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Improvement of the wear resistance of hot forging dies using a locally selective deposition technology with transition layers
CIRP Annals - Manufacturing Technology 65 (2016) 257–260
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Improvement of the wear resistance of hot forging dies using a locally selective deposition technology with transition layers Dong-Gyu Ahn a,*, Ho-Jin Lee a, Jong-Rae Cho b, Dae-Seon Guk a a
Department of Mechanical Engineering, Chosun University, Gwang-ju, Republic of Korea Department of Mechanical Engineering, Korea Maritime & Ocean University, Busan, Republic of Korea Submitted by Dong-Yol Yang(1), Daejeon, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Locally selective deposition Super-alloy Wear resistance Direct energy deposition process Hot forging die
A locally selective deposition technology with transition layers using a direct energy deposition process is investigated to improve the wear resistance of hot forging dies. A transition layer between the deposited region and the substrate is created to reduce mechanical and thermal problems as a buffer in interface regions. Design data of transition layers are obtained from experiments. Numerical analyses have been performed for deposited regions of dies. From the hot forging experiment, it has been shown that the proposed locally selective deposition technology with a buffer layer can help to dramatically improve the wear resistance of hot forging dies. ß 2016
1. Introduction Severe working conditions of the hot forging process lead to premature failure and short service life of hot forging dies [1– 4]. Several researchers have reported that the predominant failure mode of hot forging dies is the wear of die surfaces [1–4]. The wear is caused by a local bonding between the hot workpiece and the die, and a thermal softening phenomenon of the die [1,4]. Recently, interest in the development of a novel overlay coating technology has steadily increased to improve the wear resistance of hot forging dies via the deposition of a material with wear resistance at the elevated temperature on the dies [1–4]. Since the development of direct metal 3D printing processes, several overlay coating technologies for hot working dies have been investigated [4,5]. Most of the research works related to the overlay coating technology have focused on the investigation of the effects of the deposited material, the substrate material and the layer thickness on the wear characteristics of the coated region [2–5]. Several literatures have reported that cracks can be created in the vicinity of the overlay coated layer and joined regions between the coated layer and the substrate when a typical overlay coating technology with two layers is applied to the die [4,6]. Advanced overlay coating technologies with multi-layers have been introduced to overcome demerits of the typical overlay coating technology [7,8]. Smurov et al. have introduced the concept of inter-layers consisting of multi-layers with linearly graded materials [7]. Ocylok et al. have proposed a different type of advanced overlay coating technology with interlayers including a buffer layer and a linearly graded layer [8]. In this work, a locally selective deposition technology with a transition layer (TL) using a direct energy deposition (DED) process is investigated to improve the wear resistance of hot forging dies. * Corresponding author. Tel.: +82 622307043; fax: +82 622307234. E-mail address: [email protected] (D.-G. Ahn). http://dx.doi.org/10.1016/j.cirp.2016.04.013 0007-8506/ß 2016
The design data of TLs are obtained from experiments. The proposed technology is applied to the hot forging die of the axle shaft. Benefits of the proposed technology are discussed using the results of the hot forging experiments. 2. Concept of a locally selective deposition technology Fig. 1 illustrates the concept of a locally selective deposition technology with the TL. The DED process fabricates parts via lineby-line deposition of beads [9]. The beads are created from a laser cladding process using a coaxial nozzle. The proposed technology creates the TL between the deposited layer and the substrate to reduce the thermal fatigue induced by differences of thermal properties. The worn regions of hot forging dies are estimated via numerical analyses and experiments. After the removal of the worn regions, TLs are created on the substrate via the deposition of the mixed powder consisting of the substrate and the deposited material. The proposed technology can minimize inter-layers in the TL due to dilution layers in the vicinity of joined regions. The
Fig. 1. Concept of a locally selective deposition technology.
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deposited layer is created on the TL. The TL and the deposited layer are fabricated by the DED process. Finally, post-processing is performed to create dies with the desired quality. 3. Experiments and numerical analyses Wear experiments are performed at the elevated temperature to obtain a proper material combination and thickness of the TL. Fig. 2 shows the design concept of the TL and the specimen. In case of Type 1, Type 2 and Type 3 specimens, a single inter-layer with the mixed material is created between the substrate and the deposited layer as the TL. In the case of Type 4 specimen, three inter-layers with the graded material combination are formed between the substrate and the deposited layer as the TL.
Fig. 2. Design concept of the transition layer and the specimen.
