Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV FORMING COLUMN CRYSTAL MICROSTRUCTURE SAMPLE BASED ON THE OPTIMIZED SINGLE CLADDING TRACK CHARACTERISTICS TẠO MẪU NHANH TỔ CHỨC TINH THỂ HÌNH TRỤ TRÊN CƠ SỞ TỐI ƯU HÓA ĐẶC TÍNH CỦA ĐƯỜNG QUÉT ĐƠN 1Đỗ Xuân Tươi, 1Đoàn Tất Khoa, 2Nguyễn Anh Tú Học viện Kỹ thuật quân Trung tâm 80 - Cục tác chiến điện tử - BTTM dxtuoi76@gmail.com ABSTRACT This paper investigated the influence of the cladding conditions on the characteristics of single cladding track, which is elementary unit in Direct Laser Forming (DLF) Single cladding tracks were clad with different technology parameter sets, in which laser power, scanning speed and powder feeding rate were all changed The characteristics of all tracks were measured Based on the analysis of the results, the forming conditions, which can make samples with column crystal microstructure, were obtained The results showed that from determining single cladding track characteristics, obtaining them by optimization of technology parameters, DZ125L super-alloy sample with column crystal microstructure can be formed by DLF It can be used in the approach to form parts which have the microstructure of layer by layer epitaxial growth of column crystal Keywords: Direct Laser Forming, single cladding track, column crystal microstructure, epitaxial growth, DZ125L super-alloy TÓM TẮT Bài báo nghiên cứu ảnh hưởng tham số công nghệ đến đặc tính đường quét đơn, phần tử trình tạo mẫu nhanh trực tiếp Laser (DLF) Các đường quét đơn tạo với tham số công nghệ khác cách thay đổi công suất Laser, tốc độ quét tốc độ cấp bột kim loại Đặc tính đường quét đơn đo đạc phân tích, từ xác định dải tham số công nghệ để tạo mẫu thí nghiệm có tổ chức tinh thể hình trụ Kết nghiên cứu rằng, tối ưu hóa tham số công nghệ tạo mẫu thí nghiệm từ hợp kim chịu nhiệt DZ125L với tổ chức tinh thể hình trụ phát triển liên tục sử dụng công nghệ tạo mẫu nhanh trực tiếp Laser Đây kết quan trọng giúp cho trình nghiên cứu tạo mẫu nhanh chi tiết yêu cầu có tổ chức tinh thể hình trụ phát triển liên tục Từ khóa: tạo mẫu trực tiếp Laser, đường quét đơn, tổ chức tinh thể hình trụ, phát triển liên tục, hợp kim chịu nhiệt DZ125L INTRODUCTION Direct Laser Forming (DLF) is a novel layer additive manufacturing technology There are some other similar technologies using the same principle as DLF are Laser Engineered Net Shaping (LENS), Laser Direct Metal Forming (LDMF), Direct Metal Deposition (DMD),…, which base on rapid prototyping and laser cladding technique [1-3, 7] In DLF, dense metal parts are fabricated directly from CAD files line by line and layer by layer without constraints on part shape and powder material and without using any tooling, and it has been a hot topic in the advanced manufacturing fields The DLF supports many types of metals including stainless steels (316 and 304); Ni based super-alloys (Inconel 625, 690, and 614 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV 718, FGH95, DZ408, DZ125L); cobalt-chrome; and Ti-6Al-4V titanium alloy Parts formed by DLF, as by any other technologies, are all required to get suitable morphology accuracy, microstructure evolution and material performance,.etc For some components like turbine blade, which usually formed by Ni based super-alloys and used in aircraft engine and turbine machines, microstructure evolution of material is the key factor, it needs columnar crystal microstructure At the beginning, turbine blade was manufactured by forging, it was then successfully casted in U.S.A in 1950, and the forging process was gradually replaced by cast process In recent years, investigation into manufacturing turbine blade by laser application processes has been carried out all over the world, including of feeding powder and spreading powder methods It showed that turbine blade can be fabricated by Selective Laser Melting SLM, by DLF [4-5] or by DMD [6] However, the mechanical performance of the parts fabricated by the above-mentioned methods has not been reported sufficiently There were some investigations in repairing and coating on directionally solidified Ni based super-alloys by DLF [8-10], there were also reports in fabrication of directional solidification Ni based super-alloy samples [11], whereas all of them showed samples with six to eight layers high At present, forming column crystal metal part by DLF still remains a great challenge DZ125L is a high performance Ni based super-alloy, designed in China for turbine application in advanced gas turbine engines Using DZ125L for the experiments, this paper systematically investigated