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Research on Preheating of Titanium Alloy Powder in Electron Beam Melting Technology
Rare Metal Materials and Engineering Volume 40, Issue 12, December 2011 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2011, 40(12): 2072-2075.
ARTICLE
Research on Preheating of Titanium Alloy Powder in Electron Beam Melting Technology He Weiwei,
Jia Wenpeng,
Liu Haiyan,
Tang Huiping,
Kang Xinting,
Huang Yu
State Key Laboratory of Porous Metals Materials, Northwest Institute for Non-ferrous Metal Research, Xi’an 710016, China
Abstract: The titanium alloy powder preheating process in electron beam melting (EBM) was investigated, and the connection mode among powder particles after preheating was analyzed. The results show that when the Ti6Al4V powder is preheated to above 600 °C during EBM process, a powder aggregation is formed. The sintering mechanism is that small particles partially or completely melt and play the role of binder to bond the majority of big particles together; this will be not only to help the particles to hold their places, withstanding the impact force of electron beam, but also to prevent the spheroidization effect in the prototyped surface. Using Ti6Al4V powder as starting material and adopting complete preheating of the powder layer, the column samples with full interlayer bonding and excellent mechanical properties are produced. Key words: Ti6Al4V; forming; powder; preheating; sintering
Electron beam melting (EBM) is a new solid-freeform-fabrication technology, by which parts are built layer by layer with the help of an electron beam selectively melting metal powder, and as loose powder supports all downward-facing surfaces, highly complex geometric shapes can be built. In addition, a high building velocity can be obtained since this process make use of some advantages of electron beam, such as high energy density, high adsorption rate and high scanning speed. Lastly, the parts with excellent properties can be attained as the entire process takes place under vacuum, which provides a clean atmosphere to prevent the contamination of the metal parts during the forming process[1-3]. Compared with the traditional process of powder metallurgy and mechanical processing, the EBM technology offers a high building speed and a high level of geometric freedom together with first-class material properties, which has a wide array of applications in automotive and aerospace industry as well as biomedical engineering[3,4]. In the process of electron beam melting, the loose powder particles are easily pushed away by the impact of the high energy electron beam called powder blown phenomena; furthermore, if the temperature of the powder around molten pool is too low, the spheroidization phenomenon easily emerges for
bad wetting properties between powder particles and metal molten pool. This will cause poor forming precision or even failure of forming. Peter Heinl[5] and Cormier D[6] used powder material preheating process to cope with powder blown and spheroidization phenomenon. Qi Haibo[7-9] used various shape particles mixture powder as material and preheated them to solve the same question. All these indicated that preheating will be useful to eliminate powder blown and spheroidization effect, but the mechanism of forming process affected by preheating is still not known clearly. In this paper, using Ti6Al4V powder as starting material, the research on the preheating of powder in EBM process was carried out, the connection mode among powder particles was analyzed, and the mechanism for eliminating powder blown and preventing spheroidization effect was illustrated. At last, using preheating process, the column samples with full interlayer bonding were prepared and mechanical properties were also analyzed.
1 Experiment The starting material was pre-alloyed Ti-6Al-4V powder which was prepared by gas atomizing with particle size less than 100 µm. All experiments were carried out in the Arcam
Received date: December 11, 2010 Foundation item: National Key Technologies R&D Program (2007BAE07B05) Corresponding author: He Weiwei, Candidate for Ph. D., State Key Laboratory of Porous Metals Materials, Northwest Institute for Nonferrous Metal Research, Xi’an 710016, P. R. China; Tang Huiping, Professor, Ph. D., Tel: 0086-29-86231095 Copyright © 2011, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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He Weiwei et al. / Rare Metal Materials and Engineering, 2011, 40(12): 2072-2075
S12 system, whose EB power can be up to 4 kW and melting chamber internal pressure up to 10-2-10-4 Pa. A steel start plate with dimensions of 150 mm×150 mm×10 mm was embedded into the powder bed that spread on the building platform. The plate was heated by the electron beam gun until the temperature was above 600 °C. Then a 0.1 mm thick layer of titanium alloy powder was spread on the plate. The area of powder preheated was a square of 150 mm×150 mm, scanning each layer by electron beam was carried out on the scanned area with the line scanning mode, and the scanning direction would be alternated after each layer with 90°. In each layer, the area of preheating powder was initially scanned for at least 40 times using a beam velocity of 8000 mm/s. From the first scan to the last scan, the beam current was linearly ramped up until the temperature of powder above 600 °C. Then the specific area powder that was related to testing bars cross-section information was melted by increasing of the beam power and decreasing of the scan speed. The building platform was lowered by an amount equal to one layer thickness, a new layer of metal powder was spread, and the process was repeated until the column samples were complete.