SKD61 and Stellite21 are chosen as materials of the substrate and the deposited layer, respectively [2,3]. A pin-on-disk type wear experiment is performed, as shown in Fig. 3. The dimensions of specimens are 30 mm 30 mm 10 mm [3,4]. In order to create the TL, the mixed material with the desired combination of the Stellite21 and the SKD61 is deposited on the heat treated SKD61 block using a DED process. Subsequently, the deposited layer is created by deposition of Stellite21 on the TL using the DED process. Finally, the desired specimen is fabricated from machining of the deposited specimen. The furnace temperature, the normal load, the rotational speed, and the experimental time of the wear experiment are set to be 800 8C, 147 N, 200 RPM, and 1800 s, respectively [3,4]. The material of the pin is Sialon [3,4]. Line and area analyses are performed using an energy dispersive spectrometer (EDS). The hardness is measured by a Microvickers hardness tester. Friction induced work hardening (FIWH) is estimated to examine the influence of the design of the TL on the work hardening of the worn surface [3,4]. Morphologies of specimens are observed by a scanning electron microscope.
Fig. 4. Design of hot forging die and model of FEA.
4. Results and discussion 4.1. Deposition characteristics of specimen Fig. 5 shows the shape of specimens and the roughness of the deposited region. The maximum roughness of the top surface of the deposited specimen ranges from 95 mm to 170 mm, as shown in Fig. 5. Hardness of the top surface of the specimen ranges from 566 Hv to 627 Hv. Fig. 6 shows the results of line and area analyses. A sudden change of counts of cobalt (Co), chromium (Cr) and iron (Fe) components is observed in the vicinity of the joined region between the TL and the substrate for the case of the Type 1 specimen, while a sudden change of counts of those components is found in the vicinity of the joined region between the Stetllite21 deposited region and the TL for the case of the Type 3 specimen. Unlike Type 1 and Type 3 specimens, a quasi-graded TL with a smooth change of components is formed when the Type 2 specimen is adopted. The desired TL with linearly graded components is created for the case of the Type 4 specimen.
Fig. 5. Shape of specimens and surface roughness of the deposited region.
Fig. 3. Set-up of wear experiments at the elevated temperature.
Hot forging experiments for an axle shaft are carried out to investigate the applicability and the benefits of the proposed locally selective deposition technology. Fig. 4 illustrates the design of the hot forging die and the model of the hot forging analysis. The wear map of the hot forging die is estimated through threedimensional finite element analysis (FEA). The FEA is performed by DEFORM 3D software. Considering the symmetry of the dies, a quarter model is used to perform numerical analyses. Materials of the workpiece and the die are SCM440 and SKD61, respectively. Excessively worn regions of the die are predicted using a wear map. The die with the deposited layer and the TL is experiments are performed using a screw press with the payload of 600 tons. The ram speed of the press is 420 mm/s. The die temperature and the friction factor are set to be 300 8C and 0.5, respectively. Finally, the wear resistance of the designed die is compared to that of the conventional die.
Fig. 6. Results of line and area analyses: (a) line and (b) area.
4.2. Hardness and wear characteristics of specimen The hardness of the unworn region of the top surface increases after wear experiments. The increment of the hardness ranges from 73 Hv to 110 Hv. This results from age hardening of Stellite21 [3]. Fig. 7 shows the effects of the TL design on the hardness and the friction induced work hardening of the worn surface. The hardness of the worn surface greatly increases as compared to the initial hardness before the wear experiments. This is attributed to the occurrence of friction induced work hardening in the worn region
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Fig. 7. Hardness and FIWHs of the worn surface for different specimens.
due to the low stacking fault energy of Stellite21 [2–4]. The FIWH of the Types 1, 2 and 3 specimens is in the range of 24.8–25.8%, while the FIWH of the Type 4 specimen is nearly 18.7%. Fig. 8 shows the effects of the TL design on wear characteristics including weight losses, worn depths and worn widths of specimens. Weight losses of Types 1, 2 and 4 specimens are greater than that of the Type 2 specimen. In addition, worn depths of Types 1, 3 and 4 specimens are deeper than that of the Type 2 specimen by factors of nearly 1.6 times, 1.2 times and 1.4 times, respectively. The worn width of Type 3 and Type 4 specimens is almost identical to that of the Type 2 specimen, while the worn width of the Type 1 specimen is greater than that of the Type 2 specimen by the factor of 1.3 times. Fig. 9 shows morphologies of joined regions. Cracks appear in the vicinity of the joined region between the TL and the substrate for the Type 3 specimen. In addition, cracks are observed in the vicinity of the joined region between inter-layers for the Type 4 specimen.
