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Investigation of die for pressure die-casting of Mg-alloy
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Engineering Failure Analysis 16 (2009) 176–181 www.elsevier.com/locate/engfailanal
Investigation of die for pressure die-casting of Mg-alloy M. Torkar *, M. Godec, M. Lamut Institute of Metals and Technology, P.O. Box 431, Lepi pot 11, SI-1000 Ljubljana, Slovenia Received 11 February 2008; accepted 17 February 2008 Available online 23 February 2008
Abstract Investigated was a die for the pressure die-casting of Mg-alloy. Die was made of martensitic chromium hot-work steel grade and the surface was gas nitrided. Due damaged surface the die was removed from the production line after 20.000 shots Light microscopy and electron microscopy studies were performed. Mechanical properties as well as fracture toughness of the basic material were determined. Presented are results of the investigations. It was established that the base material properties correspond to declared properties. The corrosion phenomena in the form of pits, were observed on all surfaces of the die. Metallography revealed that nitrided layer consisted only of diffusion layer, without a c0 compound layer. The depth of the pits was limited to the thickness of nitrided layer and in the pits the presence of Ca, Cl and S was detected. No cracks were observed in nitrided layer and all edges of pits were rounded. The damages were caused by a combined mechanism of corrosion–erosion. The main reason for the damages were not optimised nitride layer properties. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Failure of tool; Nitrided layer; Metallography; Corrosion; Erosion
1. Introduction Different surfacing techniques are applied for surface protection and prolongation of the pressure die service life time. Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range (500–550 °C). During nitriding of steels two different structures are formed from surface to core, known as the c0 compound layer and e diffusion region. The compound layer consists of iron nitrides e phase (e-Fe2–3N), gamma phase (c0 -Fe4N) or a mixed phase (e + c0 ) developed at the surface. Wear resistance of the compound layer depends on many factors such as compound layer composition (epsilon/ gamma) compound layer thickness, mode of mechanical loading, etc. [1–4]. On the other hand, the diffusion region brings about an improvement of fatigue strength when compared to an untreated material. In this structure, N atoms also dissolved interstitially in excess in the ferritic lattice, give rise to formation of nitride precipitates [5].
*
Corresponding author. Tel.: +386 1 4701980; fax: +386 1 4701939. E-mail address: [email protected] (M. Torkar).
1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.02.001
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Nitrogen has partial solubility in iron. It can form a solid solution with ferrite at nitrogen contents up to about 6%. At about 6% N, a compound called gamma prime (c0 ), with a composition of Fe4N is formed. At nitrogen contents greater than 8%, the equilibrium reaction product is e compound, Fe3N. The outermost surface can be all c0 and if this is the case, it is referred to as the white layer. Such a surface layer is often undesirable: it is very hard profiles but is so brittle that it may spall in use. Usually it is removed; or special nitriding processes are used to reduce this layer or make it less brittle. The e zone is hardened by the formation of the Fe3N compound, and below this layer there is some solid solution strengthening from the nitrogen in solid solution. Thus, nitriding is similar to carburizing in that surface composition is altered, but different in that nitrogen is added into ferrite instead of austenite. Because nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite, nitriding can be accomplished with a minimum of distortion and with excellent dimensional control. The mechanism of nitriding is generally known, but the specific reactions that occur in different steels and with different nitriding media are not always known. The higher the nitrogen concentration, the harder and more brittle the compound (i.e. e nitrides are much harder than c0 nitrides) layer is formed. Nitriding is a common practice in surface protection against wear and erosion [6,7]. Different nitriding techniques are applied as gas nitriding, pulsed plasma ion nitriding and others. Dependent on the process parameters the nitrided layer consists of diffusion layer and thin compound layer on the top of diffusion layer. The compound layer can delay the nucleation of heat cracks and hold back the propagation of heat cracks from surface to substrate because of its high hardness and strength. On the other hand, the heat checking expands faster with the compound layer on the surface than that without it. The behaviour of the nitrided tool, during exploitation, depends much on the properties of nitrided layer. Expected life time of such dies is up to 160.000 shots, but often appears failures in the 15.000–65.000 shot range. The main reason of failures is in corrosion