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Assessment of the powder extrusion of silicon–aluminium alloy
Journal of Materials Processing Technology 114 (2001) 18±21
Assessment of the powder extrusion of silicon±aluminium alloy H. So*, W.C. Li, H.K. Hsieh Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan Received 5 June 1999
Abstract The silicon±aluminium alloys are, in general, brittle in behaviour and have comparably low strengths. However, Sumitomo Electric Industries claimed that they had developed the so-called Sumi-Although alloys, which had tensile strengths ranging from 400 to 500 MPa. Such alloys were suitable to be used for load bearing machine components. By performing the conventional processes of compaction, sintering and extrusion, this study assesses the important factors those affect the strength as well as other mechanical properties of the Al±Si powder forgings. The results showed that by mechanical compression, it was dif®cult to obtain green compacts of considerable strength if the particle sizes of the powder were small, e.g., less than the value of 45 mm. The tensile strengths of the sintered preforms as well as extruded products were mainly affected by the silicon grain sizes. Large silicon grains contained voids of various sizes, and the voids could not be closed up in plastic deformation. In addition, many cracks could form in the silicon grains during a plastic deformation process. The cracks and the voids in the silicon grains remarkably decreased the strengths of the alloys. Therefore, by controlling the grain sizes of powder forgings of Al±Si alloys to be as small as possible, one can obtain higher tensile strengths of the alloys. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Powder extrusion; Al±Si alloys; Strength; Grain size
1. Introduction The technique of powder forging or extrusion has been developed for more than two decades and, many commercial powder forged products have also been widely used [1,2]. It was because aluminium alloys had comparably low strengths, the powder forging technique applied for aluminium alloys to produce load bearing machine components was not common 10 years ago, until Kawase et al. [3] and Akechi [4] introduced new applications of this technique for aluminium alloys. They claimed that the P/M forged SumiAlthough alloys had a tensile strength ranging from 400 to 500 MPa and had a coef®cient of thermal expansion comparable to that of cast iron. The other method used for enhance the high temperature stability as well as the tensile strength of Alalloys was called mechanical alloying, proposed by Brun et al. [5]. They added ®ne oxide and carbide dispersoids to aluminium powder in powder processing and then they used such powder mixtures to obtain some M/A forged products. They found that the mechanical properties of the M/A forgings were higher by 50% than those of the as-atomised powder forgin gs at temperatures ranging from 200 to 3508C. *
Corresponding author. Tel.: 886-2-2363-1922; fax: 886-2-2363-1755. E-mail address: [email protected] (H. So).
This study assesses the important factors that affect the strength of powder forged Al±Si alloys, which, in general, are brittle alloys. 2. Experiments 2.1. Material compositions Two sets of atomised and uncoated aluminium alloy powder were used. The chemical compositions of the selected powder are listed in Table 1. The particle size of the powder were divided into three groups, namely, ®ne (<45 mm), medium (45±120 mm) and coarse (120±300 mm). In addition to these three groups of powder, various weight percentages selecting from these three groups of powder were mixed to obtain another three groups of mixtures. The percentages of each particle size in mixtures are listed in Table 2. 2.2. Compacting process By using Alloy 1 (Table 1), every 10 g of powder mixture, respectively, containing 1.5, 1 and 0.5 wt.% of wax was compressed into a 12 mm diameter cylinder. It was found that only the powder mixtures containing 1±1.5 wt.% wax
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 3 5 - X
H. So et al. / Journal of Materials Processing Technology 114 (2001) 18±21 Table 1 Chemical compositions of the powder
Table 3 Maximum allowable pressure to obtain green compacts of considerable strengths
Elements Alloy 1 (wt.%) Alloy 2 (wt.%)
Si
Cu
Mg
Mn
Fe
Al
24.04 23.84
2.56 2.76
1.04 1.20
0.50 <0.02
0.16 1.14
Bal. Bal.