Fig. 10. Results of FEA: (a) temperature, (b) pressure, (c) sliding velocity, (d) wear map, and (e) die design.
Archard’s wear model is used to estimate the wear map [10]. The wear coefficient of SKD61 is cited from reference 10. Fig. 10(d) shows the estimated wear map of the hot forging die for the axle shaft. Top and interface regions are predicted as excessively worn regions. The maximum wear appears in the vicinity of the top region. In terms of a local bonding mechanism of wear, the possibility of the wear augments when the pressure and the temperature increase. Hence, the deposited region of the die is determined as the top region of the die, as shown in Fig. 10(e). 4.4. Design and manufacture of the hot forging die
Fig. 8. Wear characteristics for different specimens.
Fig. 9. Formation of cracks in the vicinity of joined regions.
From the results of the experiments, the Type 2 specimen with a thickness of 1 mm and Stellite21 of 50 wt.% is determined to be a suitable TL design. The wear resistance of the specimen with the Type 2 design of the TL is compared to that of the SKD61 specimen, as shown in Table 1. The worn volume of the Type 2 specimen is smaller than that of the SKD61 specimen by nearly 76.9%. Softening of the worn region takes place for the case of the SKD61 specimen, unlike the Type 2 specimen. These results reveal that the proposed technology with the TL can improve the wear resistance of hot forging dies.
Fig. 11 shows the design and fabrication procedures of the hot forging die. Using the results of wear experiments and numerical analyses, a hot forging die with the deposited layer and the TL is designed, as shown in Fig. 11(a). In order to consider postprocessing of the deposited die, the offset geometry is created from the reference geometry of the die. The offset is applied to the application region of the DED process. The offset distance is nearly 1.0 mm. Considering the distance from the external surface of the deposited layer to that of the TL, the TL design with a trapezoidal shape is contrived. The thickness of the TL is 1 mm. Using the results of a previous research, the distance from the external surface of the deposited layer to the external surface of the TL ranges from 1.8 mm to 2.0 mm, exclusive of border regions [3]. The substrate of the die is created by machining of SKD61. The deposited layer and the TL are deposited by the DED process, as shown in Fig. 11(b). Two hoppers are used to selectively supply the Stellite21 powder and the mixed powder with Stellite21 of 50 wt.% and SKD61 of 50 wt.%. After creation of the deposited region, a vacuum heat treatment is performed to harden the deposited region and the substrate together. Finally, post-processing is performed to obtain the final shape and the desired surface roughness of the die, as shown in Fig. 11(b).
Table 1 Comparison of wear resistance of Type 2 specimen with that of SKD61. Specimen
Worn volume
FIWH
Type 2 SKD61 [3]
4.6 mm3 20.1 mm3
25.7% 4.5%
4.3. Wear map of the hot forging die Fig. 10 shows the results of FEA for a hot forging process of the axle shaft. Temperature, pressure and sliding velocity distributions are estimated to create the wear map of the die, as shown in Figs. 10(a)–(c). The maximum temperature and the maximum pressure appear in the vicinity of the top region of the die. The sliding velocity of the top region is smaller than the remaining region of the die. The maximum pressure and the maximum sliding velocity are nearly 791 MPa and 734 mm/s, respectively. The
Fig. 11. Design and fabrication of the die: (a) design and (b) fabrication.
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4.5. Wear resistance and benefit of the fabricated hot forging die Fig. 12 shows the results of hot forging experiments for the axle shaft. 10,000 axle shafts are successfully produced by the designed die. The shape of the top region of the designed die is nearly identical to the initial shape of the die when 10,000 parts are forged, while the shape of the top region of the conventional die is changed to a flat shape when 5000 parts are forged, as shown in Fig. 12(a). In order to quantitatively investigate the wear resistance of the designed die, the worn height and the worn volume of the designed die are compared to those of the conventional die, as shown in Fig. 12(b). The worn volume is estimated using the worn height and the die design. The worn height of the conventional die after the production of 5000 parts is nearly 0.80 mm, while that of the designed die after the production of 10,000 parts is nearly 0.13 mm. In addition, the worn volume of designed die after the production of 10,000 parts is nearly 37 mm3, while that of the conventional die after the production of 5000 parts is nearly 577 mm3. The worn height and the worn volume of the designed die after the production of 10,000 parts are 6 times and 15 times as small as those of the conventional die after the production of 5000 parts, respectively. From these results, it is shown that the designed die can dramatically improve the wear resistance of hot forging dies.
