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Laser forming of aluminium and aluminium alloys — microstructural investigation
Journal of Materials Processing Technology 115 (2001) 159±165
Laser forming of aluminium and aluminium alloys Ð microstructural investigation M. Merklein*, T. Hennige, M. Geiger University of Erlangen-Nuremberg, Egerlandstr. 11, 91058 Erlangen, Germany
Abstract This paper shows the microstructural development and changes in mechanical properties aluminium and aluminium alloys undergo during the process of laser forming. The resulting structures, grain structure and microstructure were analysed both quantitatively and qualitatively by optical and electron microscopy in order to examine the kind of existing structure zones and their extent. Zones with different substructures can be seen depending on the selected test parameters. The degree of substructural development seems to be a function of all in¯uences and cannot entirely be explained by laser parameters and material characteristics. Obviously, during laser forming several different inhomogeneous particle and dislocation structures develop which are responsible for the strength of the whole composite on the one hand, and for the strength of individual small components of the whole on the other hand. Thus, hardness tests of laser-formed specimens prove the existence of different zones depending on the material and heat treatment. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser forming; Aluminium and aluminium alloys; Microstructure; Hardness test; Rapid prototyping
1. Introduction Laser forming is a non-contact forming technology which gained in signi®cance over the last few years in several ®elds of industrial manufacturing. The main application of this ¯exible forming technology is in the ®eld of rapid prototyping and adjustment purposes (accurate adjustment of electronic microcomponents as well as sheet metal or sections) where its lack of forming tools proves as extremely advantageous [1±3]. The principles and models of laser forming and laser bending are discussed in detail in [4,5]. For laser forming different types of lasers, such as Nd:YAG laser, CO2 laser and excimer laser can be used. Decisive conditions for laser forming are a high temperature limited by surface melting and a high temperature gradient. Both demands are ful®led by the high energy density of most kinds of lasers. The in¯uence of the applied laser type on the experimental results can be put down to the fact that the speci®c wavelength of a laser in¯uences the interaction attributed to the material and the selected laser parameters [6]. There are many results of research activities about the in¯uence of geometrical and energetic parameters. Furthermore, there are many descriptions of the in¯uence and the *
Corresponding author. Tel.: 49-9131-85-27961; fax: 49-9131-930142. E-mail address: [email protected] (M. Merklein).
dependence of the bending angle on the material properties, for example, the coef®cient of thermal expansion, the heat capacity, the Young's modulus or the density. So, often the result of laser treatment is only assessed by the bending angle or the costs of the process. The structural and microstrucural changes inside the material should be considered for research into long-term effects. Therefore, this work demonstrates the changes in microstructure and mechanical properties of some aluminium alloys by laser forming. 2. Experimental procedure 2.1. Materials The materials to be investigated are AA1050 and AA6082. The thickness of the examined specimens is 0.99 and 1.02 mm, respectively, at a width of 80 mm. The heat treatment conditions of AA1050 are soft, annealed and solution heat treated with a subsequent natural ageing (T 41) or a following arti®cial ageing (T 61) for AA6082. The chemical composition of AA1050 and the AlMgSi1 alloy used in the present work is listed in Table 1. Fig. 1 shows the initial conditions of both alloys. The grain structure of AA1050 has a cellular substructure comprising a large number of dislocations. Hardly any dislocations are visible in the inside, however. Both ageing
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 7 5 9 - 2
160
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Table 1 Chemical composition of AA1050 and AA6082 in weight percent Element
Al99.5 AlMgSi1
Al
Si
Fe
Cu
Mn
Mg
Zn
V
Ti
Min. 99.5 Rem
0.25 0.7±1.3
0.4
0.05
0.05 0.4±1.0
0.05 0.6±1.2
0.05
0.05
0.03
Fig. 1. Initial condition of: (a) AA1050; (b) AA6082 T41; (c) AA6082 T61 .
