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Thermal fatigue failure of brass die-casting dies
Engineering Failure Analysis 20 (2012) 137–146
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Thermal fatigue failure of brass die-casting dies Dhouha Mellouli a,⇑, Nader Haddar a, Alain Köster b, Hassine Ferid Ayedi a a b
Laboratory of Industrial Chemistry and Materials, National Engineering School of Sfax, Box 1173, W3038 Sfax, Tunisia Centre des Matériaux Pierre Marie Fourt, Ecole des Mines de Paris, France
a r t i c l e
i n f o
Article history: Received 17 June 2011 Received in revised form 27 October 2011 Accepted 7 November 2011 Available online 12 November 2011 Keywords: Hot-working tool steel Thermal fatigue Failure mechanisms Die casting die Brass
a b s t r a c t This research has been conducted to elucidate the mechanisms of brass die casting failure. A die was examined and we have evaluated the causes of crack failure mechanisms after use in brass die casting. The dominating failure mechanism in the investigated die was thermal fatigue cracking. Crack initiation is associated to accumulation of the local plastic strain that occurs during each casting cycle. The crack growth is facilitated by a number of elements: oxidation of the cracks’ surfaces, filling of brass and softening of the die material. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Die-casting dies offer an interesting medium for the investigation of thermal fatigue failure. In fact, thermal fatigue cracking, due to thermal cycling, may significantly reduce the lifetime of the die. Cracks degrade not only the surface quality of dies but also consequently the casting surfaces. Thermal fatigue cracking is often observed on the tool surface as a network of fine cracks, called heat checking. Stress cracking, another variant of thermal fatigue cracks, may be observed as individual and clearly pronounced cracks in areas exposed to local stress concentrations in excess of die material yield strength [1]. Subsequently, the crack growth was pronounced by thermal fatigue, erosion of the melt flow, corrosion, oxidation, soldering of metal to the die surface, deformation of die contact surfaces and gross fracture [2,3]. The formation of thermal fatigue cracks may lead to a loss of surface material as small fragments splinter off from the surface. To endure these severe conditions the tools are made of hot-work tool steel, designed to have an adequate combination of hot strength, toughness and ductility, as well as thermal conductivity and thermal expansion [4,5]. Die maintenance may be done by grinding or welding if the surface quality or dimensions of the castings are no longer sufficient. However, the tool and service costs constitute a remarkable part of the production costs in die casting and there are numerous approaches to optimize the lifetime of the dies. In general, die life may be enhanced by geometric factors in die design (governing stresses and thermal gradients), die material considerations (e.g. machinability, heat treatment, toughness, resistance to wear and heat checking), processing conditions (e.g. preheating, heating and cooling cycles, machine closing force, lubricants, service intervals), and die surface considerations [6]. Surface treatments, such as nitriding, are often applied for casting dies to reduce abrasive wear and improve thermal fatigue resistance. In recent years, the application of hard coatings deposited by physical vapor deposition (PVD) techniques has been reported in the literature [7,8]. ⇑ Corresponding author. E-mail address: [email protected] (D. Mellouli). 1350-6307/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2011.11.006
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D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
The objective of this paper is to investigate and characterize the thermal fatigue failure of a prematurely failed brass diecasting dies. We measured and investigated the mechanical and chemical properties of the dies materials. Then, we inspected the die-casting dies cracks on the surface. Finally, we evaluated both the hardness changes as well as the growth and density of cracks in order to elucidate and explain the life-limiting failure mechanisms in field-tested brass die casting dies. 2. Materials and methods 2.1. Experimental techniques Identification of the failure mechanisms was realized by macroscopic examinations. The surface and fractured samples were examined using Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Mechanical properties were identified using monotonic uniaxial tensile tests at different temperatures with standard flat tensile specimens (5 2 mm2 as section). In order to investigate the fragilisation of the steel tool, we employed Charpy tests [9,10]. Standard Charpy V-notch specimens with a (10 10 mm2) section, a central 45° V-notch of 2 mm depth and a 0.25 mm notch root radius, were used. Hardness profiles were measured using Vickers micro-hardness machine on polished cross-sections with a load of 1 kg. 2.2. Experimental material 2.2.1. Chemical investigation and microstructure The chemical analysis of the steel die and H13 were carried out using an X-ray fluorescence spectrometer (Table 1). These results are similar to previous research analysis [11]. Fig. 1 shows the structure of the steel die, indicating that the steel mold has a martensitic structure. 2.2.2. Mechanicals properties Fig. 2 shows the evolution of mechanicals properties versus the temperature of the mold steel samples. The investigation demonstrates that the tensile strength (Rm), tensile yield strength (Rp0.2) (Fig. 2a) and Young modulus (Fig. 2b) decrease when the temperature of the steel increases. However, the reduction of the area at tensile fracture increases as the temperature increases (Fig. 2a). It is desirable that the modulus of elasticity of the die material be as small as possible. In fact, the higher modulus of the steel involved the higher thermo-mechanical stresses [12]. The Charpy-V fracture toughness was determined on three CVN samples at room temperature. The measured values were 14 ± 2 J, which is lower than normal (28–35 J) for this type of steel [13]. Fig. 3 shows examples of fracture surfaces from Charpy-V tests, clearly indicating the brittle nature of the specimen material.
