- Email: [email protected]
Effects of Weaving Laser on Weld Microstructure and Crack for Al 6k21-T4 Alloy
J. Mater. Sci. Technol., 2011, 27(1), 93-96.
Effects of Weaving Laser on Weld Microstructure and Crack for Al 6k21-T4 Alloy B.H. Kim1) , N.H. Kang1)† , W.T. Oh2) , C.H. Kim3) , and J.H. Kim3) , Y.S. Kim4) and Y.H. Park1) 1) Department of Materials Science and Engineering, Pusan National University, San30, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea 2) Department of Materials and Components Engineering, Dong-Eui University, Busan 614-714, Korea 3) Advanced Joining Technology Team, KITECH, 7-47, Songdo-dong, Yeonsu-gu, Incheon 406-840, Korea 4) Division for Dongnam Area Technology Service, KITECH, Jisa-dong, Ganseo-gu, Busan 618-230, Korea [Manuscript received April 28, 2010]
For Al 6k21-T4 overlap weld joint, the shear-tensile strength by using the weaving laser was improved as compared to the case of linear laser. For the specimen of low strength, the porosity was distributed continuously along the intersection between the plates and fusion line. However, for the optimized welding condition, large oval-shaped porosities were located only in the advancing track of the concave part. Therefore, the continuity of cracks and porosities played a key role to determine the strength. And, the weaving width was also the important parameter to control the strength. Furthermore, the concave part had more significant hot and cold cracking in the weld and heat-affected zone (HAZ), respectively, than the convex part. KEY WORDS: Weaving laser; Weld microstructure; Void; Al alloy
1. Introduction Light weight vehicles by using aluminum have been actively studied due to environmental issues that emissions control of carbon dioxide are significantly controlled for the next decade[1] . There are several assembly processes for aluminum: arc welding[2–4] , resistance welding[5] , laser welding[6–10] , and friction stir welding[11–16] . When the arc welding is applied to the aluminum thin plate, critical defects such as deformation, cracks and voids are normally occurred. For the resistance welding of aluminum plate, aluminum sticks to the electrode made by copper-based alloys, therefore contaminating the Cu electrode, increasing the electrode consumption, and finally producing the poor soundness of weld surface. Recently, friction stir welding (FSW) receives significant attention on the joining of low melting temperature materials, e.g., aluminum (Al). It is because FSW is the † Corresponding author. Prof., Ph.D.; Tel.: +82 51 510 3027; Fax: +82 51 514 4457; E-mail address: [email protected] (N.Y. Kang).
method of solid state joining that can reduce solidification cracks, voids, and heat deformation. However, FSW needs further improvement for the mass production of Al-body vehicles, such as high productivity and feasibility of welding design. Laser welding process has many advantages for the vehicle assembly: rapid welding speed, deep weld penetration, and low heat input[17,18] . But, laser welding has a problem to overcome for low melting temperature materials such as aluminium and magnesium: poor soundness of welding surface due to high density heat source and poor welding strength due to voids and cracks. Studies for improving crack formation and weld strength were conducted by the arc oscillation during the welding[19,20] . However, there is no study applying the weaving process to laser welding, as far as the authors acknowledged. Therefore, this study investigated the effect of weaving laser on the property of aluminum overlap welding: microstructure of heat-affected zone (HAZ) and weld, void, crack, and weld strength.