conditions of AA6082, naturally aged (T41) and arti®cially aged (T61), have a grain structure without any de®ned substructure. The precipitations lie in disorder in the whole grain structure. The dislocation density of AA6082 T61 is comparatively low, whereas the dislocation density of AA6082 T41 is signi®cantly higher. 2.2. Test conditions and laser forming mechanism The process of laser forming depends on many different parameters, which can be divided into material parameters and parameters of the laser system. For the research presented in this paper, a 1 kW Nd:YAG laser (Haas, Germany) operating in a continuous wave mode (cw mode) was used. Laser bending, as described in this work, implies the irradiation of a single line. The development of the bending angle aB for each set of laser parameters is a function of the number of irradiations. Table 2 shows the applied parameters (laser power, processing velocity, focus diameter and the number of irradiations) relevant for this paper. In order to increase the coupling of the laser energy, a graphite coating is applied to the test specimens. Analytical calculations of the temperature ®eld for all test specimens show that the selected parameters are characteristic of the temperature-gradient-mechanism (TGM). For
example, Fig. 2 shows calculated temperature ®elds for AA1050, AA6082 T41 and AA6082 T61. The applied laser parameters in this case are laser power PL 535 W, focus diameter dL 3 mm and processing velocity vL 85 mm=s. Furthermore, there are differences in temperature at the irradiated surface in the range of 308C applied to AA1050 and AA6082 T41. For all materials, the absorption coef®cient is assumed to be 60% because of the graphite coating on the surface of the sheet specimen. The hardness tests are carried out with a microhardness testing system (Fischerscope HV 100). The determination of hardness values is symbolised by the universal hardness HU. The type of the indentor is a variation of the standard Vickers indentor, showing a pyramidal angle of 1368. The test load for all materials and specimens was 50 mN, the creep
Table 2 Applied laser parameters of a 1 kW Nd:YAG laser Set
PL (W)
vL (mm/s)
dL (mm)
Number of irradiations
1 2
535 650
85 119
3 3
1, 10 and 30 1, 10 and 30
Fig. 2. Calculation of the temperature field for AA1050, AA6082 T41 and AA6082 T61 (PL 535 W, dL 3 mm).
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
161
duration 10 s. After measuring the bending angle, the test sheets were cut into smaller parts in order to prepare specimens for investigation of mechanical changes. For this purpose, metallographic microsections were made by means of mechanical grinding and polishing to a ®nal polishing agent grain size of 1 mm. All hardness measurements were done after several days in order to reach a steady state condition of precipitation hardening. 2.3. Structural and microstructural research The structural research was done with a scanning electron microscope (SEM, Philips). The microstructural research was carried out in a 120 kV and a 200 kV transmission electron microscope (TEM, Philips). 3. Results and discussion 3.1. Development of the bending angle Figs. 3 and 4 show the results of the experiments on laser bending with regard to the bending angle and with an increasing number of irradiations. Each symbol represents the arithmetic average of ®ve individual specimens. In all the cases, the standard deviation is smaller than the size of the symbols. Both ®gures show that the application of the bending angle versus the number of irradiations has a degressive form for the aluminium alloy AA6082. This degressive form is, on the one hand, a consequence of work hardening on the lower side of the sheet, and of the decreasing absorption, on the other hand. Because of the missing coating with graphite between two irradiations, the absorption coef®cient decreases with increasing number of irradiations. In contrast to the degressive form of both aluminium alloys, pure aluminium has a linear correlation for each of the applied laser parameters. Therefore, it seems that the interaction between the work hardening and the absorption differs for AA1050 and AA6082.
Fig. 3. Development of the bending angle as a function of the number of irradiations.
Fig. 4. Development of the bending angle as a function of the number of irradiations.