fatigue fracture mechanism. Growth of short fatigue cracks [8], originated in small corrosion pits, determines the ultimate life of a die or other engineering component [9]. Investigated was a die for the die-casting of Mg-alloy AM60 (5.6–6.4% Al, 0.26–0.50% Mn, max. 0.20% Zn, bal. Mg). The temperature of the melt was 670–690 °C and the working temperature of the die was 240 °C. The surface of the die was treated by gas nitriding and the declared hardness of the die was 47 HRc. During operation the lubricants Petrofer 820 SLO or Trenex Mg 1611 were applied. After 20.000 shots the damages were observed on the all die surface. The die operated in campaigns. That means the die was heated to working temperature and cooled down to room temperature several times. The damages in the form of pits and wider corroded–eroded areas were observed on the surfaces of the die cavity as well as on other surfaces, not in contact with the melt. No cracks due fatigue or corrosion fatigue were observed. The aim of the investigation was to determine the reasons for the appearance of pits and corrosion–erosion damaged areas in nitrided layer, that shortened the life time of the die for pressure die-casting of Mg-alloy. 2. Experimental The surface of the die was careful visual and macroscopic surveyed and samples from damaged areas were cut for metallographic investigation. The reproduction of pits from the die appeared also on the surface of the castings. The samples for metallography were cut from the die by wire-erosion process. The chemical analysis of material was performed, the hardness in the cross-section was measured and the Charpy fracture toughness was determined. Chemical composition of material was determined by optical emission spectrometer ARL-OES-3460. Vickers hardness tests were undertaken through the nitrided layer. The profile of the hardness (HV) was measured by Durimet Leitz measuring device. The hardness of the base material was determined by Rockwell (HrC) device and the fracture toughness was tested with Charpy device on Charpy V-notched samples (CVN). For metallography the light microscope Microphot FXA, Nikon with 3CCD videocamera Hitachi-C20A and computer programme analysis was applied. The damaged surface was observed by scanning electron
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microscope (SEM) Jeol JSM35 and analyses by wavelength dispersive spectrometry (WDS) were performed on damaged and not damaged surfaces. 3. Results with discussion Chemical composition (wt.%) of the tool steel was 0.37% C, 0.13% Si, 0.51% Mn, 0.007% P, 0.002% S, 4.86% Cr, 2.37% Mo, 0.56% V and it corresponds the composition of tool steel for hot work. The profile of Vickers hardness HV0.3 through the nitrided layer was measured perpendicular to the surface and with 0.02 mm of distance among the prints. The following values were measured from the surface: 1083, 1277, 1163, 1252, 1142, 977, 784, 564 HV0.3. From the profile of hardness determined thickness of nitrided layer was Nht 0.14 mm. The Rockwell – C hardness of base material of the die was 45.8 HRc, but on the CVN samples, cut off the die, the measured hardness was 44.5 HRc. It was established, that the measured hardness values HRc, in general, were for 2.5 HRc lower as it was declared by the user of the investigated die. Fracture toughness Charpy – V, was determined on three CVN samples at 20 °C. The measured values were 22 J, 21 J and 21 J, that is 10–12 J lower than normal (28–35 J) for this type of steel. The die did not show mechanical failure so the further investigation of the microstructure, to reveal the reasons for lower fracture
Fig. 1. Surface of the tool with pits.
Fig. 2. Detail of tool with pits in engraved area.
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toughness, was not performed. The absence of mechanical failures confirmed that pits in the cavity originated from a combined corrosion–erosion processes. On the other surfaces of the die the pits formed due corrosion process. The overview by naked eye revealed that pits (Figs. 1 and 2) are present on the contiguous surfaces of the both die parts as well as on ingraved areas of the die. The reproduction of pits was evident also on the surface of the casting (Fig. 3). Metallography of the cross-section (Fig. 4) reveals that the nitrided layer consists only of a diffusion layer. No compound layer on the surface of the nitride layer was observed. By optical microscope measured thickness of the nitride layer was around 0.14 mm that agree with the thickness determined from the profile of hardness. Light microscopy studies confirmed that pits are present in the nitrided layer (Figs. 5–7). Cross-section in the eroded region revealed that in pits the nitrided layer was removed till the basic tool steel (Figs. 5 and 6). No rests of fatigue or other cracks were detected in the nitrided layer and on the edges of the pits (Fig. 6). The rests of not damaged nitrided areas, like islands, were also observed within the damaged areas (Fig. 7). The absence of both cracks and sharp edges indicates that the main reasons for the pits formation were combined erosion– corrosion phenomena.