B.H. Kim et al.: J. Mater. Sci. Technol., 2011, 27(1), 93–96
94
Fig. 1 Strength for weaving welded specimens as a function of weaving conditions
2. Experimental The programmable focusing optics was used for laser optical apparatus. Nd:YAG disk laser of 4 kW maximum power was used for the study and depth of focus was fixed to 450 mm. Laser head was connected to six-axis robot to control easily. Workpiece for the welding was 6k21-T4 aluminum alloy (1 wt% Si, 0.6 wt% Mg) with a dimension of 50 mm×100 mm×1 mm. Overlap joint was used for laser welding with 30 mm overlap space width. Weaving width varied from 1 mm to 2 mm and weaving frequency from 25 Hz to 40 Hz. Welding speed and laser power was fixed to 3 m/min and 3 kW, respectively. Shear-tensile test was performed on the welded specimen. To understand the variation of the joint strength, the weaving and linearly welded specimens were chosen to examine internal defects such as voids and cracks by using 3-dimensional X-ray CT test. 3. Results and Discussion 3.1 Behavior of shear-tensile strength and weld width with respect to weaving conditions Shear-tensile strength was measured as a function of weaving width and frequency. The strength is shown in Fig. 1. All specimens were equally frac-
tured at the heat affected zone (HAZ). Regardless of weaving condition, bead surface was smooth with no solidification cracks in the center of weldment and no undercut in the edge of weldment. For 1 mm weaving width, the minimum strength (3.5 kN) was measured at 25 Hz (No. 1 specimen). As the frequency increased from 25 Hz to 40 Hz, the strength increased and stabilized to 3.8∼3.9 kN. For 2 mm weaving width, the maximum strength (4.4 kN) appeared at No. 5 specimen that was tested at 25 Hz. It was interesting to note that the strength decreased continuously as the frequency increased from 25 Hz to 40 Hz. The larger weaving width produced larger shear-tensile strength, however the frequency did not change the strength significantly. The strength of base metal was 7.3 kN and that of linear laser welding was 3.4 kN. The weaving laser significantly enhanced the shear-tensile strength as compared to the linear welding. To correlate the strength with weaving condition systematically, a weld dimension was analyzed on the weld cross-section. Full penetration welds were produced for all welding conditions. Therefore, only the weld width was considered to determine the weld dimension. No. 5 specimen made by 2 mm weaving width and 25 Hz frequency indicated the largest width and the maximum shear-tensile strength. And, No. 1 specimen and linear weld that had the minimum shear-tensile strength produced the smallest width. Therefore, the increase of shear-tensile strength was directly correlated with the increase of the weld width that was changed with respect to the weaving conditions. 3.2 Behavior of void formation with respect to weaving conditions Nondestructive scans of X-ray CT test are shown in Fig. 2. Regardless of weaving conditions, porosities were located at the intersection of the plates and fusion line. Except for No. 5 specimen, small porosities were distributed continuously along the intersection and configured to a rope-link. This continuity of porosity was seen more significantly in linear welding. Although No. 5 specimen had the largest size of oval-shaped porosity, the continuity of porosity was not seen severely. Regarding the largest strength in
Fig. 2 X-ray CT test of void: measured from weld top-surface
B.H. Kim et al.: J. Mater. Sci. Technol., 2011, 27(1), 93–96
Fig. 3 Tendency of void location as a function of welding direction (No. 5)
95
more significantly to the laser heat, accumulating the heat more significantly. On the other hand, the conductive heat from the convex parts was more easily released to the base metal, i.e., dissipating the heat more easily. A schematic diagram is given in Fig. 3 to explain the heat accumulation for the concave parts and the heat dissipation for the convex parts. That is why the concave parts should have lower cooling rate than the convex parts, therefore delaying the solidification process and producing the void in the concave parts. However, as the frequency increased from 25 Hz to 40 Hz, the distance between the consecutive concave parts decreased to form the continuity of porosity. 3.3 Behavior of microstructure and crack with respect to weaving conditions
Fig. 4 Solidification and cold cracks at (a) concave part with respect to (b) convex part