the wear of the die, the relationship between the forged time and the depth of groove is investigated via regression analyses, as shown in Fig. 13(b). The quadratic function is used as the regression function. Service limits of the conventional die and the designed die are estimated to be nearly 3180 shots and 22,850 shots, respectively. From these results, it is demonstrated that the designed die can dramatically improve the service life of the hot forging die and the quality of the product. 5. Conclusion A locally selective deposition technology with a TL using a DED process has been investigated to improve the wear resistance of a hot forging die. The TL between the deposited region and the substrate is created to reduce a thermal fatigue phenomenon. The influence of the TL design on deposition, hardness, and wear characteristics of the specimen has been investigated via experiments. From the results of the experiments, a single layer type of TL with a thickness of 1 mm and a 50% weight ratio of Stellite21 has been determined to be a suitable TL design. Through the comparison of wear characteristics of the deposited specimen with the TL and those of the SKD61 specimen, it has been shown that the proposed technology can improve the wear resistance of hot forging dies. Hot forging experiments for an axle shaft have been carried out to investigate the applicability and the benefits of the proposed technology. The wear map of the hot forging die has been created to estimate the deposited regions of the die via FEA. The design methodology and the fabrication procedure for the die have been discussed. The wear resistance of the designed die has been compared to that of the conventional die from viewpoints of wear characteristics of the die and quality of the product. From the results of the comparison, it has been shown that the proposed technology can dramatically improve the wear resistance of hot forging dies and the quality of the product. In the future, additional hot forging experiments are needed to investigate a practical service life of the designed die. Acknowledgements
Fig. 12. Wear characteristics for different dies: (a) exterior of the die and (b) worn height and worn volume of the die.
Fig. 13 shows axle shafts produced with designed and conventional dies. The deformed shape of the groove region of the 1000th forged part is almost identical to that of the 10,000th forged part when the designed die is used. However, the groove shape of the 5000th forged part is conspicuously changed due to the wear of the die when the conventional die is applied. Unlike the designed die, the depth of groove is rapidly reduced after the production of 2000 parts when the conventional die is used. In order to estimate the service limit of the die from the viewpoint of
Fig. 13. Comparison of the product quality of the designed die and that of the conventional die: (a) deformed shapes and (b) depth of groove.
This work was supported by Basic Science Research Program through National Research Foundation of Korea funded by the Ministry of Education of Korea (NRF-2015R1D1A3A03016692).
References [1] Lange K, Cesr L, Geiger M, Kals JAS (1992) Tool Life and Tool Quality in Bulk Metal Forming. Annals of CIRP 41(2):667–675. [2] Kashani H, Amadeh A, Vatanara MR (2008) Improvement of Wear Resistance of Hot Working Tool Steel by Hardfacing Part2 – Case Study. Material Science and Technology 24(3):356–360. [3] Park N-R, Ahn D-G (2014) A Study on the Effects of Hardfacing Thickness on Wear Characteristics of Stellite21 Hardfaced STD61 Hot-working Tool Steel at the Elevated Temperature. International Journal of Precision Engineering and Manufacturing 15(5):941–944. [4] Park N-R, Ahn D-G (2014) Wear Characteristics of Stellite6 and NOREM02 Hardfaced SKD61 Hot Working Tool Steel at the Elevated Temperature. International Journal of Precision Engineering and Manufacturing 15(12): 2549–2558. [5] Fessler J, Nickel A, Link G, Prize F, Fussell P (1997) Functional Gradient Metallic Prototypes through Shape Deposition Manufacturing. Proceedings of Solid Freeform Fabrication Symposium, 521–528. [6] Ko¨hler H, Partes K, Kornmeier JR, Vollersten F (2012) Residual Stresses in Steel Specimens Induced by Laser Cladding and Their Effect on Fatigue Strength. Physics Procedia 39:354–361. [7] Smurov I (2008) Laser Cladding and Laser Assisted Direct Manufacturing. Surface Coating Technology 202(18):4496–4502. [8] Ocylok S, Weisheit A, Kelbassa I (2010) Functionally Graded Multi-layers by Laser Cladding for Increased Wear and Corrosion Protection. Physics Procedia 5(A):359–367. [9] Yoon HS, Lee JY, Kim HS, Kim ES, Shin YJ, Chu WS, Ahn SH (2014) A Comparison of Energy Consumption in Bulk Forming, Subtractive, and Additive Processes: Review and Case Study, International. Journal of Precision Engineering and Manufacturing-Green Technology 1(3):261–279. [10] Lee RS, Jou JL (2003) Application of Numerical Simulation for Wear Analysis of Warm forging die. Journal of Material Processing Technology 140(1–3):43–48.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Improvement of the wear resistance of hot forging dies using a locally selective deposition technology with transition layers Dong-Gyu Ahn a,*, Ho-Jin Lee a, Jong-Rae Cho b, Dae-Seon Guk a a