Fig. 3. Detail of the casting with reproduction of pits on the surface.
Fig. 4. Cross-section of nitrided layer. Only diffusion layer is observed, without compound layer on the top of diffusion layer.
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Fig. 5. Cross-section of nitrided layer with pits and a part of not damaged nitrided layer.
Fig. 6. Detail of the cross-section of nitrided layer with wide pit. No fatigue cracks are present in the nitride layer or in the base material.
Fig. 7. Damaged surface of nitrided layer. Darker islands are not damaged areas in nitrided layer (SEM).
To clear the corrosion process it is necessary to analyse the corrosion products. Analysing of corrosion products [8] enables to identify the corrosion process. The pits were analysed by WDS on the both surfaces; working surface of the cavity and surface not in the contact with the melt. The analysis confirmed traces of Ca, Cl and S in the pits. Similar concentrations were
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obtained on both surfaces. Calcium carbonates originated from cooling water. Coolant chemicals and mineral deposits precipitate from the water when vapor is formed due to overheating of the die. It is supposed that smaller pits were formed due pitting corrosion at presence of chloride ions during cooling down and still stand of the tool, but larger pits in the die cavity were formed by combined mechanisms of erosion–corrosion. No evidents were found for the corrosion fatigue. 4. Conclusions In the present work the damaged surface of the die for pressure die-casting of Mg-alloy has been studied. The most important conclusions obtained are: Chemical analysis confirmed that the die was made of a martensitic chromium hot-work tool steel grade. From the measured hardness HRc it can be conclude that the steel was annealed to working hardness in the temperature region within 600 °C and 610 °C, that means in the lower region of hardness, as recommended for die-casting die for Mg-alloys. The depth of nitride layer Nht 0.14 mm was adequate to recommendation of steel producer and in accordance with the common practice. At Charpy impact test measured absorbed energy was 10–12 J lower as expected for this type of steel. To declare a reason for lower values of absorbed energy, a more detailed analysis of the microstructure should be done. The depth of the pits was limited to the thickness of nitrided layer. Metallography and scanning electron microscopy revealed that a combination of pitting corrosion and corrosion–erosion of nitrided layer was the main reason for the formation of the surface defects of the die. References [1] Ratajski J, Tacikowski J, Somers MAJ. Development of compound layer of iron (carbo) nitrides during nitriding of steel. Surf Eng 2003;19(4):285–91. [2] Tercelj M, Smolej A, Fajfar P, Turk R. Laboratory assessment of wear on nitrided surfaces of dies for hot extrusion of aluminium. Tribol Int 2007;40:374–84. [3] Karamisß MB, Gercßekciog˘lu E. Wear behaviour of plasma nitrided steels at ambient and elevated temperatures. Wear 2000;243:76–84. [4] Kang H, Park IW, Jae JS, Kang SS. A study on a die wear model considering thermal softening: (I) construction of the wear model. J Mater Process Technol 1999;96:53–8. [5] Sjo¨strom J, Bergstro¨m J. Thermal fatigue testing of chromium martensitic hot-work tool steel after different austenitizing treatments. J Mater Process Technol 2004;153–154:1089–96. [6] Castro G, Fernandez-Vicente A, Cid J. Influence of the time in the wear behaviour of an AISI H13 during a crankshaft forging process. Wear 2007;263(7–12):1375–85. [7] Godec M. Cooling cracks on nitrided pins. Eng Fail Anal 2002;9(6):665–71. [8] Wei Ying-hui, Yin Guo-sheng, Hou Li-feng, Xu Bing-she, Ichinose Hideki. Formation mechanism of pits on the surface of thin-wall die-casting magnesium alloy components. Eng Fail Anal 2006;13(4):558–64. [9] Murtaza G, Akid R. Empirical corrosion fatigue life prediction models of a high strength steel. Eng Fract Mech 2000;67(5):461–74.