the influence of the main forming condition parameters as laser power, scanning speed, powder feed rate on the characteristics of single cladding track, the elementary unit in DLF It also developed an analysis method to find out optimal forming parameters The result showed that by determining characteristics of single cladding track, obtaining them by optimized technological parameters, DZ125L super-alloy thin wall part with more than twenty layers column crystal microstructure could be formed by DLF EXPERIMENTAL PROCEDURE 2.1 Materials and equipment The experiments were carried out by the system as shown in Figure The DLF process was performed by independently developed XJTU-I machines which includes a Nd:YAG laser with a kW maximum output power (wavelength 1063 nm, spot diameter of 0.48 mm, the laser beam was guided to the workstation through an optical fiber and focused by an optic) and a three-axis CNC linkage worktable and a powder feeder with a coaxial feeding nozzle and a gas protection device The processing chamber of the system was protected from oxidation by argon gas Figure Schematic diagram of experiment setup 615 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV The powder used was Ni based super-alloy DZ125L with spherical shape and smooth surface Additionally, the DZ125L particle size distributes of about 30-60 µm and the mean particle size of about 45 µm The substrate was the same material, machined from single crystalline cast ingots, with the orientation normal to the surface, and its dimension was Φ50x3.5 The compositions of the powder and the substrate are shown in Table Material Table The compositions of the powder and the substrate (wt %) C Cr Co Mo W Al Ti Ta B Ni Substrate 0.07 9.09 10.00 2.09 7.17 4.48 3.05 3.64 0.011 Balance Powder 0.09 9.70 9.64 2.18 7.14 4.90 3.12 3.78 0.015 Balance 2.2 Processing Before doing the experiments, the powder was dried in a vacuum oven at 2000C for hours to remove the moisture and improve the flow ability The substrate was polished and then cleaned by acetone before being installed on the forming worktable and leveled Single cladding tracks (20 mm in length) were prepared (Figure 2a) using different laser powers P, scanning speeds V and powder feeding rates Mp (as shown in Table 2) Three separate tracks were prepared with each parameter set of (M p , P, V) and the values of each parameter in the set of (M P , P, V) were selected according to the preliminary basic research results Thin wall samples were formed based on the analysis of single cladding tracks characteristics (Figure 2b, 2c) No pre-heat or post-deposition heat treatment of the deposited samples was applied The samples were cut for characterization with wire EDM All samples were mounted and prepared in accordance with standard metallographic procedures The topography characteristics and the microstructure characteristics of each sample were analyzed using VH3000 optical microscope (made in Japan by the KEYENCE) The cross section characteristic parameters of each single track was measured, the results were calculated to find out the mean values Finally, showing the influence of the process parameters on the single cladding track characteristic diagrams were obtained (Figure and Figure 5) Technology parameters, from which can clad single tracks with suitable morphology and microstructure characteristics, were obtained from analyzing the diagrams, after that thin wall sample with column crystal microstructure was formed Table Technology parameters for single cladding tracks in DLF M p (g.min-1) P (W) 4.9* 190, 210, 230, 250, 270 V (mm.s-1) 6, 8, 10, 12 Shielding gas (l.min-1) 4** Protecting gas (l.min-1) 6** Track number of each parameter set * The preliminary basic research indicated that 4.9 g.min- remained stable cladding tracks, less than or more than this value released poor samples ** Shielding gas and protecting gas outputs influence very little to the single cladding track morphology and they can be ignored 616 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV Figure Sketch of single cladding tracks (a), thin wall samples (b and c) RESULTS AND DISCUSSION 3.1 Influence of the process parameters on the single cladding track characteristics The cladding track morphology is described with the cladding track width W (µm), the cladding track height H (µm), the cladding track depth h (µm) and the height of column crystal microstructure zone h ds (µm) All of them are defined in the cladding profile as shown in Figure The single track morphology parameters were measured after the experiments