting porosity.
Acknowledgements This work was financially supported by the Polish Ministry of Science and Higher Education. The research was performed
Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003
6
archives of civil and mechanical engineering ] (]]]]) ]]]–]]]
within the framework of the statutory activity of the Instytut Spawalnictwa and was partly supported by the Grant no. N507 295439 realized in the AGH University of Science and Technology Faculty of Metals Engineering and Industrial Computer Science.
references
[1] A. Arora, R. Nandan, A.P. Reynolds, T. DebRoy, Torque, power requirement and stir zone geometry in friction stir welding through modelling and experiments, Scripta Materialia 60 (2009) 13–16. [2] C. Hamilton, S. Dymek, I. Kalemba, M. Blicharski, Friction stir welding of aluminium 7136-T76511 extrusions, Science and Technology of Welding and Joining 13 (8) (2008) 714–720. [3] I. Kalemba, S. Dymek, C. Hamilton, M. Blicharski, Microstructure evaluation in friction stir welded aluminium alloys, Archives of Metallurgy and Materials 54 (1) (2009) 75–82. [4] D. Lohwasser, Z. Chen, Friction stir welding: From basics to applications, Woodhead Publishing, Cambridge, 2009. [5] R.S. Mishra, Friction stir welding and processing, Materials Science and Engineering: R: Reports 50 (2005) 1–78. [6] R.S. Mishra, M.W. Mahoney, Friction Stir Welding and Friction Stir Processing, ASM International, Materials Park (Ohio), 2007.
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Please cite this article as: M.St. We˛glowski, S. DymekRelationship between friction stir processing parameters and torque, temperature and the penetration depth of the tool, Archives of Civil and Mechanical Engineering (2013), http://dx.doi.org/ 10.1016/j.acme.2013.01.003