No. 5 and the lowest strength in linear welding, the continuity of porosity had more important effects on the decrease of shear-tensile strength rather than the size of porosity. Analyzing No. 5 specimen in more detail, the distribution and size of void had a definite tendency with respect to the weaving frequency and welding direction instead of showing the continuity. Void areas are shown in red circles on the macro-etched weld surface (Fig. 3). Large oval-shaped porosities were presented along the advancing fusion line in concave part which heat input was maximized at. Before the laser source came directly to the advancing track, the concave parts received the conductive heat from the retreating part of previous frequency. And, the heat dissipation from the advancing track was overlapped with that from the retreating part of previous frequency. Therefore, comparing with the convex parts, the concave parts in the welding track were exposed
Crack initiation and propagation during the solidification showed the same behavior on the weld crosssection as a function of weaving conditions. Mainly, large voids were located in the intersection of the overlap plates and fusion line. And, solidification cracks were initiated from the large void in the intersection and propagated to the weldment passing through the small porosities that was formed during the laser welding. The void dimension was the important factor to determine the crack initiation. The void continuity was the significant factor to control the crack propagation. For the Al 6k21-T4 alloy, the continuity of porosity seems to be more dominant to determine the shear-tensile strength. Therefore, the linear weld and No. 1 specimen indicated the low shear-tensile strength because they had the continuity of porosity. Figure 4 shows the microstructure at the concave parts and the convex parts. The concave parts received the heat accumulation and had the larger heat input, as compared to the convex parts. That was why several solidification cracks were observed in the weld only at the concave parts. In fact, the solidification crack observed in the concave weld part should be the part of cracks propagated from the large void, which was located in the intersection. Furthermore, the concave parts in the HAZ showed the liquation and/or cold cracks more significantly, comparing with the convex parts. The HAZ was more closely investigated for the composition analysis by chemical etching method and electron dispersive spectroscopy (EDS), as shown in Fig. 5. Along the HAZ displayed the tarnished region close to the fusion line. The composition of the tarnished region was revealed to be Mg2 Si intermetallic compounds. During the cooling post to laser heating, magnesium and silicon was possibly segregated to the grain boundary in the HAZ and the Mg2 Si intermetallics were formed. The Mg2 Si intermetallics have very brittle property, therefore creating the cold cracks in the HAZ. In some cases for Al multi-pass welding, the low melting temperature materials
96
B.H. Kim et al.: J. Mater. Sci. Technol., 2011, 27(1), 93–96
Acknowledgements This research was supported by a grant from the Ministry of Knowledge Economy, Republic of Korea, and by NCRC (National Core Research Center) Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (20100001-222). REFERENCES
Fig. 5 SEM microstructure and compositional analysis of No. 5 weld surface: the numbers indicates the weight percent of magnesium and silicon, respectively
segregated to the grain boundary produced the liquation crack during the subsequent heating and cooling. However, for the study, one-pass welding was performed, so the liquation crack was not produced as far as the authors observed. Moreover, the weld part showed the high content of silicon because the low melting temperature materials should be vaporized due to the intense laser heat. 4. Conclusions (1) The weaving laser produced the larger sheartensile strength than the linear welding. (2) For the same frequency condition, the wider weaving width produced the larger weld width and shear-tensile strength. (3) For the study, the weaving condition (2 mm weaving width and 25 Hz frequency) produced the maximum shear-tensile strength in which large ovalshaped porosities were located along the advancing track in the concave part. The concave part received more heat accumulation than the convex part. (4) Solidification cracks and cold cracks were observed more significantly at the concave part rather than the convex part. Solidification cracks started from the large voids in the intersection of the plates and fusion line, and then propagated into the weld along the small voids that was formed during the laser welding. (5) The size of cracks and porosities had a minor effect on the shear-tensile strength and the continuity of the porosity was dominant to decide the strength.
