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Reliability evaluation of a laser repaired die-casting die
Materials Science and Engineering A 483–484 (2008) 343–345
Reliability evaluation of a laser repaired die-casting die Richun Song, Satoshi Hanaki ∗ , Masato Yamashita, Hitoshi Uchida Graduate School of Engineering, University of Hyogo, Shosha, Himeji, Hyogo 671-2201, Japan Received 6 June 2006; received in revised form 4 October 2006; accepted 19 October 2006
Abstract The fatigue life of the die-casting dies depends on the heat checks, which are characterized by fine shallow cracks on the surface due to cyclic thermal stress. According to the recent study, laser-processing is effective to repair cracks and extend the life of dies. The purpose of this study is to develop a reliability evaluation method for laser repaired die to validate the effect of repair technique. The proposed method was applied for simple die model. First, the effect of laser-melting process on hot work tool steel (SKD6) has been evaluated by rotating bending fatigue test. As a result, the fatigue strength of the laser-processed specimen decreases remarkably in compared with that of base metal. However, it can be recovered by heat treatment at 773 K. This could be due to a high yield strength resulted in a secondary-hardening effect. At the same time, cyclic thermal stress applied to the die was evaluated by finite element methods and failure probabilities for required number of cycles were calculated. As a result, the failure probability for laser-processed die increased remarkably in compared with that of non-processed ones. However, it can recover 773 K tempered die. © 2007 Elsevier B.V. All rights reserved. Keywords: Hot work tool steel; Laser-processing; S–N curve; Fatigue life; Reliability
1. Introduction Die-casting has advantages in high size accuracy in compared with other casting and widely applied to automobile parts manufacturing. Since metallic molds of die-casting are used at high temperature repeatedly, crack damage on the surface of molds, so-called heat check has been a problem in service [1]. The repair by arc welding has been commonly performed for tools, machine elements and so forth. However, this method has a disadvantage that it may change the metallographic structure as a result of heating over a wide range. Recently, a method of locally melting selected area using laser beam has been attracted attention. Ernst et al. [2] have applied laser-melting process to repair a die surface with notch. Brown et al. [3] has reported that the laser-melting repair of fatigue cracks in ship steel could reduce the cost and the repair time. The purpose of this study is to develop a reliability evaluation method for laser repaired die to validate the effect of the repair technique. Because the cyclic thermal stress applied to the die will change due to the shape of die or the thermal condition in casting process, experimental ∗
Corresponding author. Tel.: +81 792 67 4837; fax: +81 792 67 4837. E-mail address: [email protected] (S. Hanaki).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.207
approach requires a lot of cost and time. In proposed method, cyclic thermal stress applied to the die is estimated by FEM and fatigue strength is evaluated by experiment. From these results, the failure probability of the laser repaired die is calculated. 2. Evaluation of fatigue strength 2.1. Specimens and testing apparatus The material investigated in this study is a commercial hot work tool steel (JIS-SKD6), which is equivalent to H11 (AISI), used in aluminum alloy die-casting. This material was austenitized at 1303 K for 3.6 ks and then cooled in air. Afterwards, the material was machined to the rotary bending test piece which had a ringed U groove as shown in Fig. 1(a). Using this specimen, rotating bending fatigue tests were carried out under the constant stress amplitude. The rotation speed was 175 rpm. The grooves of some specimens were processed using Nd:YAG laser at 0.5 kW power. Macroscopic views of test specimen are shown in Fig. 1(b). The processing was carried out by one pass spiral rotating, rotating and transversing simultaneously, to avoid overlapping of the start–stop position. The pheripheric and transfer speed of rotation were 10 and 5 mm/s, respectively. In previous work [4], the authors have demon-
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R. Song et al. / Materials Science and Engineering A 483–484 (2008) 343–345
Fig. 1. Scheme of fatigue test specimen: (a) macroscopic view; (b) magnified view of U-notch part after laser-processing. Fig. 3. S–N plots of SKD6.
strated that cracks can be disappeared by laser-melting process using pre-cracked specimen. It is consider that the effect of precrack can be removed by laser-melting process and the fatigue tests were carried out for non-cracked specimens. 2.2. Secondary hardening effect Because the laser-processed structure is expected to become brittle due to rapid cooling, recovery by annealing near secondary hardening temperature was tried. The relation between the hardness and tensile strength to tempering temperature was shown in Fig. 2. It can be seen that the hardness rises gradually with the increase of heat-treating temperature and shows a slightly secondary hardening peak at 773 K. On the basis of this experimental result, three types of specimens are prepared: (1) base metal (non-processed), (2) as-laser condition and (3) heated at 773 K after laser-processing (henceforth, this material is described to be 773 K tempered material).