3.2. Microhardness tests Figs. 5 and 6 show the two-dimensional hardness distribution in representative cross-sections of the investigated alloys after 30 successive irradiations. Each plot consists of 1500±2000 individual, equally spaced hardness measurements. In order to enhance the visibility of laser-induced hardness changes, the difference between the hardness values of the initial state and the laser-bent state of the alloys were plotted using isolines. The hardness of AA1050 increases with the number of irradiations. In Fig. 5, the heat-affected zone after 30 irradiations is plotted. It can be seen that the difference in hardness varies between 50 and 100 N/mm2, which is equivalent to an increase in hardness of about 15±30% in comparison to the initial hardness of 342 9 N=mm2 . Obviously, there is a variation in hardness. On the irradiated surface, the increase in hardness is less than that on the lower surface of the sheet. This means that the work hardening is not a constant in all parts of the specimen, it differs depending on parameters like dislocation density, temperature and relevant strain or stress, for example. Fig. 6a shows the hardness scan ®eld of AA6082 T41 after 30 irradiations. The laser parameters of the examined specimen are 650 W and a path feed rate of 119 mm/s. The value of the initial hardness is 1127 12 N=mm2 . The hardness plot indicates a slight decrease of approximately 5±10%. The hardness change of AA6082 T41 is low in comparison to the much stronger decrease measured for AA6082 T61 (Fig. 6b). AA6082 T41 has a microstructure with coherent precipitations, producing only elastic strains inside the crystal lattice. Inspite of this, the appearance of semicoherent precipitations of Mg2Si is characteristic for AA6082 T61. These precipiations cause plastic strains inside the crystal lattice, so precipitation hardening is decided for the mechanical behaviour of the material. The laser parameters applied to the AA6082 T61 specimen are exactly the same as the ones used for the specimen of Fig. 6a. The decrease of about 30% can be explained by the dissolving of the b0 -precipitations and the subsequent
162
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Fig. 5. Hardness test of AA1050 (set 1).
natural ageing in the heat-affected zone. As the initial hardness of 1400 28 N=mm2 of AA6082 T61 is reduced by the laser treatment to values comparable to the initial state of AA6082 T41, the above stated reason for the hardness decrease seems to be correct. Additionally, a heat-affected zone, lying in an area of the specimen which is not irradiated, can be found. That area is
characterised by a decrease of hardness of approximately 100 N/mm2. 3.3. Structural and microstructural development The structural and microstructural development must be seen as a consequence of deformation interacting with
Fig. 6. (a) Hardness test of AA6082 T41 (set 2); (b) hardness test of AA6082 T61 (set 2).
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
163
Fig. 7. Structural development in AA1050, PL 650 W, N 30.
partial heating in the heat-affected zone. On the one hand, laser forming in¯uences the precipitations responsible for the strength of hardenable aluminium alloys and on the other hand, a de®nite dislocation substructure may be generated by laser bending. During laser forming, the grain structure is deformed inhomogeneously, leading to the formation of zones with different grain sizes and grain orientations (Fig. 7). The initial state of AA1050 has a grain structure with a dislocation substructure that can be compared with a cell structure (Fig. 1). In contrast to this appearance, the heataffected zone of a specimen that has been irradiated only once (Fig. 8) already shows a clearly different dislocation structure. The thickness of the cell wall increases, the size of the cells becomes inhomogenous and in a few cells an extensive dislocation motion can be seen. After 30 irradiations, an inhomogeneous subgrain structure is produced (Fig. 9). Nearly all dislocations are situated in the subgrain borders, in the inner area of the subgrains hardly any dislocations can be seen. Considering the increased hardness, it seems reasonable to assume that laser bending causes work hardening and, of course, increasing dislocation density. Regarding the second aluminium alloy AA6082, two different effects can be observed. The initial state of the arti®cially aged condition of AA6082 is mainly given by metastable semicoherent precipitations of the type Mg2Si. In the heat-affected zone these precipitations are dissolved only after one irradiation. Similar to AA1050, repeated irradiations produce an inhomogeneous subgrain structure in the heat-affected zone lying directly below the irradiated surface (up to 500 mm), the dislocation density inside the subgrains is low (Fig. 10). The subgrain structure of the naturally aged aluminium alloy AA6082 (T41) also becomes inhomogeneous after 30 irradiations. The dislocation density inside the subgrains seems to be higher than that in the arti®cially aged specimen (Fig. 11). Investigations of TEM specimens taken from the part of the specimen which is near the lower surface
Fig. 8. Inhomogeneous cell structure in AA1050.