[1 ] G.S. Cole and A.M. Sherman: Mater. Charact., 1995, 35(1), 3. [2 ] N. Geoffroy, E. Vittecoq, A. Birr, F. de Mestral and J.M. Martin: Scripta Mater., 2007, 57(4), 349. [3 ] L.R. Hwang, C.H. Gung and T.S. Shih: J. Mater. Process. Technol., 2001, 116(2-3), 101. [4 ] P.P. Lean, L. Gil and A. Ure˜ na: J. Mater. Process. Technol., 2003, 143-144, 846. [5 ] J.H. Han, P. Shan and S.S. Hu: Mater. Sci. Eng. A, 2006, 435-436, 204. [6 ] D. Fabr´egue, A. Deschamps, M. Su´ery and H. Proudhon: Scripta Mater., 2008, 59(3), 324. [7 ] E. Cicalˇ a, G. Duffet, H. Andrzejewski, D. Grevey and S. Ignat: Mater. Sci. Eng. A, 2005, 395(1-2), 1. [8 ] Y.W. Shi, F. Zhong, X.Y. Li, S.L. Gong and L. Chen: Mater. Sci. Eng. A, 2007, 465(1-2), 153. [9 ] M. Arone, D. Cerniglia and V. Nigrelli: J. Mater. Process. Technol., 2006, 176(1-3), 95. [10] R. Braun: Mater. Sci. Eng. A, 2006, 426(1-2), 250. [11] F.C. Liu and Z.Y. Ma: Scripta Mater., 2008, 59(8), 882. [12] X. Sauvage, A. D´ed´e, A. Cabello Mu˜ noz and B. Huneau: Mater. Sci. Eng. A, 2008, 491(1-2), 364. [13] S.T. Amancio-Filho, S. Sheikhi, J.F. dos Santos and C. Bolfarini: J. Mater. Process. Technol., 2008, 206(13), 132. [14] C.G. Derry, J.D. Robson: Mater. Sci. Eng. A, 2008, 490(1-2), 328. [15] S.R. Ren, Z.Y. Ma and L.Q. Chen: Mater. Sci. Eng. A, 2008, 479(1-2), 293. [16] S.A. Khodir and T. Shibayanagi: Mater. Sci. Eng. B, 2008, 148(1-3), 82. [17] E. Schubert, M. Klassen, I. Zerner, C. Walz and G. Sepold: J. Mater. Process. Technol., 2001, 115(1), 2. [18] K. Behler, J. Berkmanns, A. Ehrhardt and W. Frohn: Mater. Des., 1997, 18(4-6), 261. [19] A. Kumar, P. Shailesh and S. Sundarrajan: Mater. Des., 2008, 29(10), 1904. [20] K. Sivaprasad, S. Ganesh Sundara Raman, P. Mastanaiah and G. Madhusudhan Reddy: Mater. Sci. Eng. A, 2006, 428(1-2) 327.
Effects of Weaving Laser on Weld Microstructure and Crack for Al 6k21-T4 Alloy B.H. Kim1) , N.H. Kang1)† , W.T. Oh2) , C.H. Kim3) , and J.H. Kim3) , Y.S. Kim4) and Y.H. Park1) 1) Department of Materials Science and Engineering, Pusan National University, San30, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea 2) Department of Materials and Components Engineering, Dong-Eui University, Busan 614-714, Korea 3) Advanced Joining Technology Team, KITECH, 7-47, Songdo-dong, Yeonsu-gu, Incheon 406-840, Korea 4) Division for Dongnam Area Technology Service, KITECH, Jisa-dong, Ganseo-gu, Busan 618-230, Korea [Manuscript received April 28, 2010]
For Al 6k21-T4 overlap weld joint, the shear-tensile strength by using the weaving laser was improved as compared to the case of linear laser. For the specimen of low strength, the porosity was distributed continuously along the intersection between the plates and fusion line. However, for the optimized welding condition, large oval-shaped porosities were located only in the advancing track of the concave part. Therefore, the continuity of cracks and porosities played a key role to determine the strength. And, the weaving width was also the important parameter to control the strength. Furthermore, the concave part had more significant hot and cold cracking in the weld and heat-affected zone (HAZ), respectively, than the convex part. KEY WORDS: Weaving laser; Weld microstructure; Void; Al alloy
1. Introduction Light weight vehicles by using aluminum have been actively studied due to environmental issues that emissions control of carbon dioxide are significantly controlled for the next decade[1] . There are several assembly processes for aluminum: arc welding[2–4] , resistance welding[5] , laser welding[6–10] , and friction stir welding[11–16] . When the arc welding is applied to the aluminum thin plate, critical defects such as deformation, cracks and voids are normally occurred. For the resistance welding of aluminum plate, aluminum sticks to the electrode made by copper-based alloys, therefore contaminating the Cu electrode, increasing the electrode consumption, and finally producing the poor soundness of weld surface. Recently, friction stir welding (FSW) receives significant attention on the joining of low melting temperature materials, e.g., aluminum (Al). It is because FSW is the † Corresponding author. Prof., Ph.D.; Tel.: +82 51 510 3027; Fax: +82 51 514 4457; E-mail address: [email protected] (N.Y. Kang).