2
Results and Discussion
A cubic powder aggregation is formed in the powder bed after electron beam preheating and selective melting processes. Fig.1a is the photograph of cubic aggregation, and the column samples are inbuilt in this aggregation. Since preheating process is adopted, a certain extent inter-particle cohesion is generated in the powder bed, and a cake-like cubic aggregation is formed. This powder aggregation material has certain strength, but it can be broken up by bead blasting with Ti6Al4V powder or other appropriate blast media. Fig.1b shows the column samples that emerge after the cubic aggregate is crashed down. It is very essential for the powder to generate certain inter-particle cohesion in the EBM process, and this inter-particle cohesion can withstand the impact of the large current electron beam when selective melting area is melted. The powder cubic aggregation which can be easily crashed into dispersive powder particles and be reused is not the true sintering. Through analyzing the chemical composition of the recycled powder, and comparing with the initial powder, the results are presented in Table 1, which shows that there are almost no changes in the chemical composition of the recycled powder, especially the concerned oxygen content. The preheating doesn’t change the chemical composition of the powder, and the powder can be reused. Fig.2 shows the connection mode among powder particles of cubic aggregation and a typical powder particle after the aggregation is crashed. It can be seen from Fig.2a that Ti-6Al-4V powder is consisted of large spherical particles and small satellite particles; generally, there is no neck growth between large particles, but most satellite small particles are sintered with at least one large parti-
a
b
Fig.1 Photographs of the cubic aggregation of powder material (a) and the TC4 column sample (b) after cubic aggregate powder material is crashed down Table 1 Chemical analysis for Ti6Al4V powder Element
Ti
Al
V
Fe
C
N
O
H
Recycled Balanced 6.52 3.89 0.07 0.01 0.006 0.13 0.002 powder Initial powder
Balanced 6.55 3.93 0.05 0.02 0.005 0.13 0.002
a
b
Fig.2 SEM images of powder aggregation (a) and a typical powder particle (b)
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He Weiwei et al. / Rare Metal Materials and Engineering, 2011, 40(12): 2072-2075
cle. The arrows pointed in Fig.2a also show clearly that small satellite particles in interspace are connected to large spherical particles in presintering mode, which forms a skeleton to bond the large particles together. This connection mode is frangible, but tough enough to hold powder and resist the impact of electron beam during EBM process. When EBM process is finished, the powder aggregation can be broken up by exterior force and the embedded samples are free. The SEM micrograph of a typical powder particle is shown in Fig.2b. In Fig.2b, the powder particle has several scars resulting from detaching of bonding particles from its surface, which indicates that it is sintered with two particles at least. It is known that the contact of particles can be built as soon as they are heated, and the area of surface in contact will increase with the force of capillary or interface. The effect of preheating powder in EBM process is almost the same as that of the first stage of traditional sintering, while the preheating procedure only heats the upper thin layer powder and brings them to bond with the formed aggregation. The Ti6Al4V powder consists of large spherical particles and satellite small particles located in the interspace, and the property of the satellite particles with high specific area can promote presinter process. When electrons with the speed of 0.1-0.5 times of light hit the powder particles in the building platform, they release kinetic energy mostly as thermal energy, while the