(500±1000 mm) show no subgrains but a grain structure including a very high dislocation density without de®nite structuring (Fig. 12). The hindered softening of the heataffected zone in the upper area caused by subgrain formation
Fig. 9. Subgrain structure in AA1050, N 30.
164
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Fig. 10. Subgrains in AA6082 T61 with a comparatively low dislocation density.
Fig. 12. Grain with extremely high dislocation density in the lower part of the laser treated zone.
can be veri®ed by the results of the hardness tests. In contrast, the increase of the dislocation density in the lower part of the heat-affected zone cannot compensate the softening. As a result of this, a decrease in hardness of about 100 N/mm2 can be measured. These results, in comparison with microhardness tests, suggest that the differences in substructural development can be traced back to ageing and the different in¯uence of laser power. While the test specimens warm up in the same way, the arti®cially aged alloy consumes a lot of energy for dissolution of precipitations, whereas the naturally aged specimen uses all the energy for production and reaction of thermal stresses and plastic deformation. 4. Conclusions
Fig. 11. Elongated, inhomogeneous subgrain structure in AA6082 T41.
This paper shows how laser forming with a Nd:YAG laser in¯uences both aluminium and aluminium alloys. Laser parameters, not leading to surface melting, produce bending angles ranging between 18 (one irradiation) and approximately 508 (30 irradiations). This bending involves a deformation of the grain structure inside the treated zone. In combination with heating, changes in the mechanical behaviour as well as in the microstructures can be observed. Soft and annealed aluminium, like AA1050, shows hardening. This work hardening is proved by hardness tests of deformed specimens and by electron microscopy explaining the dislocation motion and changes in microstructure.
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Arti®cially aged AA6082 may be bent in the same way as aluminium and naturally aged AlMgSi1. The effectiveness of the process is nearly the same regardless of the used alloy. The hardness produced by arti®cial ageing is lost directly in the heat-affected zone and in the surrounding area. The hardness value after laser forming is comparable to the naturally aged specimens. These effects are justi®ed by the dissolution of Mg2Si precipitations and the development of an energetically convenient dislocation substructure. In contrast, the hardness of the naturally aged AA6082 decreases slightly in spite of an increasing dislocation density in the lower heat-affected zone and the development of a subgrain structure in the upper heat-affected zone. These results also show that further investigations on the extension of the described zones are necessary. A more intensive assessment of the microstructural changes is important in order to ensure an exact description and calculation of the laser-induced structural and mechanical changes.
165
Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft (DFG) for the ®nancial support of the research project Ge530/34 from which the presented results were obtained. References [1] M. Geiger, J. Kraus, F. Vollertsen, Laserstrahlumformen raÈumlicher Bauteile, BaÈnder Bleche Rohre 11 (1994) 26±37. [2] W. Hoving, Accurate manipulation using laser technology, in: Proceedings of the SPIE Europto Series: Lasers in Material Processing, Vol. 3097, 1997, pp. 284±295. [3] J. Magee, K.G. Watkins, M. Steen, Advances in laser forming, J. Laser Appl. 10 (6) (1998) 235±246. [4] F. Vollertsen, Mechanisms and models for laser forming, in: M. Geiger, F. Vollertsen (Eds.), Laser Assisted Net Shape Engineering, Bamberg, Meisenbach, 1994, pp. 345±360. [5] F. Vollertsen, Laserstrahlumformen. LasergestuÈtzte Formgebung: Verfahren, Mechanismen, Modellierung, Meisenbach, Bamberg, 1996. [6] W.M. Steen, Laser Material Processing, Springer, London, 1991.