Table 2 Particle size percentage in powder mixtures (wt.%) Mixture (%) A Fine Medium Coarse
100
B
100
19
C
D
E
F
50
25 50 25
20 80
100
50
could be formed into green compacts successfully. Mixtures containing 0.5 wt.% wax could not be formed into green compacts of considerable strength. The mixture of ®ne particle size (Mixture F in Table 2) was dif®cult to form into green compacts successfully, even under a high compacting pressure. It was also found that the suitable compacting pressure employed to obtain a green compact of suf®cient strength was different for different powder mixtures. Table 3 indicates the maximum pressure employed in the compacting process for various powder mixtures. If the nominal pressure exceeds the values listed in Table 3, cracks will form on the free surface of the green compacts, when they are drawn from the die. When the compacting pressure was lower than that listed in Table 3 by 20%, both the density and strength of the green compacts would be too low to be handled. When Alloy 2 (Table 1) was used, it was not possible to obtain any green compacts of considerable strength. 2.3. Sintering process The atmosphere used in the sintering process of the silicon±aluminium green compacts was restricted to gasi®ed nitrogen from the liquid state. Neither hydrogen nor nitrogen in a gas state could be used as the sintering atmosphere. If these gases should be used, one would obtain metallic
Powder mixture Max. allowable pressure (MPa)
A
C
D
E
321.5
309.6
340
350
aluminium precipitated from the green compacts. Fig. 1 presents a photograph of the result of using such an unfavourable condition. The sintering temperature is also important in grain-size control. The higher the temperature, the larger the grain size of the silicon. If the sintered preforms contain big grains, their strengths will decrease correspondingly. It was found that the condition at a sintering temperature lower than 6008C and in an atmosphere of gasi®ed nitrogen from the liquid state might be suitable. Fig. 2 shows some typical micrographs of the sintered preforms. The particle-like grains in the photographs are silicon particles. 2.4. Extrusion process Hot forward extrusion of sintered preforms was conducted to produce circular cylinders. Two extrusion ratios of 1.5 and 3 were, respectively, employed. The extruded cylinders were used for tension tests. In order to study the formability of the sintered preforms, backward extrusion was also conducted to form cup-like products. 3. Results and discussion 3.1. Density change The theoretical densities of the powder mixtures are listed in Table 4. The densities of sintered preforms and extruded cylinders are listed in Table 5. Because most of the voids in the aluminium matrix of the preforms can be closed up under plastic deformation, the densities of the extruded products depend upon the amount of plastic deformation as well as upon the extrusion ratio. The maximum density of the extruded cylinders reached only as high as 90% of the theoretical densities of the powder mixtures. This low
Fig. 1. A photograph showing the preforms sintered in a nitrogen gas atmosphere. Note: the globules of precipitated metallic aluminium.
20
H. So et al. / Journal of Materials Processing Technology 114 (2001) 18±21
Fig. 3. A typical micrograph of an extruded product.
3.2. Strengths of the sintered and the extruded products
Fig. 2. Micrographs showing alloys sintered at various sintering temperatures: (a) 6408C; (b) 6908C. Table 4 Theoretical density of the powder Particle size Density
Fine
Medium
Coarse
2.651
2.631
2.5824
Table 5 The ratios of densities of sintered and extruded products to the theoretical density of the powder mixtures
Sintered Extruded R 1 Extruded R 3
A
C
D
E
0.723 0.825 0.862
0.701 0.795 0.842
0.744 0.843 0.887
0.755 0.853 0.905
density was mainly caused by the existence of voids and cracks in the silicon grains. The voids originally existing in the silicon grains could not be eliminated by plastic deformation during the extrusion process. In contrast, subjected to plastic deformation, the silicon grains of the sintered preforms might produce cracks. Fig. 3 indicates a typical micrograph of an extruded product, where one can see many cracks existing in the silicon grains.