method of solid state joining that can reduce solidification cracks, voids, and heat deformation. However, FSW needs further improvement for the mass production of Al-body vehicles, such as high productivity and feasibility of welding design. Laser welding process has many advantages for the vehicle assembly: rapid welding speed, deep weld penetration, and low heat input[17,18] . But, laser welding has a problem to overcome for low melting temperature materials such as aluminium and magnesium: poor soundness of welding surface due to high density heat source and poor welding strength due to voids and cracks. Studies for improving crack formation and weld strength were conducted by the arc oscillation during the welding[19,20] . However, there is no study applying the weaving process to laser welding, as far as the authors acknowledged. Therefore, this study investigated the effect of weaving laser on the property of aluminum overlap welding: microstructure of heat-affected zone (HAZ) and weld, void, crack, and weld strength.
B.H. Kim et al.: J. Mater. Sci. Technol., 2011, 27(1), 93–96
94
Fig. 1 Strength for weaving welded specimens as a function of weaving conditions
2. Experimental The programmable focusing optics was used for laser optical apparatus. Nd:YAG disk laser of 4 kW maximum power was used for the study and depth of focus was fixed to 450 mm. Laser head was connected to six-axis robot to control easily. Workpiece for the welding was 6k21-T4 aluminum alloy (1 wt% Si, 0.6 wt% Mg) with a dimension of 50 mm×100 mm×1 mm. Overlap joint was used for laser welding with 30 mm overlap space width. Weaving width varied from 1 mm to 2 mm and weaving frequency from 25 Hz to 40 Hz. Welding speed and laser power was fixed to 3 m/min and 3 kW, respectively. Shear-tensile test was performed on the welded specimen. To understand the variation of the joint strength, the weaving and linearly welded specimens were chosen to examine internal defects such as voids and cracks by using 3-dimensional X-ray CT test. 3. Results and Discussion 3.1 Behavior of shear-tensile strength and weld width with respect to weaving conditions Shear-tensile strength was measured as a function of weaving width and frequency. The strength is shown in Fig. 1. All specimens were equally frac-
tured at the heat affected zone (HAZ). Regardless of weaving condition, bead surface was smooth with no solidification cracks in the center of weldment and no undercut in the edge of weldment. For 1 mm weaving width, the minimum strength (3.5 kN) was measured at 25 Hz (No. 1 specimen). As the frequency increased from 25 Hz to 40 Hz, the strength increased and stabilized to 3.8∼3.9 kN. For 2 mm weaving width, the maximum strength (4.4 kN) appeared at No. 5 specimen that was tested at 25 Hz. It was interesting to note that the strength decreased continuously as the frequency increased from 25 Hz to 40 Hz. The larger weaving width produced larger shear-tensile strength, however the frequency did not change the strength significantly. The strength of base metal was 7.3 kN and that of linear laser welding was 3.4 kN. The weaving laser significantly enhanced the shear-tensile strength as compared to the linear welding. To correlate the strength with weaving condition systematically, a weld dimension was analyzed on the weld cross-section. Full penetration welds were produced for all welding conditions. Therefore, only the weld width was considered to determine the weld dimension. No. 5 specimen made by 2 mm weaving width and 25 Hz frequency indicated the largest width and the maximum shear-tensile strength. And, No. 1 specimen and linear weld that had the minimum shear-tensile strength produced the smallest width. Therefore, the increase of shear-tensile strength was directly correlated with the increase of the weld width that was changed with respect to the weaving conditions. 3.2 Behavior of void formation with respect to weaving conditions Nondestructive scans of X-ray CT test are shown in Fig. 2. Regardless of weaving conditions, porosities were located at the intersection of the plates and fusion line. Except for No. 5 specimen, small porosities were distributed continuously along the intersection and configured to a rope-link. This continuity of porosity was seen more significantly in linear welding. Although No. 5 specimen had the largest size of oval-shaped porosity, the continuity of porosity was not seen severely. Regarding the largest strength in
Fig. 2 X-ray CT test of void: measured from weld top-surface
B.H. Kim et al.: J. Mater. Sci. Technol., 2011, 27(1), 93–96
Fig. 3 Tendency of void location as a function of welding direction (No. 5)
95
more significantly to the laser heat, accumulating the heat more significantly. On the other hand, the conductive heat from the convex parts was more easily released to the base metal, i.e., dissipating the heat more easily. A schematic diagram is given in Fig. 3 to explain the heat accumulation for the concave parts and the heat dissipation for the convex parts. That is why the concave parts should have lower cooling rate than the convex parts, therefore delaying the solidification process and producing the void in the concave parts. However, as the frequency increased from 25 Hz to 40 Hz, the distance between the consecutive concave parts decreased to form the continuity of porosity. 3.3 Behavior of microstructure and crack with respect to weaving conditions