Engineering Failure Analysis 16 (2009) 176–181 www.elsevier.com/locate/engfailanal
Investigation of die for pressure die-casting of Mg-alloy M. Torkar *, M. Godec, M. Lamut Institute of Metals and Technology, P.O. Box 431, Lepi pot 11, SI-1000 Ljubljana, Slovenia Received 11 February 2008; accepted 17 February 2008 Available online 23 February 2008
Abstract Investigated was a die for the pressure die-casting of Mg-alloy. Die was made of martensitic chromium hot-work steel grade and the surface was gas nitrided. Due damaged surface the die was removed from the production line after 20.000 shots Light microscopy and electron microscopy studies were performed. Mechanical properties as well as fracture toughness of the basic material were determined. Presented are results of the investigations. It was established that the base material properties correspond to declared properties. The corrosion phenomena in the form of pits, were observed on all surfaces of the die. Metallography revealed that nitrided layer consisted only of diffusion layer, without a c0 compound layer. The depth of the pits was limited to the thickness of nitrided layer and in the pits the presence of Ca, Cl and S was detected. No cracks were observed in nitrided layer and all edges of pits were rounded. The damages were caused by a combined mechanism of corrosion–erosion. The main reason for the damages were not optimised nitride layer properties. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Failure of tool; Nitrided layer; Metallography; Corrosion; Erosion
1. Introduction Different surfacing techniques are applied for surface protection and prolongation of the pressure die service life time. Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range (500–550 °C). During nitriding of steels two different structures are formed from surface to core, known as the c0 compound layer and e diffusion region. The compound layer consists of iron nitrides e phase (e-Fe2–3N), gamma phase (c0 -Fe4N) or a mixed phase (e + c0 ) developed at the surface. Wear resistance of the compound layer depends on many factors such as compound layer composition (epsilon/ gamma) compound layer thickness, mode of mechanical loading, etc. [1–4]. On the other hand, the diffusion region brings about an improvement of fatigue strength when compared to an untreated material. In this structure, N atoms also dissolved interstitially in excess in the ferritic lattice, give rise to formation of nitride precipitates [5].
*
Corresponding author. Tel.: +386 1 4701980; fax: +386 1 4701939. E-mail address: [email protected] (M. Torkar).
1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.02.001
M. Torkar et al. / Engineering Failure Analysis 16 (2009) 176–181
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Nitrogen has partial solubility in iron. It can form a solid solution with ferrite at nitrogen contents up to about 6%. At about 6% N, a compound called gamma prime (c0 ), with a composition of Fe4N is formed. At nitrogen contents greater than 8%, the equilibrium reaction product is e compound, Fe3N. The outermost surface can be all c0 and if this is the case, it is referred to as the white layer. Such a surface layer is often undesirable: it is very hard profiles but is so brittle that it may spall in use. Usually it is removed; or special nitriding processes are used to reduce this layer or make it less brittle. The e zone is hardened by the formation of the Fe3N compound, and below this layer there is some solid solution strengthening from the nitrogen in solid solution. Thus, nitriding is similar to carburizing in that surface composition is altered, but different in that nitrogen is added into ferrite instead of austenite. Because nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite, nitriding can be accomplished with a minimum of distortion and with excellent dimensional control. The mechanism of nitriding is generally known, but the specific reactions that occur in different steels and with different nitriding media are not always known. The higher the nitrogen concentration, the harder and more brittle the compound (i.e. e nitrides are much harder than c0 nitrides) layer is formed. Nitriding is a common practice in surface protection against wear and erosion [6,7]. Different nitriding techniques are applied as gas nitriding, pulsed plasma ion nitriding and others. Dependent on the process parameters the nitrided layer consists of diffusion layer and thin compound layer on the top of diffusion layer. The compound layer can delay the nucleation of heat cracks and hold back the propagation of heat cracks from surface to substrate because of its high hardness and strength. On the other hand, the heat checking expands faster with the compound layer on the surface than that without it. The behaviour of the nitrided tool, during exploitation, depends much on the properties of nitrided layer. Expected life time of such dies is up to 160.000 shots, but often appears failures in the 15.000–65.000 shot range. The main reason of failures is in corrosion fatigue fracture mechanism. Growth of short fatigue cracks [8], originated in