The sintered preforms exhibited limited ductility when they were subjected to tensile tests. The ultimate tensile strength of the sintered preforms depended on the silicon grain sizes and the porosity distribution in the aluminium matrix. It was found that the grain sizes of the silicon crystals could be controlled by the particle size of the powder mixture as well as by the sintering temperature, as mentioned previously. The higher the sintering temperature, the larger the grain sizes; the larger the powder particle, also the larger the grain sizes. When the silicon grains were large, the number and size of the voids found in the grains were corresponding large. Voids either in the aluminium matrix or in the silicon grains could reduce the tensile strength of the sintered preforms. When these sintered preforms were subjected to an extrusion process, the silicon grains might produce cracks, although most of the voids in the aluminium matrix were closed up (Fig. 3). In addition, the existence of cracks in the silicon grains caused the extruded products to behave as brittle bodies. The fracture strengths and maximum elongation of the extruded products from various powder mixtures are listed in Table 6. In this table, one can ®nd that the extrusion ratio has a considerable effect on the strength. Increase of the extrusion ratio can close up the voids in the aluminium matrix and increase the strength correspondingly. However, when subjected to an extrusion process the voids in the silicon grains could not be closed up and cracks were produced, as mentioned previously. Table 6 Tensile strengths and fracture strains of extruded products Mixtures
A C D E
Extrusion R 1
Extrusion R 3
Tensile strength (MPa)
Fracture strain
Tensile strength (MPa)
Fracture strain
125.7 115.7 143.7 158.9
0.0257 0.0250 0.0256 0.0225
189.8 169.8 204.2 212.3
0.0175 0.0156 0.0175 0.0213
H. So et al. / Journal of Materials Processing Technology 114 (2001) 18±21
21
In a cup extrusion process with a suitable lubrication condition, cracks were not found on the free surface of the cup products, when the dimensional ratio of cup height to wall thickness was less than a value of 3.5. No back pressure was needed in the process. If the height to thickness ratio exceeds the value of 3.5, cracks will be produced on the free surface of the cups. 4. Conclusions Fig. 4. A micrograph showing a typical sintered preform that had been produced by cold isostatic pressing.
Consequently, the tensile strength of the extruded products did not increase remarkably. In addition, the maximum elongation would decrease as the extrusion ratio increased due to cracking of the silicon grains. The green compacts formed by the cold isostatic pressing process had higher tensile strengths than those produced by mechanical compaction. In addition, the grain size of the sintered preforms was smaller than that for mechanical compaction. Fig. 4 shows a typical micrograph of a sintered preform from the cold isostatic pressing process. 3.3. Cup extrusion Sintered cylinders of 10 mm diameter and 5 mm length were used for conducting a hot inverted extrusion. The purpose of cup extrusion was to evaluate the formability of the sintered preforms. It was found that when the working temperature was maintained at 4758C and a suitable lubrication condition was ensured, cups of 3.5 mm height and 1 mm wall thickness could be produced without any cracks being formed on the free surface of the cups.
To obtain near-net shape or net-shape products by the powder forging of Al±Si alloy is not dif®cult. However, how high the required tensile strength is. From the present study, some conclusions can be drawn. 1. The overall particle size of the alloy powder should not be smaller than the value of 45 mm, as it is dif®cult to obtain green compacts of considerable strength if the powder has small particle sizes. 2. The atmosphere in the sintering process of green compacts is important. It was found that nitrogen gasi®ed from the liquid state seems suitable. 3. A suitable amount of lubricant should be used in a mechanical compacting process, but not in cold isostatic pressing. 4. The strengths of the ®nal products depend mainly on the silicon grain sizes. Large grains can accommodate more and larger voids, but such a situation is harmful. Further, cracks are easy to form in the silicon grains under plastic deformation, which can reduce the tensile strength and the maximum elongation of forgings. Therefore, the silicon grain sizes should be controlled to be as small as possible. 5. Introducing suitable lubrication conditions rather than using back pressure can eliminate the cracks producing on the free surface of forged products.
3.4. Comparison with existing data
References
The tensile strength of the present powder forged products was low in comparison with the data claimed by Sumitomo Electric Industries, being only as high as 60% of that they claimed. The low strengths of the present products were obviously caused by the comparably large silicon grains in the sintered preforms. When these sintered preforms were subjected to an extrusion process, the silicon grains did not deform plastically, but fractured.
[1] P.K. Johnson, Int. J. Powder Metall. 26 (3) (1990) 271±276. [2] G. Sutradhar, A.K. Jha, S. Kumar, J. Mater. Process. Technol. 41 (1994) 143±169. [3] K. Kawase, M. Otsuki, T. Kohno, K. Morimoto, M. Kobayashi, Proceedings of the 1993 Powder Metallurgy World Congress, pp. 899±902. [4] K. Akechi, US Patent No. 4,838,936 (June 1989). [5] P. Le Brun, L. Froyen, B. Munar, L. Delaey, Scand. J. Metall. 19 (1990) 19±22.