energy of the electron beam is not strong enough to melt all powder particles in the procedure of preheating, so those particles with smaller diameter and some bulges or external points of bulky particles partially or completely melt rapidly for their higher surface energy, while many bulk particles don’t melt for more fusing energy needed. With the temperature rising, it is helpful for the melted particles to bond the bulk grains together and form a powder aggregation. Forming of the aggregation can freeze the powder particles, and improve their ability of resisting the impact of electric beam. At the same time, the powder surface is activated and the wetting ability is increased. High temperature makes powder surface wettable to the melting pool, avoiding spheroidization effects. All these ensure the stability of EBM process. What’s more, a persistent thermal field formed in the powder bed through preheating process, giving heat treating of the product in building procedure, will result in the excellent mechanical performance of the final product. Using the preheating process, column samples with approximate dimension of Φ8 mm×55 mm can be finally prepared after building up the fused structure layer by layer. The longitudinal and transverse sections of each bar are prepared for light microscopy using standard metallographic techniques. These characterizations show that the microstructure is extremely fine but inhomogeneous. As shown in Fig.3a, the columnar grains dominate in the entirely longitudinal specimen, whose size varies from the edge to the centre and from the bottom to the top of the sample. The width of columnar grains
in the sample varies from 0.02 to 0.2 mm, and the height from 0.5 to 15 mm, i.e., the height of the entire sample. There are no layer bands in the EBM processed samples, whose microstructures are nearly full dense. From Fig.3b, the microstructure of transverse specimen is mainly basket-weave, and the size of α and β laths vary at different locations. Through characterizing the mechanical properties of EBM samples, the ultimate tensile and yield strengths of untreated specimen are 1010 and 950 MPa, respectively, which clearly reach the limits of ASM handbook (volume 2) for wrought Ti6Al4V alloy (930-1015 MPa and 860-965 MPa). The breaking elongation is 15.0%, and it surpasses ASM limit (10%-14%). It indicates that the EBSM processed Ti6Al4V alloy hold excellent mechanical properties. The SEM fracture surfaces of the tensile tested specimen are provided in Fig.4. Many typical dimples appear in the SEM morphology of the tensile fracture surface, and the fracture is ductile rupture. a
b
20 μm Fig.3 Optical morphologies of the longitudinal (a) and transverse (b) specimens from the tested sample
Fig.4 SEM fracture surface of the tensile specimen
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He Weiwei et al. / Rare Metal Materials and Engineering, 2011, 40(12): 2072-2075
3 Conclusions Preheating the powder at a proper temperature, the particles will be slightly sintered, and a cubic aggregation is produced. In the cubic aggregation, the satellite small particles are partially or completely melt, and the little molten metal drops play the role of binder to bond the majority of big particles together and form the powder block, which not only helps the particles to hold their places to withstand the impact force of electron beam, but also improves the wetting ability between powder particles and molten metal pool, and prevents droplets formed in the surface prototyped. Several Ti6Al4V column samples are prepared by this process. The samples exhibit full interlayer bonding almost without pores, and their tensile performance is in the same range as respective values derived from the conventionally wrought annealed titanium alloy, indicating excellent mechanical properties.