Laser forming of aluminium and aluminium alloys Ð microstructural investigation M. Merklein*, T. Hennige, M. Geiger University of Erlangen-Nuremberg, Egerlandstr. 11, 91058 Erlangen, Germany
Abstract This paper shows the microstructural development and changes in mechanical properties aluminium and aluminium alloys undergo during the process of laser forming. The resulting structures, grain structure and microstructure were analysed both quantitatively and qualitatively by optical and electron microscopy in order to examine the kind of existing structure zones and their extent. Zones with different substructures can be seen depending on the selected test parameters. The degree of substructural development seems to be a function of all in¯uences and cannot entirely be explained by laser parameters and material characteristics. Obviously, during laser forming several different inhomogeneous particle and dislocation structures develop which are responsible for the strength of the whole composite on the one hand, and for the strength of individual small components of the whole on the other hand. Thus, hardness tests of laser-formed specimens prove the existence of different zones depending on the material and heat treatment. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser forming; Aluminium and aluminium alloys; Microstructure; Hardness test; Rapid prototyping
1. Introduction Laser forming is a non-contact forming technology which gained in signi®cance over the last few years in several ®elds of industrial manufacturing. The main application of this ¯exible forming technology is in the ®eld of rapid prototyping and adjustment purposes (accurate adjustment of electronic microcomponents as well as sheet metal or sections) where its lack of forming tools proves as extremely advantageous [1±3]. The principles and models of laser forming and laser bending are discussed in detail in [4,5]. For laser forming different types of lasers, such as Nd:YAG laser, CO2 laser and excimer laser can be used. Decisive conditions for laser forming are a high temperature limited by surface melting and a high temperature gradient. Both demands are ful®led by the high energy density of most kinds of lasers. The in¯uence of the applied laser type on the experimental results can be put down to the fact that the speci®c wavelength of a laser in¯uences the interaction attributed to the material and the selected laser parameters [6]. There are many results of research activities about the in¯uence of geometrical and energetic parameters. Furthermore, there are many descriptions of the in¯uence and the *
Corresponding author. Tel.: 49-9131-85-27961; fax: 49-9131-930142. E-mail address: [email protected] (M. Merklein).
dependence of the bending angle on the material properties, for example, the coef®cient of thermal expansion, the heat capacity, the Young's modulus or the density. So, often the result of laser treatment is only assessed by the bending angle or the costs of the process. The structural and microstrucural changes inside the material should be considered for research into long-term effects. Therefore, this work demonstrates the changes in microstructure and mechanical properties of some aluminium alloys by laser forming. 2. Experimental procedure 2.1. Materials The materials to be investigated are AA1050 and AA6082. The thickness of the examined specimens is 0.99 and 1.02 mm, respectively, at a width of 80 mm. The heat treatment conditions of AA1050 are soft, annealed and solution heat treated with a subsequent natural ageing (T 41) or a following arti®cial ageing (T 61) for AA6082. The chemical composition of AA1050 and the AlMgSi1 alloy used in the present work is listed in Table 1. Fig. 1 shows the initial conditions of both alloys. The grain structure of AA1050 has a cellular substructure comprising a large number of dislocations. Hardly any dislocations are visible in the inside, however. Both ageing
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 7 5 9 - 2
160
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Table 1 Chemical composition of AA1050 and AA6082 in weight percent Element
Al99.5 AlMgSi1
Al
Si
Fe
Cu
Mn
Mg
Zn
V
Ti
Min. 99.5 Rem
0.25 0.7±1.3
0.4
0.05
0.05 0.4±1.0
0.05 0.6±1.2
0.05
0.05
0.03
Fig. 1. Initial condition of: (a) AA1050; (b) AA6082 T41; (c) AA6082 T61 .