Assessment of the powder extrusion of silicon±aluminium alloy H. So*, W.C. Li, H.K. Hsieh Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan Received 5 June 1999
Abstract The silicon±aluminium alloys are, in general, brittle in behaviour and have comparably low strengths. However, Sumitomo Electric Industries claimed that they had developed the so-called Sumi-Although alloys, which had tensile strengths ranging from 400 to 500 MPa. Such alloys were suitable to be used for load bearing machine components. By performing the conventional processes of compaction, sintering and extrusion, this study assesses the important factors those affect the strength as well as other mechanical properties of the Al±Si powder forgings. The results showed that by mechanical compression, it was dif®cult to obtain green compacts of considerable strength if the particle sizes of the powder were small, e.g., less than the value of 45 mm. The tensile strengths of the sintered preforms as well as extruded products were mainly affected by the silicon grain sizes. Large silicon grains contained voids of various sizes, and the voids could not be closed up in plastic deformation. In addition, many cracks could form in the silicon grains during a plastic deformation process. The cracks and the voids in the silicon grains remarkably decreased the strengths of the alloys. Therefore, by controlling the grain sizes of powder forgings of Al±Si alloys to be as small as possible, one can obtain higher tensile strengths of the alloys. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Powder extrusion; Al±Si alloys; Strength; Grain size
1. Introduction The technique of powder forging or extrusion has been developed for more than two decades and, many commercial powder forged products have also been widely used [1,2]. It was because aluminium alloys had comparably low strengths, the powder forging technique applied for aluminium alloys to produce load bearing machine components was not common 10 years ago, until Kawase et al. [3] and Akechi [4] introduced new applications of this technique for aluminium alloys. They claimed that the P/M forged SumiAlthough alloys had a tensile strength ranging from 400 to 500 MPa and had a coef®cient of thermal expansion comparable to that of cast iron. The other method used for enhance the high temperature stability as well as the tensile strength of Alalloys was called mechanical alloying, proposed by Brun et al. [5]. They added ®ne oxide and carbide dispersoids to aluminium powder in powder processing and then they used such powder mixtures to obtain some M/A forged products. They found that the mechanical properties of the M/A forgings were higher by 50% than those of the as-atomised powder forgin gs at temperatures ranging from 200 to 3508C. *
Corresponding author. Tel.: 886-2-2363-1922; fax: 886-2-2363-1755. E-mail address: [email protected] (H. So).
This study assesses the important factors that affect the strength of powder forged Al±Si alloys, which, in general, are brittle alloys. 2. Experiments 2.1. Material compositions Two sets of atomised and uncoated aluminium alloy powder were used. The chemical compositions of the selected powder are listed in Table 1. The particle size of the powder were divided into three groups, namely, ®ne (<45 mm), medium (45±120 mm) and coarse (120±300 mm). In addition to these three groups of powder, various weight percentages selecting from these three groups of powder were mixed to obtain another three groups of mixtures. The percentages of each particle size in mixtures are listed in Table 2. 2.2. Compacting process By using Alloy 1 (Table 1), every 10 g of powder mixture, respectively, containing 1.5, 1 and 0.5 wt.% of wax was compressed into a 12 mm diameter cylinder. It was found that only the powder mixtures containing 1±1.5 wt.% wax
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 3 5 - X
H. So et al. / Journal of Materials Processing Technology 114 (2001) 18±21 Table 1 Chemical compositions of the powder
Table 3 Maximum allowable pressure to obtain green compacts of considerable strengths
Elements Alloy 1 (wt.%) Alloy 2 (wt.%)
Si
Cu
Mg
Mn
Fe
Al
24.04 23.84
2.56 2.76
1.04 1.20
0.50 <0.02
0.16 1.14
Bal. Bal.