From each set of (M P , P, V), which used for three separate tracks, we got three value sets of (W, H, h and h ds ) These values then calculated to find out each mean value of W, H, h and h ds , and finally expressed by the diagrams in Figure Figure Single cladding track cross section parameters After polishing and etching the samples, directional dendrite structure in the substrate was released The refined dendrite structure in the deposit zone showed an epitaxial growth parallel with the substrate dendrite direction The epitaxial growth also verified the nature of bonding at the interface to be metallurgical Compared with the dendrite size of the substrate metal, the primary dendrite size in the deposit zone is approximately 50-70 times smaller From Figure we can see that, with different technology parameters used, single cladding track cross sections with different characteristic values were released The result showed that the increase in laser power resulted in the increase in the cladding track width W and the cladding track depth h (Figure 4a and 4c) The increase in laser power resulted in the decrease in the height of column crystal microstructure zone h ds (Figure 4d) For the cladding track height H, its value was relatively stable when the scanning speed of mm.s-1 or 10 mm.s-1, too small when the scanning speed of 12 mm.s-1 and big when the scanning speed of mm.s-1 (Figure 4c) The height of column crystal microstructure zone h ds is the most important parameter, it must be relatively bigger than zero to make the subsequent layer with an epitaxial growth parallel to the previous ones Figure 4d also indicated that laser power should not be too high, and the suitable value is not higher than 250W 617 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV Figure Influence of the main process parameters on the single cladding track characteristics (a) cladding track width W (µm), (b) cladding track height H (µm), (c) cladding track depth h (µm), (d) the height of column crystal microstructure zone h ds (µm) Figure The difference values of (H-h ds -h) according to different technology parameter sets Moreover, the dendrite layer (multidirectional zone) and an inter-dendrite shrinkage should be thoroughly re-melted to ensure the densification and epitaxial growth between the depositing layer and the previous one From Figure 3, the difference value of (H-h ds ) must be smaller than h, it means that the difference value of (H-h ds -h) must be negative The calculation result, which is shown in Figure 5, indicated that the suitable condition is obtained when the scanning speed is 10 mm.s-1 or 12 mm.s-1 with laser power less than 270W Low scanning speed of mm.s-1 or mm.s-1 (with laser power less than 230W) is not suitable, mm.s-1 scanning speed is suitable only when laser power relatively high (in range of 230W - 255W) 3.2 DZ125L super-alloy thin wall part forming From above analysis, DZ125L super-alloy thin wall parts were formed (Figure and Figure 6) The forming conditions with different technology parameter sets were used as shown in Table 618 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV Samples Table Technology parameters for thin wall samples Laser Scanning Powder Caring Protect Standoff power velocity Feed rate gas rate Gas rate distance (W) (mm) (mm.s-1) (g.min-1) (l.min-1) (l.min-1) 200 250 270 240, 230, 220, 210 10 10 10 10 4.9 4.9 4.9 4.9 4 4 6 6 0.1 0.1 0.1 0.1 Number of layers 20 20 20 10,10, 20,40 Figure The first layers of thin wall samples (a) sample 1, (b) sample 3, (c) sample 2, (d) sample The results showed a strong metallurgical bonding has been achieved between the thin wall and the substrate With sample (Figure 2b and Figure 6a), although the technology parameters met the requirements from h ds and (H-h-h ds ), but with the small values of W, H and h, the forming quality was very poor Therefore its microstructure was also very poor and without column crystal structure (Figure 6a) Lowering of laser power also resulted in poor samples With 270W and higher laser power, thin wall samples could be formed smoothly, there was directional tendency of the microstructure in several first layers, but it did not show epitaxial column crystal structure (Figure 6b) The samples formed with the laser power of 250W and lower showed good appearances, without any crack, and the microstructure achieved directional column crystal, which shown in Figure and Figure 6c, 6d With proper deposition parameters, it was able to produce continued directional growth in the deposition zone over twenty layers of sample (Figure 