Department of Mechanical Engineering, Chosun University, Gwang-ju, Republic of Korea Department of Mechanical Engineering, Korea Maritime & Ocean University, Busan, Republic of Korea Submitted by Dong-Yol Yang(1), Daejeon, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Locally selective deposition Super-alloy Wear resistance Direct energy deposition process Hot forging die
A locally selective deposition technology with transition layers using a direct energy deposition process is investigated to improve the wear resistance of hot forging dies. A transition layer between the deposited region and the substrate is created to reduce mechanical and thermal problems as a buffer in interface regions. Design data of transition layers are obtained from experiments. Numerical analyses have been performed for deposited regions of dies. From the hot forging experiment, it has been shown that the proposed locally selective deposition technology with a buffer layer can help to dramatically improve the wear resistance of hot forging dies. ß 2016
1. Introduction Severe working conditions of the hot forging process lead to premature failure and short service life of hot forging dies [1– 4]. Several researchers have reported that the predominant failure mode of hot forging dies is the wear of die surfaces [1–4]. The wear is caused by a local bonding between the hot workpiece and the die, and a thermal softening phenomenon of the die [1,4]. Recently, interest in the development of a novel overlay coating technology has steadily increased to improve the wear resistance of hot forging dies via the deposition of a material with wear resistance at the elevated temperature on the dies [1–4]. Since the development of direct metal 3D printing processes, several overlay coating technologies for hot working dies have been investigated [4,5]. Most of the research works related to the overlay coating technology have focused on the investigation of the effects of the deposited material, the substrate material and the layer thickness on the wear characteristics of the coated region [2–5]. Several literatures have reported that cracks can be created in the vicinity of the overlay coated layer and joined regions between the coated layer and the substrate when a typical overlay coating technology with two layers is applied to the die [4,6]. Advanced overlay coating technologies with multi-layers have been introduced to overcome demerits of the typical overlay coating technology [7,8]. Smurov et al. have introduced the concept of inter-layers consisting of multi-layers with linearly graded materials [7]. Ocylok et al. have proposed a different type of advanced overlay coating technology with interlayers including a buffer layer and a linearly graded layer [8]. In this work, a locally selective deposition technology with a transition layer (TL) using a direct energy deposition (DED) process is investigated to improve the wear resistance of hot forging dies. * Corresponding author. Tel.: +82 622307043; fax: +82 622307234. E-mail address: [email protected] (D.-G. Ahn). http://dx.doi.org/10.1016/j.cirp.2016.04.013 0007-8506/ß 2016
The design data of TLs are obtained from experiments. The proposed technology is applied to the hot forging die of the axle shaft. Benefits of the proposed technology are discussed using the results of the hot forging experiments. 2. Concept of a locally selective deposition technology Fig. 1 illustrates the concept of a locally selective deposition technology with the TL. The DED process fabricates parts via lineby-line deposition of beads [9]. The beads are created from a laser cladding process using a coaxial nozzle. The proposed technology creates the TL between the deposited layer and the substrate to reduce the thermal fatigue induced by differences of thermal properties. The worn regions of hot forging dies are estimated via numerical analyses and experiments. After the removal of the worn regions, TLs are created on the substrate via the deposition of the mixed powder consisting of the substrate and the deposited material. The proposed technology can minimize inter-layers in the TL due to dilution layers in the vicinity of joined regions. The
Fig. 1. Concept of a locally selective deposition technology.
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deposited layer is created on the TL. The TL and the deposited layer are fabricated by the DED process. Finally, post-processing is performed to create dies with the desired quality. 3. Experiments and numerical analyses Wear experiments are performed at the elevated temperature to obtain a proper material combination and thickness of the TL. Fig. 2 shows the design concept of the TL and the specimen. In case of Type 1, Type 2 and Type 3 specimens, a single inter-layer with the mixed material is created between the substrate and the deposited layer as the TL. In the case of Type 4 specimen, three inter-layers with the graded material combination are formed between the substrate and the deposited layer as the TL.
Fig. 2. Design concept of the transition layer and the specimen.