1 Arcam S E. U S Patent[P]. No. 2006157454, 2006, 7: 20 2 Yang Xin, Tang Huiping, He Weiwei. Titanium Industry Progress [J], 2007, 24(3): 10 3 Denis Cormier, Ola Harrysson, Tushar Mahale. Journal of the Chinese Institute of Industrial Engineers[J], 2003, 20: 193 4 Ola L A Harrysson, Omer Cansizoglu, Denis J Marcellin. Materials Science and Engineering C[J], 2008, 28: 366 5 Peter Heinl, Andreas Rottmair, Carolin Körner. Advanced Engineering Materials[J], 2007, 9: 360 6 Cormier D, Harrysson O, West H. Rapid Prototyping J[J], 2004, 10: 35 7 Qi Haibo, Yan Yongnian, Lin Feng. J Tsinghua Univ (Sci ＆ Tech) [J], 2005, 45(8): 1012 8 Qi Haibo, Yan Yongnian, Lin Feng. J Engineering Manufacture, Proceedings Part B[J], 2006, 220(B11): 1845 9 Yan Yongnian, Qi Haibo, Lin Feng. Chinese Journal of Mechanical Engineering[J], 2007, 43(6): 87
References
2075
ARTICLE
Research on Preheating of Titanium Alloy Powder in Electron Beam Melting Technology He Weiwei,
Jia Wenpeng,
Liu Haiyan,
Tang Huiping,
Kang Xinting,
Huang Yu
State Key Laboratory of Porous Metals Materials, Northwest Institute for Non-ferrous Metal Research, Xi’an 710016, China
Abstract: The titanium alloy powder preheating process in electron beam melting (EBM) was investigated, and the connection mode among powder particles after preheating was analyzed. The results show that when the Ti6Al4V powder is preheated to above 600 °C during EBM process, a powder aggregation is formed. The sintering mechanism is that small particles partially or completely melt and play the role of binder to bond the majority of big particles together; this will be not only to help the particles to hold their places, withstanding the impact force of electron beam, but also to prevent the spheroidization effect in the prototyped surface. Using Ti6Al4V powder as starting material and adopting complete preheating of the powder layer, the column samples with full interlayer bonding and excellent mechanical properties are produced. Key words: Ti6Al4V; forming; powder; preheating; sintering
Electron beam melting (EBM) is a new solid-freeform-fabrication technology, by which parts are built layer by layer with the help of an electron beam selectively melting metal powder, and as loose powder supports all downward-facing surfaces, highly complex geometric shapes can be built. In addition, a high building velocity can be obtained since this process make use of some advantages of electron beam, such as high energy density, high adsorption rate and high scanning speed. Lastly, the parts with excellent properties can be attained as the entire process takes place under vacuum, which provides a clean atmosphere to prevent the contamination of the metal parts during the forming process[1-3]. Compared with the traditional process of powder metallurgy and mechanical processing, the EBM technology offers a high building speed and a high level of geometric freedom together with first-class material properties, which has a wide array of applications in automotive and aerospace industry as well as biomedical engineering[3,4]. In the process of electron beam melting, the loose powder particles are easily pushed away by the impact of the high energy electron beam called powder blown phenomena; furthermore, if the temperature of the powder around molten pool is too low, the spheroidization phenomenon easily emerges for
bad wetting properties between powder particles and metal molten pool. This will cause poor forming precision or even failure of forming. Peter Heinl[5] and Cormier D[6] used powder material preheating process to cope with powder blown and spheroidization phenomenon. Qi Haibo[7-9] used various shape particles mixture powder as material and preheated them to solve the same question. All these indicated that preheating will be useful to eliminate powder blown and spheroidization effect, but the mechanism of forming process affected by preheating is still not known clearly. In this paper, using Ti6Al4V powder as starting material, the research on the preheating of powder in EBM process was carried out, the connection mode among powder particles was analyzed, and the mechanism for eliminating powder blown and preventing spheroidization effect was illustrated. At last, using preheating process, the column samples with full interlayer bonding were prepared and mechanical properties were also analyzed.
1 Experiment The starting material was pre-alloyed Ti-6Al-4V powder which was prepared by gas atomizing with particle size less than 100 µm. All experiments were carried out in the Arcam
Received date: December 11, 2010 Foundation item: National Key Technologies R&D Program (2007BAE07B05) Corresponding author: He Weiwei, Candidate for Ph. D., State Key Laboratory of Porous Metals Materials, Northwest Institute for Nonferrous Metal Research, Xi’an 710016, P. R. China; Tang Huiping, Professor, Ph. D., Tel: 0086-29-86231095 Copyright © 2011, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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He Weiwei et al. / Rare Metal Materials and Engineering, 2011, 40(12): 2072-2075
S12 system, whose EB power can be up to 4 kW and melting chamber internal pressure up to 10-2-10-4 Pa. A steel start plate with dimensions of 150 mm×150 mm×10 mm was embedded into the powder bed that spread on the building platform. The plate was heated by the electron beam gun until the temperature was above 600 °C. Then a 0.1 mm thick layer of titanium alloy powder was spread on the plate. The area of powder preheated was a square of 150 mm×150 mm, scanning each layer by electron beam was carried out on the scanned area with the line scanning mode, and the scanning direction would be alternated after each layer with 90°. In each layer, the area of preheating powder was initially scanned for at least 40 times using a beam velocity of 8000 mm/s. From the first scan to the last scan, the beam current was linearly ramped up until the temperature of powder above 600 °C. Then the specific area powder that was related to testing bars cross-section information was melted by increasing of the beam power and decreasing of the scan speed. The building platform was lowered by an amount equal to one layer thickness, a new layer of metal powder was spread, and the process was repeated until the column samples were complete.