small corrosion pits, determines the ultimate life of a die or other engineering component [9]. Investigated was a die for the die-casting of Mg-alloy AM60 (5.6–6.4% Al, 0.26–0.50% Mn, max. 0.20% Zn, bal. Mg). The temperature of the melt was 670–690 °C and the working temperature of the die was 240 °C. The surface of the die was treated by gas nitriding and the declared hardness of the die was 47 HRc. During operation the lubricants Petrofer 820 SLO or Trenex Mg 1611 were applied. After 20.000 shots the damages were observed on the all die surface. The die operated in campaigns. That means the die was heated to working temperature and cooled down to room temperature several times. The damages in the form of pits and wider corroded–eroded areas were observed on the surfaces of the die cavity as well as on other surfaces, not in contact with the melt. No cracks due fatigue or corrosion fatigue were observed. The aim of the investigation was to determine the reasons for the appearance of pits and corrosion–erosion damaged areas in nitrided layer, that shortened the life time of the die for pressure die-casting of Mg-alloy. 2. Experimental The surface of the die was careful visual and macroscopic surveyed and samples from damaged areas were cut for metallographic investigation. The reproduction of pits from the die appeared also on the surface of the castings. The samples for metallography were cut from the die by wire-erosion process. The chemical analysis of material was performed, the hardness in the cross-section was measured and the Charpy fracture toughness was determined. Chemical composition of material was determined by optical emission spectrometer ARL-OES-3460. Vickers hardness tests were undertaken through the nitrided layer. The profile of the hardness (HV) was measured by Durimet Leitz measuring device. The hardness of the base material was determined by Rockwell (HrC) device and the fracture toughness was tested with Charpy device on Charpy V-notched samples (CVN). For metallography the light microscope Microphot FXA, Nikon with 3CCD videocamera Hitachi-C20A and computer programme analysis was applied. The damaged surface was observed by scanning electron
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M. Torkar et al. / Engineering Failure Analysis 16 (2009) 176–181
microscope (SEM) Jeol JSM35 and analyses by wavelength dispersive spectrometry (WDS) were performed on damaged and not damaged surfaces. 3. Results with discussion Chemical composition (wt.%) of the tool steel was 0.37% C, 0.13% Si, 0.51% Mn, 0.007% P, 0.002% S, 4.86% Cr, 2.37% Mo, 0.56% V and it corresponds the composition of tool steel for hot work. The profile of Vickers hardness HV0.3 through the nitrided layer was measured perpendicular to the surface and with 0.02 mm of distance among the prints. The following values were measured from the surface: 1083, 1277, 1163, 1252, 1142, 977, 784, 564 HV0.3. From the profile of hardness determined thickness of nitrided layer was Nht 0.14 mm. The Rockwell – C hardness of base material of the die was 45.8 HRc, but on the CVN samples, cut off the die, the measured hardness was 44.5 HRc. It was established, that the measured hardness values HRc, in general, were for 2.5 HRc lower as it was declared by the user of the investigated die. Fracture toughness Charpy – V, was determined on three CVN samples at 20 °C. The measured values were 22 J, 21 J and 21 J, that is 10–12 J lower than normal (28–35 J) for this type of steel. The die did not show mechanical failure so the further investigation of the microstructure, to reveal the reasons for lower fracture
Fig. 1. Surface of the tool with pits.
Fig. 2. Detail of tool with pits in engraved area.
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toughness, was not performed. The absence of mechanical failures confirmed that pits in the cavity originated from a combined corrosion–erosion processes. On the other surfaces of the die the pits formed due corrosion process. The overview by naked eye revealed that pits (Figs. 1 and 2) are present on the contiguous surfaces of the both die parts as well as on ingraved areas of the die. The reproduction of pits was evident also on the surface of the casting (Fig. 3). Metallography of the cross-section (Fig. 4) reveals that the nitrided layer consists only of a diffusion layer. No compound layer on the surface of the nitride layer was observed. By optical microscope measured thickness of the nitride layer was around 0.14 mm that agree with the thickness determined from the profile of hardness. Light microscopy studies confirmed that pits are present in the nitrided layer (Figs. 5–7). Cross-section in the eroded region revealed that in pits the nitrided layer was removed till the basic tool steel (Figs. 5 and 6). No rests of fatigue or other cracks were detected in the nitrided layer and on the edges of the pits (Fig. 6). The rests of not damaged nitrided areas, like islands, were also observed within the damaged areas (Fig. 7). The absence of both cracks and sharp edges indicates that the main reasons for the pits formation were combined erosion– corrosion phenomena.