SKD61 and Stellite21 are chosen as materials of the substrate and the deposited layer, respectively [2,3]. A pin-on-disk type wear experiment is performed, as shown in Fig. 3. The dimensions of specimens are 30 mm 30 mm 10 mm [3,4]. In order to create the TL, the mixed material with the desired combination of the Stellite21 and the SKD61 is deposited on the heat treated SKD61 block using a DED process. Subsequently, the deposited layer is created by deposition of Stellite21 on the TL using the DED process. Finally, the desired specimen is fabricated from machining of the deposited specimen. The furnace temperature, the normal load, the rotational speed, and the experimental time of the wear experiment are set to be 800 8C, 147 N, 200 RPM, and 1800 s, respectively [3,4]. The material of the pin is Sialon [3,4]. Line and area analyses are performed using an energy dispersive spectrometer (EDS). The hardness is measured by a Microvickers hardness tester. Friction induced work hardening (FIWH) is estimated to examine the influence of the design of the TL on the work hardening of the worn surface [3,4]. Morphologies of specimens are observed by a scanning electron microscope.
Fig. 4. Design of hot forging die and model of FEA.
4. Results and discussion 4.1. Deposition characteristics of specimen Fig. 5 shows the shape of specimens and the roughness of the deposited region. The maximum roughness of the top surface of the deposited specimen ranges from 95 mm to 170 mm, as shown in Fig. 5. Hardness of the top surface of the specimen ranges from 566 Hv to 627 Hv. Fig. 6 shows the results of line and area analyses. A sudden change of counts of cobalt (Co), chromium (Cr) and iron (Fe) components is observed in the vicinity of the joined region between the TL and the substrate for the case of the Type 1 specimen, while a sudden change of counts of those components is found in the vicinity of the joined region between the Stetllite21 deposited region and the TL for the case of the Type 3 specimen. Unlike Type 1 and Type 3 specimens, a quasi-graded TL with a smooth change of components is formed when the Type 2 specimen is adopted. The desired TL with linearly graded components is created for the case of the Type 4 specimen.
Fig. 5. Shape of specimens and surface roughness of the deposited region.
Fig. 3. Set-up of wear experiments at the elevated temperature.
Hot forging experiments for an axle shaft are carried out to investigate the applicability and the benefits of the proposed locally selective deposition technology. Fig. 4 illustrates the design of the hot forging die and the model of the hot forging analysis. The wear map of the hot forging die is estimated through threedimensional finite element analysis (FEA). The FEA is performed by DEFORM 3D software. Considering the symmetry of the dies, a quarter model is used to perform numerical analyses. Materials of the workpiece and the die are SCM440 and SKD61, respectively. Excessively worn regions of the die are predicted using a wear map. The die with the deposited layer and the TL is experiments are performed using a screw press with the payload of 600 tons. The ram speed of the press is 420 mm/s. The die temperature and the friction factor are set to be 300 8C and 0.5, respectively. Finally, the wear resistance of the designed die is compared to that of the conventional die.
Fig. 6. Results of line and area analyses: (a) line and (b) area.
4.2. Hardness and wear characteristics of specimen The hardness of the unworn region of the top surface increases after wear experiments. The increment of the hardness ranges from 73 Hv to 110 Hv. This results from age hardening of Stellite21 [3]. Fig. 7 shows the effects of the TL design on the hardness and the friction induced work hardening of the worn surface. The hardness of the worn surface greatly increases as compared to the initial hardness before the wear experiments. This is attributed to the occurrence of friction induced work hardening in the worn region
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Fig. 7. Hardness and FIWHs of the worn surface for different specimens.
due to the low stacking fault energy of Stellite21 [2–4]. The FIWH of the Types 1, 2 and 3 specimens is in the range of 24.8–25.8%, while the FIWH of the Type 4 specimen is nearly 18.7%. Fig. 8 shows the effects of the TL design on wear characteristics including weight losses, worn depths and worn widths of specimens. Weight losses of Types 1, 2 and 4 specimens are greater than that of the Type 2 specimen. In addition, worn depths of Types 1, 3 and 4 specimens are deeper than that of the Type 2 specimen by factors of nearly 1.6 times, 1.2 times and 1.4 times, respectively. The worn width of Type 3 and Type 4 specimens is almost identical to that of the Type 2 specimen, while the worn width of the Type 1 specimen is greater than that of the Type 2 specimen by the factor of 1.3 times. Fig. 9 shows morphologies of joined regions. Cracks appear in the vicinity of the joined region between the TL and the substrate for the Type 3 specimen. In addition, cracks are observed in the vicinity of the joined region between inter-layers for the Type 4 specimen.
Fig. 10. Results of FEA: (a) temperature, (b) pressure, (c) sliding velocity, (d) wear map, and (e) die design.