conditions of AA6082, naturally aged (T41) and arti®cially aged (T61), have a grain structure without any de®ned substructure. The precipitations lie in disorder in the whole grain structure. The dislocation density of AA6082 T61 is comparatively low, whereas the dislocation density of AA6082 T41 is signi®cantly higher. 2.2. Test conditions and laser forming mechanism The process of laser forming depends on many different parameters, which can be divided into material parameters and parameters of the laser system. For the research presented in this paper, a 1 kW Nd:YAG laser (Haas, Germany) operating in a continuous wave mode (cw mode) was used. Laser bending, as described in this work, implies the irradiation of a single line. The development of the bending angle aB for each set of laser parameters is a function of the number of irradiations. Table 2 shows the applied parameters (laser power, processing velocity, focus diameter and the number of irradiations) relevant for this paper. In order to increase the coupling of the laser energy, a graphite coating is applied to the test specimens. Analytical calculations of the temperature ®eld for all test specimens show that the selected parameters are characteristic of the temperature-gradient-mechanism (TGM). For
example, Fig. 2 shows calculated temperature ®elds for AA1050, AA6082 T41 and AA6082 T61. The applied laser parameters in this case are laser power PL 535 W, focus diameter dL 3 mm and processing velocity vL 85 mm=s. Furthermore, there are differences in temperature at the irradiated surface in the range of 308C applied to AA1050 and AA6082 T41. For all materials, the absorption coef®cient is assumed to be 60% because of the graphite coating on the surface of the sheet specimen. The hardness tests are carried out with a microhardness testing system (Fischerscope HV 100). The determination of hardness values is symbolised by the universal hardness HU. The type of the indentor is a variation of the standard Vickers indentor, showing a pyramidal angle of 1368. The test load for all materials and specimens was 50 mN, the creep
Table 2 Applied laser parameters of a 1 kW Nd:YAG laser Set
PL (W)
vL (mm/s)
dL (mm)
Number of irradiations
1 2
535 650
85 119
3 3
1, 10 and 30 1, 10 and 30
Fig. 2. Calculation of the temperature field for AA1050, AA6082 T41 and AA6082 T61 (PL 535 W, dL 3 mm).
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
161
duration 10 s. After measuring the bending angle, the test sheets were cut into smaller parts in order to prepare specimens for investigation of mechanical changes. For this purpose, metallographic microsections were made by means of mechanical grinding and polishing to a ®nal polishing agent grain size of 1 mm. All hardness measurements were done after several days in order to reach a steady state condition of precipitation hardening. 2.3. Structural and microstructural research The structural research was done with a scanning electron microscope (SEM, Philips). The microstructural research was carried out in a 120 kV and a 200 kV transmission electron microscope (TEM, Philips). 3. Results and discussion 3.1. Development of the bending angle Figs. 3 and 4 show the results of the experiments on laser bending with regard to the bending angle and with an increasing number of irradiations. Each symbol represents the arithmetic average of ®ve individual specimens. In all the cases, the standard deviation is smaller than the size of the symbols. Both ®gures show that the application of the bending angle versus the number of irradiations has a degressive form for the aluminium alloy AA6082. This degressive form is, on the one hand, a consequence of work hardening on the lower side of the sheet, and of the decreasing absorption, on the other hand. Because of the missing coating with graphite between two irradiations, the absorption coef®cient decreases with increasing number of irradiations. In contrast to the degressive form of both aluminium alloys, pure aluminium has a linear correlation for each of the applied laser parameters. Therefore, it seems that the interaction between the work hardening and the absorption differs for AA1050 and AA6082.
Fig. 3. Development of the bending angle as a function of the number of irradiations.
Fig. 4. Development of the bending angle as a function of the number of irradiations.