Table 2 Particle size percentage in powder mixtures (wt.%) Mixture (%) A Fine Medium Coarse
100
B
100
19
C
D
E
F
50
25 50 25
20 80
100
50
could be formed into green compacts successfully. Mixtures containing 0.5 wt.% wax could not be formed into green compacts of considerable strength. The mixture of ®ne particle size (Mixture F in Table 2) was dif®cult to form into green compacts successfully, even under a high compacting pressure. It was also found that the suitable compacting pressure employed to obtain a green compact of suf®cient strength was different for different powder mixtures. Table 3 indicates the maximum pressure employed in the compacting process for various powder mixtures. If the nominal pressure exceeds the values listed in Table 3, cracks will form on the free surface of the green compacts, when they are drawn from the die. When the compacting pressure was lower than that listed in Table 3 by 20%, both the density and strength of the green compacts would be too low to be handled. When Alloy 2 (Table 1) was used, it was not possible to obtain any green compacts of considerable strength. 2.3. Sintering process The atmosphere used in the sintering process of the silicon±aluminium green compacts was restricted to gasi®ed nitrogen from the liquid state. Neither hydrogen nor nitrogen in a gas state could be used as the sintering atmosphere. If these gases should be used, one would obtain metallic
Powder mixture Max. allowable pressure (MPa)
A
C
D
E
321.5
309.6
340
350
aluminium precipitated from the green compacts. Fig. 1 presents a photograph of the result of using such an unfavourable condition. The sintering temperature is also important in grain-size control. The higher the temperature, the larger the grain size of the silicon. If the sintered preforms contain big grains, their strengths will decrease correspondingly. It was found that the condition at a sintering temperature lower than 6008C and in an atmosphere of gasi®ed nitrogen from the liquid state might be suitable. Fig. 2 shows some typical micrographs of the sintered preforms. The particle-like grains in the photographs are silicon particles. 2.4. Extrusion process Hot forward extrusion of sintered preforms was conducted to produce circular cylinders. Two extrusion ratios of 1.5 and 3 were, respectively, employed. The extruded cylinders were used for tension tests. In order to study the formability of the sintered preforms, backward extrusion was also conducted to form cup-like products. 3. Results and discussion 3.1. Density change The theoretical densities of the powder mixtures are listed in Table 4. The densities of sintered preforms and extruded cylinders are listed in Table 5. Because most of the voids in the aluminium matrix of the preforms can be closed up under plastic deformation, the densities of the extruded products depend upon the amount of plastic deformation as well as upon the extrusion ratio. The maximum density of the extruded cylinders reached only as high as 90% of the theoretical densities of the powder mixtures. This low
Fig. 1. A photograph showing the preforms sintered in a nitrogen gas atmosphere. Note: the globules of precipitated metallic aluminium.
20
H. So et al. / Journal of Materials Processing Technology 114 (2001) 18±21
Fig. 3. A typical micrograph of an extruded product.
3.2. Strengths of the sintered and the extruded products
Fig. 2. Micrographs showing alloys sintered at various sintering temperatures: (a) 6408C; (b) 6908C. Table 4 Theoretical density of the powder Particle size Density
Fine
Medium
Coarse
2.651
2.631
2.5824
Table 5 The ratios of densities of sintered and extruded products to the theoretical density of the powder mixtures
Sintered Extruded R 1 Extruded R 3
A
C
D
E
0.723 0.825 0.862
0.701 0.795 0.842
0.744 0.843 0.887
0.755 0.853 0.905
density was mainly caused by the existence of voids and cracks in the silicon grains. The voids originally existing in the silicon grains could not be eliminated by plastic deformation during the extrusion process. In contrast, subjected to plastic deformation, the silicon grains of the sintered preforms might produce cracks. Fig. 3 indicates a typical micrograph of an extruded product, where one can see many cracks existing in the silicon grains.