Archard’s wear model is used to estimate the wear map [10]. The wear coefficient of SKD61 is cited from reference 10. Fig. 10(d) shows the estimated wear map of the hot forging die for the axle shaft. Top and interface regions are predicted as excessively worn regions. The maximum wear appears in the vicinity of the top region. In terms of a local bonding mechanism of wear, the possibility of the wear augments when the pressure and the temperature increase. Hence, the deposited region of the die is determined as the top region of the die, as shown in Fig. 10(e). 4.4. Design and manufacture of the hot forging die
Fig. 8. Wear characteristics for different specimens.
Fig. 9. Formation of cracks in the vicinity of joined regions.
From the results of the experiments, the Type 2 specimen with a thickness of 1 mm and Stellite21 of 50 wt.% is determined to be a suitable TL design. The wear resistance of the specimen with the Type 2 design of the TL is compared to that of the SKD61 specimen, as shown in Table 1. The worn volume of the Type 2 specimen is smaller than that of the SKD61 specimen by nearly 76.9%. Softening of the worn region takes place for the case of the SKD61 specimen, unlike the Type 2 specimen. These results reveal that the proposed technology with the TL can improve the wear resistance of hot forging dies.
Fig. 11 shows the design and fabrication procedures of the hot forging die. Using the results of wear experiments and numerical analyses, a hot forging die with the deposited layer and the TL is designed, as shown in Fig. 11(a). In order to consider postprocessing of the deposited die, the offset geometry is created from the reference geometry of the die. The offset is applied to the application region of the DED process. The offset distance is nearly 1.0 mm. Considering the distance from the external surface of the deposited layer to that of the TL, the TL design with a trapezoidal shape is contrived. The thickness of the TL is 1 mm. Using the results of a previous research, the distance from the external surface of the deposited layer to the external surface of the TL ranges from 1.8 mm to 2.0 mm, exclusive of border regions [3]. The substrate of the die is created by machining of SKD61. The deposited layer and the TL are deposited by the DED process, as shown in Fig. 11(b). Two hoppers are used to selectively supply the Stellite21 powder and the mixed powder with Stellite21 of 50 wt.% and SKD61 of 50 wt.%. After creation of the deposited region, a vacuum heat treatment is performed to harden the deposited region and the substrate together. Finally, post-processing is performed to obtain the final shape and the desired surface roughness of the die, as shown in Fig. 11(b).
Table 1 Comparison of wear resistance of Type 2 specimen with that of SKD61. Specimen
Worn volume
FIWH
Type 2 SKD61 [3]
4.6 mm3 20.1 mm3
25.7% 4.5%
4.3. Wear map of the hot forging die Fig. 10 shows the results of FEA for a hot forging process of the axle shaft. Temperature, pressure and sliding velocity distributions are estimated to create the wear map of the die, as shown in Figs. 10(a)–(c). The maximum temperature and the maximum pressure appear in the vicinity of the top region of the die. The sliding velocity of the top region is smaller than the remaining region of the die. The maximum pressure and the maximum sliding velocity are nearly 791 MPa and 734 mm/s, respectively. The
Fig. 11. Design and fabrication of the die: (a) design and (b) fabrication.
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4.5. Wear resistance and benefit of the fabricated hot forging die Fig. 12 shows the results of hot forging experiments for the axle shaft. 10,000 axle shafts are successfully produced by the designed die. The shape of the top region of the designed die is nearly identical to the initial shape of the die when 10,000 parts are forged, while the shape of the top region of the conventional die is changed to a flat shape when 5000 parts are forged, as shown in Fig. 12(a). In order to quantitatively investigate the wear resistance of the designed die, the worn height and the worn volume of the designed die are compared to those of the conventional die, as shown in Fig. 12(b). The worn volume is estimated using the worn height and the die design. The worn height of the conventional die after the production of 5000 parts is nearly 0.80 mm, while that of the designed die after the production of 10,000 parts is nearly 0.13 mm. In addition, the worn volume of designed die after the production of 10,000 parts is nearly 37 mm3, while that of the conventional die after the production of 5000 parts is nearly 577 mm3. The worn height and the worn volume of the designed die after the production of 10,000 parts are 6 times and 15 times as small as those of the conventional die after the production of 5000 parts, respectively. From these results, it is shown that the designed die can dramatically improve the wear resistance of hot forging dies.