3.2. Microhardness tests Figs. 5 and 6 show the two-dimensional hardness distribution in representative cross-sections of the investigated alloys after 30 successive irradiations. Each plot consists of 1500±2000 individual, equally spaced hardness measurements. In order to enhance the visibility of laser-induced hardness changes, the difference between the hardness values of the initial state and the laser-bent state of the alloys were plotted using isolines. The hardness of AA1050 increases with the number of irradiations. In Fig. 5, the heat-affected zone after 30 irradiations is plotted. It can be seen that the difference in hardness varies between 50 and 100 N/mm2, which is equivalent to an increase in hardness of about 15±30% in comparison to the initial hardness of 342 9 N=mm2 . Obviously, there is a variation in hardness. On the irradiated surface, the increase in hardness is less than that on the lower surface of the sheet. This means that the work hardening is not a constant in all parts of the specimen, it differs depending on parameters like dislocation density, temperature and relevant strain or stress, for example. Fig. 6a shows the hardness scan ®eld of AA6082 T41 after 30 irradiations. The laser parameters of the examined specimen are 650 W and a path feed rate of 119 mm/s. The value of the initial hardness is 1127 12 N=mm2 . The hardness plot indicates a slight decrease of approximately 5±10%. The hardness change of AA6082 T41 is low in comparison to the much stronger decrease measured for AA6082 T61 (Fig. 6b). AA6082 T41 has a microstructure with coherent precipitations, producing only elastic strains inside the crystal lattice. Inspite of this, the appearance of semicoherent precipitations of Mg2Si is characteristic for AA6082 T61. These precipiations cause plastic strains inside the crystal lattice, so precipitation hardening is decided for the mechanical behaviour of the material. The laser parameters applied to the AA6082 T61 specimen are exactly the same as the ones used for the specimen of Fig. 6a. The decrease of about 30% can be explained by the dissolving of the b0 -precipitations and the subsequent
162
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Fig. 5. Hardness test of AA1050 (set 1).
natural ageing in the heat-affected zone. As the initial hardness of 1400 28 N=mm2 of AA6082 T61 is reduced by the laser treatment to values comparable to the initial state of AA6082 T41, the above stated reason for the hardness decrease seems to be correct. Additionally, a heat-affected zone, lying in an area of the specimen which is not irradiated, can be found. That area is
characterised by a decrease of hardness of approximately 100 N/mm2. 3.3. Structural and microstructural development The structural and microstructural development must be seen as a consequence of deformation interacting with
Fig. 6. (a) Hardness test of AA6082 T41 (set 2); (b) hardness test of AA6082 T61 (set 2).
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
163
Fig. 7. Structural development in AA1050, PL 650 W, N 30.
partial heating in the heat-affected zone. On the one hand, laser forming in¯uences the precipitations responsible for the strength of hardenable aluminium alloys and on the other hand, a de®nite dislocation substructure may be generated by laser bending. During laser forming, the grain structure is deformed inhomogeneously, leading to the formation of zones with different grain sizes and grain orientations (Fig. 7). The initial state of AA1050 has a grain structure with a dislocation substructure that can be compared with a cell structure (Fig. 1). In contrast to this appearance, the heataffected zone of a specimen that has been irradiated only once (Fig. 8) already shows a clearly different dislocation structure. The thickness of the cell wall increases, the size of the cells becomes inhomogenous and in a few cells an extensive dislocation motion can be seen. After 30 irradiations, an inhomogeneous subgrain structure is produced (Fig. 9). Nearly all dislocations are situated in the subgrain borders, in the inner area of the subgrains hardly any dislocations can be seen. Considering the increased hardness, it seems reasonable to assume that laser bending causes work hardening and, of course, increasing dislocation density. Regarding the second aluminium alloy AA6082, two different effects can be observed. The initial state of the arti®cially aged condition of AA6082 is mainly given by metastable semicoherent precipitations of the type Mg2Si. In the heat-affected zone these precipitations are dissolved only after one irradiation. Similar to AA1050, repeated irradiations produce an inhomogeneous subgrain structure in the heat-affected zone lying directly below the irradiated surface (up to 500 mm), the dislocation density inside the subgrains is low (Fig. 10). The subgrain structure of the naturally aged aluminium alloy AA6082 (T41) also becomes inhomogeneous after 30 irradiations. The dislocation density inside the subgrains seems to be higher than that in the arti®cially aged specimen (Fig. 11). Investigations of TEM specimens taken from the part of the specimen which is near the lower surface
Fig. 8. Inhomogeneous cell structure in AA1050.