2
Results and Discussion
A cubic powder aggregation is formed in the powder bed after electron beam preheating and selective melting processes. Fig.1a is the photograph of cubic aggregation, and the column samples are inbuilt in this aggregation. Since preheating process is adopted, a certain extent inter-particle cohesion is generated in the powder bed, and a cake-like cubic aggregation is formed. This powder aggregation material has certain strength, but it can be broken up by bead blasting with Ti6Al4V powder or other appropriate blast media. Fig.1b shows the column samples that emerge after the cubic aggregate is crashed down. It is very essential for the powder to generate certain inter-particle cohesion in the EBM process, and this inter-particle cohesion can withstand the impact of the large current electron beam when selective melting area is melted. The powder cubic aggregation which can be easily crashed into dispersive powder particles and be reused is not the true sintering. Through analyzing the chemical composition of the recycled powder, and comparing with the initial powder, the results are presented in Table 1, which shows that there are almost no changes in the chemical composition of the recycled powder, especially the concerned oxygen content. The preheating doesn’t change the chemical composition of the powder, and the powder can be reused. Fig.2 shows the connection mode among powder particles of cubic aggregation and a typical powder particle after the aggregation is crashed. It can be seen from Fig.2a that Ti-6Al-4V powder is consisted of large spherical particles and small satellite particles; generally, there is no neck growth between large particles, but most satellite small particles are sintered with at least one large parti-
a
b
Fig.1 Photographs of the cubic aggregation of powder material (a) and the TC4 column sample (b) after cubic aggregate powder material is crashed down Table 1 Chemical analysis for Ti6Al4V powder Element
Ti
Al
V
Fe
C
N
O
H
Recycled Balanced 6.52 3.89 0.07 0.01 0.006 0.13 0.002 powder Initial powder
Balanced 6.55 3.93 0.05 0.02 0.005 0.13 0.002
a
b
Fig.2 SEM images of powder aggregation (a) and a typical powder particle (b)
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He Weiwei et al. / Rare Metal Materials and Engineering, 2011, 40(12): 2072-2075
cle. The arrows pointed in Fig.2a also show clearly that small satellite particles in interspace are connected to large spherical particles in presintering mode, which forms a skeleton to bond the large particles together. This connection mode is frangible, but tough enough to hold powder and resist the impact of electron beam during EBM process. When EBM process is finished, the powder aggregation can be broken up by exterior force and the embedded samples are free. The SEM micrograph of a typical powder particle is shown in Fig.2b. In Fig.2b, the powder particle has several scars resulting from detaching of bonding particles from its surface, which indicates that it is sintered with two particles at least. It is known that the contact of particles can be built as soon as they are heated, and the area of surface in contact will increase with the force of capillary or interface. The effect of preheating powder in EBM process is almost the same as that of the first stage of traditional sintering, while the preheating procedure only heats the upper thin layer powder and brings them to bond with the formed aggregation. The Ti6Al4V powder consists of large spherical particles and satellite small particles located in the interspace, and the property of the satellite particles with high specific area can promote presinter process. When electrons with the speed of 0.1-0.5 times of light hit the powder particles in the building platform, they release kinetic energy mostly as thermal energy, while the energy of the electron beam is not strong enough to melt all powder particles in the procedure of preheating, so those particles with smaller diameter and some bulges or external points of bulky particles partially or completely melt rapidly for their higher surface energy, while many bulk particles don’t melt for more fusing energy needed. With the temperature rising, it is helpful for the melted particles to bond the bulk grains together and