Fig. 4 Solidification and cold cracks at (a) concave part with respect to (b) convex part
No. 5 and the lowest strength in linear welding, the continuity of porosity had more important effects on the decrease of shear-tensile strength rather than the size of porosity. Analyzing No. 5 specimen in more detail, the distribution and size of void had a definite tendency with respect to the weaving frequency and welding direction instead of showing the continuity. Void areas are shown in red circles on the macro-etched weld surface (Fig. 3). Large oval-shaped porosities were presented along the advancing fusion line in concave part which heat input was maximized at. Before the laser source came directly to the advancing track, the concave parts received the conductive heat from the retreating part of previous frequency. And, the heat dissipation from the advancing track was overlapped with that from the retreating part of previous frequency. Therefore, comparing with the convex parts, the concave parts in the welding track were exposed
Crack initiation and propagation during the solidification showed the same behavior on the weld crosssection as a function of weaving conditions. Mainly, large voids were located in the intersection of the overlap plates and fusion line. And, solidification cracks were initiated from the large void in the intersection and propagated to the weldment passing through the small porosities that was formed during the laser welding. The void dimension was the important factor to determine the crack initiation. The void continuity was the significant factor to control the crack propagation. For the Al 6k21-T4 alloy, the continuity of porosity seems to be more dominant to determine the shear-tensile strength. Therefore, the linear weld and No. 1 specimen indicated the low shear-tensile strength because they had the continuity of porosity. Figure 4 shows the microstructure at the concave parts and the convex parts. The concave parts received the heat accumulation and had the larger heat input, as compared to the convex parts. That was why several solidification cracks were observed in the weld only at the concave parts. In fact, the solidification crack observed in the concave weld part should be the part of cracks propagated from the large void, which was located in the intersection. Furthermore, the concave parts in the HAZ showed the liquation and/or cold cracks more significantly, comparing with the convex parts. The HAZ was more closely investigated for the composition analysis by chemical etching method and electron dispersive spectroscopy (EDS), as shown in Fig. 5. Along the HAZ displayed the tarnished region close to the fusion line. The composition of the tarnished region was revealed to be Mg2 Si intermetallic compounds. During the cooling post to laser heating, magnesium and silicon was possibly segregated to the grain boundary in the HAZ and the Mg2 Si intermetallics were formed. The Mg2 Si intermetallics have very brittle property, therefore creating the cold cracks in the HAZ. In some cases for Al multi-pass welding, the low melting temperature materials
96
B.H. Kim et al.: J. Mater. Sci. Technol., 2011, 27(1), 93–96
Acknowledgements This research was supported by a grant from the Ministry of Knowledge Economy, Republic of Korea, and by NCRC (National Core Research Center) Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (20100001-222). REFERENCES
Fig. 5 SEM microstructure and compositional analysis of No. 5 weld surface: the numbers indicates the weight percent of magnesium and silicon, respectively
segregated to the grain boundary produced the liquation crack during the subsequent heating and cooling. However, for the study, one-pass welding was performed, so the liquation crack was not produced as far as the authors observed. Moreover, the weld part showed the high content of silicon because the low melting temperature materials should be vaporized due to the intense laser heat. 4. Conclusions (1) The weaving laser produced the larger sheartensile strength than the linear welding. (2) For the same frequency condition, the wider weaving width produced the larger weld width and shear-tensile strength. (3) For the study, the weaving condition (2 mm weaving width and 25 Hz frequency) produced the maximum shear-tensile strength in which large ovalshaped porosities were located along the advancing track in the concave part. The concave part received more heat accumulation than the convex part. (4) Solidification cracks and cold cracks were observed more significantly at the concave part rather than the convex part. Solidification cracks started from the large voids in the intersection of the plates and fusion line, and then propagated into the weld along the small voids that was formed during the laser welding. (5) The size of cracks and porosities had a minor effect on the shear-tensile strength and the continuity of the porosity was dominant to decide the strength.