(500±1000 mm) show no subgrains but a grain structure including a very high dislocation density without de®nite structuring (Fig. 12). The hindered softening of the heataffected zone in the upper area caused by subgrain formation
Fig. 9. Subgrain structure in AA1050, N 30.
164
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Fig. 10. Subgrains in AA6082 T61 with a comparatively low dislocation density.
Fig. 12. Grain with extremely high dislocation density in the lower part of the laser treated zone.
can be veri®ed by the results of the hardness tests. In contrast, the increase of the dislocation density in the lower part of the heat-affected zone cannot compensate the softening. As a result of this, a decrease in hardness of about 100 N/mm2 can be measured. These results, in comparison with microhardness tests, suggest that the differences in substructural development can be traced back to ageing and the different in¯uence of laser power. While the test specimens warm up in the same way, the arti®cially aged alloy consumes a lot of energy for dissolution of precipitations, whereas the naturally aged specimen uses all the energy for production and reaction of thermal stresses and plastic deformation. 4. Conclusions
Fig. 11. Elongated, inhomogeneous subgrain structure in AA6082 T41.
This paper shows how laser forming with a Nd:YAG laser in¯uences both aluminium and aluminium alloys. Laser parameters, not leading to surface melting, produce bending angles ranging between 18 (one irradiation) and approximately 508 (30 irradiations). This bending involves a deformation of the grain structure inside the treated zone. In combination with heating, changes in the mechanical behaviour as well as in the microstructures can be observed. Soft and annealed aluminium, like AA1050, shows hardening. This work hardening is proved by hardness tests of deformed specimens and by electron microscopy explaining the dislocation motion and changes in microstructure.
M. Merklein et al. / Journal of Materials Processing Technology 115 (2001) 159±165
Arti®cially aged AA6082 may be bent in the same way as aluminium and naturally aged AlMgSi1. The effectiveness of the process is nearly the same regardless of the used alloy. The hardness produced by arti®cial ageing is lost directly in the heat-affected zone and in the surrounding area. The hardness value after laser forming is comparable to the naturally aged specimens. These effects are justi®ed by the dissolution of Mg2Si precipitations and the development of an energetically convenient dislocation substructure. In contrast, the hardness of the naturally aged AA6082 decreases slightly in spite of an increasing dislocation density in the lower heat-affected zone and the development of a subgrain structure in the upper heat-affected zone. These results also show that further investigations on the extension of the described zones are necessary. A more intensive assessment of the microstructural changes is important in order to ensure an exact description and calculation of the laser-induced structural and mechanical changes.
165
Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft (DFG) for the ®nancial support of the research project Ge530/34 from which the presented results were obtained. References [1] M. Geiger, J. Kraus, F. Vollertsen, Laserstrahlumformen raÈumlicher Bauteile, BaÈnder Bleche Rohre 11 (1994) 26±37. [2] W. Hoving, Accurate manipulation using laser technology, in: Proceedings of the SPIE Europto Series: Lasers in Material Processing, Vol. 3097, 1997, pp. 284±295. [3] J. Magee, K.G. Watkins, M. Steen, Advances in laser forming, J. Laser Appl. 10 (6) (1998) 235±246. [4] F. Vollertsen, Mechanisms and models for laser forming, in: M. Geiger, F. Vollertsen (Eds.), Laser Assisted Net Shape Engineering, Bamberg, Meisenbach, 1994, pp. 345±360. [5] F. Vollertsen, Laserstrahlumformen. LasergestuÈtzte Formgebung: Verfahren, Mechanismen, Modellierung, Meisenbach, Bamberg, 1996. [6] W.M. Steen, Laser Material Processing, Springer, London, 1991.