Fig. 2. The effect of tempered temperature on Vickers hardness and tensile strength.
2.3. S–N curve Fig. 3 shows the obtained S–N curve. Although the fatigue strength at 107 cycles was 477 MPa in base metal, it fell even to 232 MPa in laser-processed specimen. The notch sensitivity might have risen because the structure becomes brittle due to the rapid cooling. The fatigue strength of 773 K tempered material is recovered to 395 MPa. This could be due to a high yield strength resulted in a secondary hardening effect. 3. Evaluation of thermal stress Fig. 4 shows a simple die model applied in this study. Material properties of the sample die model applied in this study are shown in Table 1. The initial temperature of die is 833 K. After the injection of melted aluminum, the die is cooled by air for 20 s. During the next 10 s, the mold is removed and the die is cooled rapidly by spray of mold lubricant. Therefore, in the analysis, the surface of die is heated at 963 K for 20 s after injection. Heat transfer coefficient around the die is 200 W/m2 K. After 20 s, this value is set to 2000 W/m2 K on the surface of die
Fig. 4. FEM model for thermal analysis.
R. Song et al. / Materials Science and Engineering A 483–484 (2008) 343–345 Table 1 Material properties of SKD6 Thermal conductivity (W/m K) Density (kg/m3 )
Specific heat (J/kg) Elastic modulus
Table 2 Failure probabilities after 105 shots 29.4 (293 K) 25.2 (873 K) 7644
Linear expansion coefficient (×10−6 K−1 )
345
10.3 (293 K) 13.2 (873 K)
A B C D E
Base metal
Laser-processed
773 K tempered
4.10 × 10−2
9.99 × 10−1
2.38 × 10−1 1.51 × 10−2 1.35 × 10−1 3.71 × 10−2 2.72 × 10−2
4.27 × 10−3 3.13 × 10−2 1.74 × 10−2 1.52 × 10−2
9.98 × 10−1 9.99 × 10−1 9.99 × 10−1 9.99 × 10−1
483
(×104
MPa)
20.6
to consider the effect of mold lubricant spray. Using the results of distribution of temperature, thermal stress was estimated. Fig. 5 shows the hysteresis principal stress at nodes of ‘A’ to ‘E’ in Fig. 4. At all nodes, thermal stress is applied just after the injection of melting aluminum and it decreased in process of time. However, tensile stress increases again by the rapid cooling at 20 s.
bility reaches maximum at point of ‘A’. The failure probability for laser-processed die increased remarkably in compared with that of non-processed ones. And, it can be decreased by heattreatment at secondary hardening temperature 773 K. Nevertheless, there remains a difference of failure probability among base metal and 773 K tempered material. However, lasermelting process can be carried out at relatively low cost and effective for the repair of heat check, which is main cause of the failure of dies. Therefore, we can conclude that the laserprocessing method is valuable for the extension of die life.
4. Estimation of failure probability
5. Conclusions
The dies used for mass production will be repaired by grinding after every 104 or 105 shots to keep the dimensional accuracy. Table 2 shows the calculated failure probabilities for point of ‘A’ to ‘E’ in Fig. 4 after 104 stress cycles. The failure proba-
Failure probabilities for required number of cycles were calculated from the results of thermal stress analysis and fatigue test. The proposed method has been applied to the simple die model and following conclusions were obtained.
Poisson’s ratio
0.28
(i) The fatigue strength of the laser-processed specimens decrease remarkably in compared with that of the base metal. However, it can be recovered to 83% of the base metal by heat treatment at secondary hardening temperature. (ii) Failure probability for laser-processed die increased remarkably in compared with that of non-processed ones. However, it can be decreased by heat-treatment at secondary hardening temperature 773 K. References
Fig. 5. Hysteresis of principle stress.
[1] T. Kanno, Introduction of Die-casting Technology, Nikkan Kougyou Shinbunnsha, Tokyo, 1997. [2] G. Ernst, A. Luftenegger, R. Ebner, Proceedings of the 5th International Conference on Tooling, Austria, 1999. [3] P.M. Brown, G. Shannon, W. Deans, J. Bird, Weld World 43 (1999) 33–39. [4] Y. Sun, S. Hanaki, H. Uchida, H. Sunada, H. Tsujii, Appl. Plasma Sci. 10 (2002) 131–138.