Fig. 3. Detail of the casting with reproduction of pits on the surface.
Fig. 4. Cross-section of nitrided layer. Only diffusion layer is observed, without compound layer on the top of diffusion layer.
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Fig. 5. Cross-section of nitrided layer with pits and a part of not damaged nitrided layer.
Fig. 6. Detail of the cross-section of nitrided layer with wide pit. No fatigue cracks are present in the nitride layer or in the base material.
Fig. 7. Damaged surface of nitrided layer. Darker islands are not damaged areas in nitrided layer (SEM).
To clear the corrosion process it is necessary to analyse the corrosion products. Analysing of corrosion products [8] enables to identify the corrosion process. The pits were analysed by WDS on the both surfaces; working surface of the cavity and surface not in the contact with the melt. The analysis confirmed traces of Ca, Cl and S in the pits. Similar concentrations were
M. Torkar et al. / Engineering Failure Analysis 16 (2009) 176–181
181
obtained on both surfaces. Calcium carbonates originated from cooling water. Coolant chemicals and mineral deposits precipitate from the water when vapor is formed due to overheating of the die. It is supposed that smaller pits were formed due pitting corrosion at presence of chloride ions during cooling down and still stand of the tool, but larger pits in the die cavity were formed by combined mechanisms of erosion–corrosion. No evidents were found for the corrosion fatigue. 4. Conclusions In the present work the damaged surface of the die for pressure die-casting of Mg-alloy has been studied. The most important conclusions obtained are: Chemical analysis confirmed that the die was made of a martensitic chromium hot-work tool steel grade. From the measured hardness HRc it can be conclude that the steel was annealed to working hardness in the temperature region within 600 °C and 610 °C, that means in the lower region of hardness, as recommended for die-casting die for Mg-alloys. The depth of nitride layer Nht 0.14 mm was adequate to recommendation of steel producer and in accordance with the common practice. At Charpy impact test measured absorbed energy was 10–12 J lower as expected for this type of steel. To declare a reason for lower values of absorbed energy, a more detailed analysis of the microstructure should be done. The depth of the pits was limited to the thickness of nitrided layer. Metallography and scanning electron microscopy revealed that a combination of pitting corrosion and corrosion–erosion of nitrided layer was the main reason for the formation of the surface defects of the die. References [1] Ratajski J, Tacikowski J, Somers MAJ. Development of compound layer of iron (carbo) nitrides during nitriding of steel. Surf Eng 2003;19(4):285–91. [2] Tercelj M, Smolej A, Fajfar P, Turk R. Laboratory assessment of wear on nitrided surfaces of dies for hot extrusion of aluminium. Tribol Int 2007;40:374–84. [3] Karamisß MB, Gercßekciog˘lu E. Wear behaviour of plasma nitrided steels at ambient and elevated temperatures. Wear 2000;243:76–84. [4] Kang H, Park IW, Jae JS, Kang SS. A study on a die wear model considering thermal softening: (I) construction of the wear model. J Mater Process Technol 1999;96:53–8. [5] Sjo¨strom J, Bergstro¨m J. Thermal fatigue testing of chromium martensitic hot-work tool steel after different austenitizing treatments. J Mater Process Technol 2004;153–154:1089–96. [6] Castro G, Fernandez-Vicente A, Cid J. Influence of the time in the wear behaviour of an AISI H13 during a crankshaft forging process. Wear 2007;263(7–12):1375–85. [7] Godec M. Cooling cracks on nitrided pins. Eng Fail Anal 2002;9(6):665–71. [8] Wei Ying-hui, Yin Guo-sheng, Hou Li-feng, Xu Bing-she, Ichinose Hideki. Formation mechanism of pits on the surface of thin-wall die-casting magnesium alloy components. Eng Fail Anal 2006;13(4):558–64. [9] Murtaza G, Akid R. Empirical corrosion fatigue life prediction models of a high strength steel. Eng Fract Mech 2000;67(5):461–74.