form a powder aggregation. Forming of the aggregation can freeze the powder particles, and improve their ability of resisting the impact of electric beam. At the same time, the powder surface is activated and the wetting ability is increased. High temperature makes powder surface wettable to the melting pool, avoiding spheroidization effects. All these ensure the stability of EBM process. What’s more, a persistent thermal field formed in the powder bed through preheating process, giving heat treating of the product in building procedure, will result in the excellent mechanical performance of the final product. Using the preheating process, column samples with approximate dimension of Φ8 mm×55 mm can be finally prepared after building up the fused structure layer by layer. The longitudinal and transverse sections of each bar are prepared for light microscopy using standard metallographic techniques. These characterizations show that the microstructure is extremely fine but inhomogeneous. As shown in Fig.3a, the columnar grains dominate in the entirely longitudinal specimen, whose size varies from the edge to the centre and from the bottom to the top of the sample. The width of columnar grains
in the sample varies from 0.02 to 0.2 mm, and the height from 0.5 to 15 mm, i.e., the height of the entire sample. There are no layer bands in the EBM processed samples, whose microstructures are nearly full dense. From Fig.3b, the microstructure of transverse specimen is mainly basket-weave, and the size of α and β laths vary at different locations. Through characterizing the mechanical properties of EBM samples, the ultimate tensile and yield strengths of untreated specimen are 1010 and 950 MPa, respectively, which clearly reach the limits of ASM handbook (volume 2) for wrought Ti6Al4V alloy (930-1015 MPa and 860-965 MPa). The breaking elongation is 15.0%, and it surpasses ASM limit (10%-14%). It indicates that the EBSM processed Ti6Al4V alloy hold excellent mechanical properties. The SEM fracture surfaces of the tensile tested specimen are provided in Fig.4. Many typical dimples appear in the SEM morphology of the tensile fracture surface, and the fracture is ductile rupture. a
b
20 μm Fig.3 Optical morphologies of the longitudinal (a) and transverse (b) specimens from the tested sample
Fig.4 SEM fracture surface of the tensile specimen
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He Weiwei et al. / Rare Metal Materials and Engineering, 2011, 40(12): 2072-2075
3 Conclusions Preheating the powder at a proper temperature, the particles will be slightly sintered, and a cubic aggregation is produced. In the cubic aggregation, the satellite small particles are partially or completely melt, and the little molten metal drops play the role of binder to bond the majority of big particles together and form the powder block, which not only helps the particles to hold their places to withstand the impact force of electron beam, but also improves the wetting ability between powder particles and molten metal pool, and prevents droplets formed in the surface prototyped. Several Ti6Al4V column samples are prepared by this process. The samples exhibit full interlayer bonding almost without pores, and their tensile performance is in the same range as respective values derived from the conventionally wrought annealed titanium alloy, indicating excellent mechanical properties.
1 Arcam S E. U S Patent[P]. No. 2006157454, 2006, 7: 20 2 Yang Xin, Tang Huiping, He Weiwei. Titanium Industry Progress [J], 2007, 24(3): 10 3 Denis Cormier, Ola Harrysson, Tushar Mahale. Journal of the Chinese Institute of Industrial Engineers[J], 2003, 20: 193 4 Ola L A Harrysson, Omer Cansizoglu, Denis J Marcellin. Materials Science and Engineering C[J], 2008, 28: 366 5 Peter Heinl, Andreas Rottmair, Carolin Körner. Advanced Engineering Materials[J], 2007, 9: 360 6 Cormier D, Harrysson O, West H. Rapid Prototyping J[J], 2004, 10: 35 7 Qi Haibo, Yan Yongnian, Lin Feng. J Tsinghua Univ (Sci ＆ Tech) [J], 2005, 45(8): 1012 8 Qi Haibo, Yan Yongnian, Lin Feng. J Engineering Manufacture, Proceedings Part B[J], 2006, 220(B11): 1845 9 Yan Yongnian, Qi Haibo, Lin Feng. Chinese Journal of Mechanical Engineering[J], 2007, 43(6): 87
References
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