Reliability evaluation of a laser repaired die-casting die Richun Song, Satoshi Hanaki ∗ , Masato Yamashita, Hitoshi Uchida Graduate School of Engineering, University of Hyogo, Shosha, Himeji, Hyogo 671-2201, Japan Received 6 June 2006; received in revised form 4 October 2006; accepted 19 October 2006
Abstract The fatigue life of the die-casting dies depends on the heat checks, which are characterized by fine shallow cracks on the surface due to cyclic thermal stress. According to the recent study, laser-processing is effective to repair cracks and extend the life of dies. The purpose of this study is to develop a reliability evaluation method for laser repaired die to validate the effect of repair technique. The proposed method was applied for simple die model. First, the effect of laser-melting process on hot work tool steel (SKD6) has been evaluated by rotating bending fatigue test. As a result, the fatigue strength of the laser-processed specimen decreases remarkably in compared with that of base metal. However, it can be recovered by heat treatment at 773 K. This could be due to a high yield strength resulted in a secondary-hardening effect. At the same time, cyclic thermal stress applied to the die was evaluated by finite element methods and failure probabilities for required number of cycles were calculated. As a result, the failure probability for laser-processed die increased remarkably in compared with that of non-processed ones. However, it can recover 773 K tempered die. © 2007 Elsevier B.V. All rights reserved. Keywords: Hot work tool steel; Laser-processing; S–N curve; Fatigue life; Reliability
1. Introduction Die-casting has advantages in high size accuracy in compared with other casting and widely applied to automobile parts manufacturing. Since metallic molds of die-casting are used at high temperature repeatedly, crack damage on the surface of molds, so-called heat check has been a problem in service [1]. The repair by arc welding has been commonly performed for tools, machine elements and so forth. However, this method has a disadvantage that it may change the metallographic structure as a result of heating over a wide range. Recently, a method of locally melting selected area using laser beam has been attracted attention. Ernst et al. [2] have applied laser-melting process to repair a die surface with notch. Brown et al. [3] has reported that the laser-melting repair of fatigue cracks in ship steel could reduce the cost and the repair time. The purpose of this study is to develop a reliability evaluation method for laser repaired die to validate the effect of the repair technique. Because the cyclic thermal stress applied to the die will change due to the shape of die or the thermal condition in casting process, experimental ∗
Corresponding author. Tel.: +81 792 67 4837; fax: +81 792 67 4837. E-mail address: [email protected] (S. Hanaki).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.207
approach requires a lot of cost and time. In proposed method, cyclic thermal stress applied to the die is estimated by FEM and fatigue strength is evaluated by experiment. From these results, the failure probability of the laser repaired die is calculated. 2. Evaluation of fatigue strength 2.1. Specimens and testing apparatus The material investigated in this study is a commercial hot work tool steel (JIS-SKD6), which is equivalent to H11 (AISI), used in aluminum alloy die-casting. This material was austenitized at 1303 K for 3.6 ks and then cooled in air. Afterwards, the material was machined to the rotary bending test piece which had a ringed U groove as shown in Fig. 1(a). Using this specimen, rotating bending fatigue tests were carried out under the constant stress amplitude. The rotation speed was 175 rpm. The grooves of some specimens were processed using Nd:YAG laser at 0.5 kW power. Macroscopic views of test specimen are shown in Fig. 1(b). The processing was carried out by one pass spiral rotating, rotating and transversing simultaneously, to avoid overlapping of the start–stop position. The pheripheric and transfer speed of rotation were 10 and 5 mm/s, respectively. In previous work [4], the authors have demon-
344
R. Song et al. / Materials Science and Engineering A 483–484 (2008) 343–345
Fig. 1. Scheme of fatigue test specimen: (a) macroscopic view; (b) magnified view of U-notch part after laser-processing. Fig. 3. S–N plots of SKD6.
strated that cracks can be disappeared by laser-melting process using pre-cracked specimen. It is consider that the effect of precrack can be removed by laser-melting process and the fatigue tests were carried out for non-cracked specimens. 2.2. Secondary hardening effect Because the laser-processed structure is expected to become brittle due to rapid cooling, recovery by annealing near secondary hardening temperature was tried. The relation between the hardness and tensile strength to tempering temperature was shown in Fig. 2. It can be seen that the hardness rises gradually with the increase of heat-treating temperature and shows a slightly secondary hardening peak at 773 K. On the basis of this experimental result, three types of specimens are prepared: (1) base metal (non-processed), (2) as-laser condition and (3) heated at 773 K after laser-processing (henceforth, this material is described to be 773 K tempered material).