[1 ] G.S. Cole and A.M. Sherman: Mater. Charact., 1995, 35(1), 3. [2 ] N. Geoffroy, E. Vittecoq, A. Birr, F. de Mestral and J.M. Martin: Scripta Mater., 2007, 57(4), 349. [3 ] L.R. Hwang, C.H. Gung and T.S. Shih: J. Mater. Process. Technol., 2001, 116(2-3), 101. [4 ] P.P. Lean, L. Gil and A. Ure˜ na: J. Mater. Process. Technol., 2003, 143-144, 846. [5 ] J.H. Han, P. Shan and S.S. Hu: Mater. Sci. Eng. A, 2006, 435-436, 204. [6 ] D. Fabr´egue, A. Deschamps, M. Su´ery and H. Proudhon: Scripta Mater., 2008, 59(3), 324. [7 ] E. Cicalˇ a, G. Duffet, H. Andrzejewski, D. Grevey and S. Ignat: Mater. Sci. Eng. A, 2005, 395(1-2), 1. [8 ] Y.W. Shi, F. Zhong, X.Y. Li, S.L. Gong and L. Chen: Mater. Sci. Eng. A, 2007, 465(1-2), 153. [9 ] M. Arone, D. Cerniglia and V. Nigrelli: J. Mater. Process. Technol., 2006, 176(1-3), 95. [10] R. Braun: Mater. Sci. Eng. A, 2006, 426(1-2), 250. [11] F.C. Liu and Z.Y. Ma: Scripta Mater., 2008, 59(8), 882. [12] X. Sauvage, A. D´ed´e, A. Cabello Mu˜ noz and B. Huneau: Mater. Sci. Eng. A, 2008, 491(1-2), 364. [13] S.T. Amancio-Filho, S. Sheikhi, J.F. dos Santos and C. Bolfarini: J. Mater. Process. Technol., 2008, 206(13), 132. [14] C.G. Derry, J.D. Robson: Mater. Sci. Eng. A, 2008, 490(1-2), 328. [15] S.R. Ren, Z.Y. Ma and L.Q. Chen: Mater. Sci. Eng. A, 2008, 479(1-2), 293. [16] S.A. Khodir and T. Shibayanagi: Mater. Sci. Eng. B, 2008, 148(1-3), 82. [17] E. Schubert, M. Klassen, I. Zerner, C. Walz and G. Sepold: J. Mater. Process. Technol., 2001, 115(1), 2. [18] K. Behler, J. Berkmanns, A. Ehrhardt and W. Frohn: Mater. Des., 1997, 18(4-6), 261. [19] A. Kumar, P. Shailesh and S. Sundarrajan: Mater. Des., 2008, 29(10), 1904. [20] K. Sivaprasad, S. Ganesh Sundara Raman, P. Mastanaiah and G. Madhusudhan Reddy: Mater. Sci. Eng. A, 2006, 428(1-2) 327.