the wear of the die, the relationship between the forged time and the depth of groove is investigated via regression analyses, as shown in Fig. 13(b). The quadratic function is used as the regression function. Service limits of the conventional die and the designed die are estimated to be nearly 3180 shots and 22,850 shots, respectively. From these results, it is demonstrated that the designed die can dramatically improve the service life of the hot forging die and the quality of the product. 5. Conclusion A locally selective deposition technology with a TL using a DED process has been investigated to improve the wear resistance of a hot forging die. The TL between the deposited region and the substrate is created to reduce a thermal fatigue phenomenon. The influence of the TL design on deposition, hardness, and wear characteristics of the specimen has been investigated via experiments. From the results of the experiments, a single layer type of TL with a thickness of 1 mm and a 50% weight ratio of Stellite21 has been determined to be a suitable TL design. Through the comparison of wear characteristics of the deposited specimen with the TL and those of the SKD61 specimen, it has been shown that the proposed technology can improve the wear resistance of hot forging dies. Hot forging experiments for an axle shaft have been carried out to investigate the applicability and the benefits of the proposed technology. The wear map of the hot forging die has been created to estimate the deposited regions of the die via FEA. The design methodology and the fabrication procedure for the die have been discussed. The wear resistance of the designed die has been compared to that of the conventional die from viewpoints of wear characteristics of the die and quality of the product. From the results of the comparison, it has been shown that the proposed technology can dramatically improve the wear resistance of hot forging dies and the quality of the product. In the future, additional hot forging experiments are needed to investigate a practical service life of the designed die. Acknowledgements
Fig. 12. Wear characteristics for different dies: (a) exterior of the die and (b) worn height and worn volume of the die.
Fig. 13 shows axle shafts produced with designed and conventional dies. The deformed shape of the groove region of the 1000th forged part is almost identical to that of the 10,000th forged part when the designed die is used. However, the groove shape of the 5000th forged part is conspicuously changed due to the wear of the die when the conventional die is applied. Unlike the designed die, the depth of groove is rapidly reduced after the production of 2000 parts when the conventional die is used. In order to estimate the service limit of the die from the viewpoint of
Fig. 13. Comparison of the product quality of the designed die and that of the conventional die: (a) deformed shapes and (b) depth of groove.
This work was supported by Basic Science Research Program through National Research Foundation of Korea funded by the Ministry of Education of Korea (NRF-2015R1D1A3A03016692).
References [1] Lange K, Cesr L, Geiger M, Kals JAS (1992) Tool Life and Tool Quality in Bulk Metal Forming. Annals of CIRP 41(2):667–675. [2] Kashani H, Amadeh A, Vatanara MR (2008) Improvement of Wear Resistance of Hot Working Tool Steel by Hardfacing Part2 – Case Study. Material Science and Technology 24(3):356–360. [3] Park N-R, Ahn D-G (2014) A Study on the Effects of Hardfacing Thickness on Wear Characteristics of Stellite21 Hardfaced STD61 Hot-working Tool Steel at the Elevated Temperature. International Journal of Precision Engineering and Manufacturing 15(5):941–944. [4] Park N-R, Ahn D-G (2014) Wear Characteristics of Stellite6 and NOREM02 Hardfaced SKD61 Hot Working Tool Steel at the Elevated Temperature. International Journal of Precision Engineering and Manufacturing 15(12): 2549–2558. [5] Fessler J, Nickel A, Link G, Prize F, Fussell P (1997) Functional Gradient Metallic Prototypes through Shape Deposition Manufacturing. Proceedings of Solid Freeform Fabrication Symposium, 521–528. [6] Ko¨hler H, Partes K, Kornmeier JR, Vollersten F (2012) Residual Stresses in Steel Specimens Induced by Laser Cladding and Their Effect on Fatigue Strength. Physics Procedia 39:354–361. [7] Smurov I (2008) Laser Cladding and Laser Assisted Direct Manufacturing. Surface Coating Technology 202(18):4496–4502. [8] Ocylok S, Weisheit A, Kelbassa I (2010) Functionally Graded Multi-layers by Laser Cladding for Increased Wear and Corrosion Protection. Physics Procedia 5(A):359–367. [9] Yoon HS, Lee JY, Kim HS, Kim ES, Shin YJ, Chu WS, Ahn SH (2014) A Comparison of Energy Consumption in Bulk Forming, Subtractive, and Additive Processes: Review and Case Study, International. Journal of Precision Engineering and Manufacturing-Green Technology 1(3):261–279. [10] Lee RS, Jou JL (2003) Application of Numerical Simulation for Wear Analysis of Warm forging die. Journal of Material Processing Technology 140(1–3):43–48.