Fig. 2. The effect of tempered temperature on Vickers hardness and tensile strength.
2.3. S–N curve Fig. 3 shows the obtained S–N curve. Although the fatigue strength at 107 cycles was 477 MPa in base metal, it fell even to 232 MPa in laser-processed specimen. The notch sensitivity might have risen because the structure becomes brittle due to the rapid cooling. The fatigue strength of 773 K tempered material is recovered to 395 MPa. This could be due to a high yield strength resulted in a secondary hardening effect. 3. Evaluation of thermal stress Fig. 4 shows a simple die model applied in this study. Material properties of the sample die model applied in this study are shown in Table 1. The initial temperature of die is 833 K. After the injection of melted aluminum, the die is cooled by air for 20 s. During the next 10 s, the mold is removed and the die is cooled rapidly by spray of mold lubricant. Therefore, in the analysis, the surface of die is heated at 963 K for 20 s after injection. Heat transfer coefficient around the die is 200 W/m2 K. After 20 s, this value is set to 2000 W/m2 K on the surface of die
Fig. 4. FEM model for thermal analysis.
R. Song et al. / Materials Science and Engineering A 483–484 (2008) 343–345 Table 1 Material properties of SKD6 Thermal conductivity (W/m K) Density (kg/m3 )
Specific heat (J/kg) Elastic modulus
Table 2 Failure probabilities after 105 shots 29.4 (293 K) 25.2 (873 K) 7644
Linear expansion coefficient (×10−6 K−1 )
345
10.3 (293 K) 13.2 (873 K)
A B C D E
Base metal
Laser-processed
773 K tempered
4.10 × 10−2
9.99 × 10−1
2.38 × 10−1 1.51 × 10−2 1.35 × 10−1 3.71 × 10−2 2.72 × 10−2
4.27 × 10−3 3.13 × 10−2 1.74 × 10−2 1.52 × 10−2
9.98 × 10−1 9.99 × 10−1 9.99 × 10−1 9.99 × 10−1
483
(×104
MPa)
20.6
to consider the effect of mold lubricant spray. Using the results of distribution of temperature, thermal stress was estimated. Fig. 5 shows the hysteresis principal stress at nodes of ‘A’ to ‘E’ in Fig. 4. At all nodes, thermal stress is applied just after the injection of melting aluminum and it decreased in process of time. However, tensile stress increases again by the rapid cooling at 20 s.
bility reaches maximum at point of ‘A’. The failure probability for laser-processed die increased remarkably in compared with that of non-processed ones. And, it can be decreased by heattreatment at secondary hardening temperature 773 K. Nevertheless, there remains a difference of failure probability among base metal and 773 K tempered material. However, lasermelting process can be carried out at relatively low cost and effective for the repair of heat check, which is main cause of the failure of dies. Therefore, we can conclude that the laserprocessing method is valuable for the extension of die life.
4. Estimation of failure probability
5. Conclusions
The dies used for mass production will be repaired by grinding after every 104 or 105 shots to keep the dimensional accuracy. Table 2 shows the calculated failure probabilities for point of ‘A’ to ‘E’ in Fig. 4 after 104 stress cycles. The failure proba-
Failure probabilities for required number of cycles were calculated from the results of thermal stress analysis and fatigue test. The proposed method has been applied to the simple die model and following conclusions were obtained.
Poisson’s ratio
0.28
(i) The fatigue strength of the laser-processed specimens decrease remarkably in compared with that of the base metal. However, it can be recovered to 83% of the base metal by heat treatment at secondary hardening temperature. (ii) Failure probability for laser-processed die increased remarkably in compared with that of non-processed ones. However, it can be decreased by heat-treatment at secondary hardening temperature 773 K. References
Fig. 5. Hysteresis of principle stress.
[1] T. Kanno, Introduction of Die-casting Technology, Nikkan Kougyou Shinbunnsha, Tokyo, 1997. [2] G. Ernst, A. Luftenegger, R. Ebner, Proceedings of the 5th International Conference on Tooling, Austria, 1999. [3] P.M. Brown, G. Shannon, W. Deans, J. Bird, Weld World 43 (1999) 33–39. [4] Y. Sun, S. Hanaki, H. Uchida, H. Sunada, H. Tsujii, Appl. Plasma Sci. 10 (2002) 131–138.