The sintered preforms exhibited limited ductility when they were subjected to tensile tests. The ultimate tensile strength of the sintered preforms depended on the silicon grain sizes and the porosity distribution in the aluminium matrix. It was found that the grain sizes of the silicon crystals could be controlled by the particle size of the powder mixture as well as by the sintering temperature, as mentioned previously. The higher the sintering temperature, the larger the grain sizes; the larger the powder particle, also the larger the grain sizes. When the silicon grains were large, the number and size of the voids found in the grains were corresponding large. Voids either in the aluminium matrix or in the silicon grains could reduce the tensile strength of the sintered preforms. When these sintered preforms were subjected to an extrusion process, the silicon grains might produce cracks, although most of the voids in the aluminium matrix were closed up (Fig. 3). In addition, the existence of cracks in the silicon grains caused the extruded products to behave as brittle bodies. The fracture strengths and maximum elongation of the extruded products from various powder mixtures are listed in Table 6. In this table, one can ®nd that the extrusion ratio has a considerable effect on the strength. Increase of the extrusion ratio can close up the voids in the aluminium matrix and increase the strength correspondingly. However, when subjected to an extrusion process the voids in the silicon grains could not be closed up and cracks were produced, as mentioned previously. Table 6 Tensile strengths and fracture strains of extruded products Mixtures
A C D E
Extrusion R 1
Extrusion R 3
Tensile strength (MPa)
Fracture strain
Tensile strength (MPa)
Fracture strain
125.7 115.7 143.7 158.9
0.0257 0.0250 0.0256 0.0225
189.8 169.8 204.2 212.3
0.0175 0.0156 0.0175 0.0213
H. So et al. / Journal of Materials Processing Technology 114 (2001) 18±21
21
In a cup extrusion process with a suitable lubrication condition, cracks were not found on the free surface of the cup products, when the dimensional ratio of cup height to wall thickness was less than a value of 3.5. No back pressure was needed in the process. If the height to thickness ratio exceeds the value of 3.5, cracks will be produced on the free surface of the cups. 4. Conclusions Fig. 4. A micrograph showing a typical sintered preform that had been produced by cold isostatic pressing.
Consequently, the tensile strength of the extruded products did not increase remarkably. In addition, the maximum elongation would decrease as the extrusion ratio increased due to cracking of the silicon grains. The green compacts formed by the cold isostatic pressing process had higher tensile strengths than those produced by mechanical compaction. In addition, the grain size of the sintered preforms was smaller than that for mechanical compaction. Fig. 4 shows a typical micrograph of a sintered preform from the cold isostatic pressing process. 3.3. Cup extrusion Sintered cylinders of 10 mm diameter and 5 mm length were used for conducting a hot inverted extrusion. The purpose of cup extrusion was to evaluate the formability of the sintered preforms. It was found that when the working temperature was maintained at 4758C and a suitable lubrication condition was ensured, cups of 3.5 mm height and 1 mm wall thickness could be produced without any cracks being formed on the free surface of the cups.
To obtain near-net shape or net-shape products by the powder forging of Al±Si alloy is not dif®cult. However, how high the required tensile strength is. From the present study, some conclusions can be drawn. 1. The overall particle size of the alloy powder should not be smaller than the value of 45 mm, as it is dif®cult to obtain green compacts of considerable strength if the powder has small particle sizes. 2. The atmosphere in the sintering process of green compacts is important. It was found that nitrogen gasi®ed from the liquid state seems suitable. 3. A suitable amount of lubricant should be used in a mechanical compacting process, but not in cold isostatic pressing. 4. The strengths of the ®nal products depend mainly on the silicon grain sizes. Large grains can accommodate more and larger voids, but such a situation is harmful. Further, cracks are easy to form in the silicon grains under plastic deformation, which can reduce the tensile strength and the maximum elongation of forgings. Therefore, the silicon grain sizes should be controlled to be as small as possible. 5. Introducing suitable lubrication conditions rather than using back pressure can eliminate the cracks producing on the free surface of forged products.
3.4. Comparison with existing data
References
The tensile strength of the present powder forged products was low in comparison with the data claimed by Sumitomo Electric Industries, being only as high as 60% of that they claimed. The low strengths of the present products were obviously caused by the comparably large silicon grains in the sintered preforms. When these sintered preforms were subjected to an extrusion process, the silicon grains did not deform plastically, but fractured.
[1] P.K. Johnson, Int. J. Powder Metall. 26 (3) (1990) 271±276. [2] G. Sutradhar, A.K. Jha, S. Kumar, J. Mater. Process. Technol. 41 (1994) 143±169. [3] K. Kawase, M. Otsuki, T. Kohno, K. Morimoto, M. Kobayashi, Proceedings of the 1993 Powder Metallurgy World Congress, pp. 899±902. [4] K. Akechi, US Patent No. 4,838,936 (June 1989). [5] P. Le Brun, L. Froyen, B. Munar, L. Delaey, Scand. J. Metall. 19 (1990) 19±22.