7) comparing to eight layers of sample As subsequent layers were deposited, the solidification substructure changed from directional columnar structure to dendrite structure and resulted in the decrease in the directional columnar zone width The change in solidification substructure can be explained by using the welding solidification theory [12] The ratio of the temperature gradient G to the growth rate R, in G/R, 619 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV governs the mode of solidification In this case, the layer deposition was accompanied by the deposition of laser heat flow Therefore, higher layer was deposited with lower ratio of G/R because the temperature of the layers increased one by one Moreover, with the increase in the deposited layer number, heat flow, dispersed into the substrate by conduction gradually decreased due to the increase in both the temperature and the distance to the substrate Meanwhile, heat flow also dispersed into the surrounding media simultaneously by convection and radiation increasingly because of the increase in temperature CONCLUSIONS Determining single cladding track characteristics, which obtained by optimizing process parameters through experiments, could help to form DZ125L super-alloy thin wall part with directional column microstructure successfully Because of the decrease in the ratio of G/R and the change of heat flow dispersing, the width of the column structure zone was decreased with the increase in deposited layer number By proper regulating laser power in accordance with the increase in deposited layer number, it has likely improved the column crystal microstructure of the part, and the column crystal structure grew with more than twenty layers in height The investigation result indicated that, laser power should not be too high and the suitable scanning speed should be 10mm.s-1 We can use the result of this research in the approach to form parts which have the microstructure of layer by layer epitaxial growth of column crystal ACKNOWLEDGEMENTS The authors gratefully acknowledge for the State Basic Research Key Projects of China through Grant no 2007CB707704; National Natural Science Foundation of China through Grant no 51005177 and no 51275392 REFERENCES [1] Wang HM, Duan G Wear and corrosion behavior of laser clad Cr3Si reinforced intermetallic composite coatings Intermetallics 2003; 11: 755-62 [2] Tan H, Chen J, Zhang FY, Lin X, Huang WD Process analysis for laser solid forming of thin-wall structure Int J Mach Tools Manu 2010; 50: 1-8 [3] Wohlers Associates, Inc Additive Manufacturing and 3D Printing State of the Industry, Annual Worldwide Progress Report ISBN 0-9754429-7-X, Wohlers report 2011 [4] http://www.fraunhofer.de/en/press/research-news/2010-2011/04/selective-laser-melting.jsp [5] http://www.mcp-group.de [6] J Choi, B Dutta, J Mazumder Spatial Control of Crystal Texture by Laser DMD Process Supplemental Proceedings: Volume 1: Fabrication, Materials, Processing and Properties TMS (The Minerals, Metals & Materials Society), 2009, 405-413 [7] Venkatakrishanan K, Sivakumar NR, Hee CW, et al Direct fabrication of surface-relief grating by interferometric technique using femtosecond laser Appl Phys A: Mater Sci Process 2003; 77(7): 959-63 620 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV [8] Leijun Li Repair of directionally solidified superalloy GTD-111 by Laser Engineered Net Shaping J Mater Sci (2006) 41: 7886-7893 [9] R Vilar, E.C Santos, P.N Ferreira, N Franco, R.C da Silva Structure of NiCrAlY coatings deposited on single-crystal alloy turbine blade material by laser cladding Acta Materialia 57 (2009) 5292–5302 [10] M Gäumann, C Bezençon, P Canalis, W Kurz Single-crystal laser deposition of superalloys: processing–microstructure maps Acta mater 49 (2001) 1051–1062 [11] Feng Liping, Huang Weidong, Lin Xin, Yang Haiou, Li Yanmin, yang Jian Laser multilayers cladding experiment on the DD3 single crystal using FGH-95 powder: Investigation on the microstructure of single crystal cladding layer Chinese journal of Aeronautics May 2002 Vol 15, No [12] Messler RW Jr (1999) Principles of welding John Wiley & Sons, p428 THÔNG TIN TÁC GIẢ Đỗ Xuân Tươi (Giảng viên - tiến sỹ; Học viện Kỹ thuật quân - Số 236 Hoàng Quốc Việt - Cầu Giấy - Hà Nội; Email: dxtuoi76@gmail.com) Đoàn Tất Khoa (Giảng viên - thạc sỹ; Học viện Kỹ thuật Quân - Số 236 Hoàng Quốc Việt - Cầu Giấy - Hà Nội; Email: doankhoactm@gmail.com) Nguyễn Anh Tú (Thạc sỹ; Trung tâm 80 - Cục tác chiến điện tử - BTTM - Số 15 Hoàng Sâm - Cầu Giấy - Hà Nội; Email: tunguyen021975@gmail.com) 621