Table 1 Chemical composition (wt.%) of the die and the H13.
Die H13
C
Si
Mn
P
S
Cr
Mo
Al
V
0.34 0.35
1.21 1.32
0.364 0.288
0.013 0.012
0.0024 0.0027
5.5 5.6
1.18 1.47
0.022 0.0
1.16 1.25
Fig. 1. The SEM microanalysis of the steel mold (Nital 3% etched).
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
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Fig. 2. Mechanical properties of mold steel showing (a) tensile strength (Rm), tensile yield strength (Rp0.2), hold yield strength (Re), reduction of area at tensile fracture (Z) versus temperature and (b) modulus of elasticity versus temperature.
Fig. 3. SEM fractographic of fracture surface of the Charpy-V sample test. (a) 20; (b) 500.
2.3. Die-casting conditions for investigative tools The die casting dies had been used for the production of brass window handles. The geometry of the studied mold is shown in Fig. 4. The field tests were made in a cold chamber machine. During brass die-casting, molten brass at a temperature of about 950 °C is injected into the mold. The cycle time for one casting was 20 s. The maximum metal pressure during injection was approximately 180 MPa. The total shot weight of each casting was 1.4 kg. Water was continuously circulated through cooling channels in the dies (20 °C < Twater < 50 °C). After the ejection of each casting, the tool surfaces were lubricated. The die lifetime was estimated to be 30,000–35,000 castings.
140
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(a)
(b)
(c)
50 mm
5 mm
10 mm
10 mm
(a)
(b)
(c)
Fig. 4. Typical macroscopic surface damages observed on the different zones (a), (b) and (c) on the casting die failures. (a) Zone A; (b) zone B; (c) zone C.
3. Results and discussion The macroscopic surface damage on the die casting was severe and thermal fatigue cracking, found at almost all sharp corners, was the most prevalent type of damage (Fig. 4). Thermal fatigue cracking, usually referred to as heat checking, often appears on the tool surface as a network of fine cracks (Fig. 4b). Another variant of thermal fatigue cracks (stress cracks) may be shown as individual and clearly pronounced cracks in areas exposed to local stress concentrations (Fig. 4a and c), the same phenomena was observed by Neu and Sehitoglu [3]. Macroscopic inspections revealed local filling of solidified brass in the cracks on the surface die (Fig. 5). Studies of polished cross-sections elucidate the morphology of the crack pattern (Fig. 5). The cracks tend to grow perpendicular to
BC
(a)
(b)
EC
Fig. 5. Polished cross-section of a steel mold (a) overview revealing typical thermal fatigue cracks. (b) Close-up of the crack indicated by the arrow in (a).
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the die surface and reach several millimeters (2.5 mm) in depth (Fig. 5a). All of these cracks are filled with brass that has a layered structure next to the steel walls and a more inhomogeneous structure in the center where it contains voids and cracks (Fig. 5b). Further analysis was conducted in order to investigate the nucleation and growth of the cracks on the surface of the die (Beginning (BC)–End (EC) of crack).
Fig. 6. EDS maps of the area indicated by frame BC in Figs. 6 and 7. The element concentration is proportional to the brightness.
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At an early stage of development the cracks are typically filled already, and especially after further development (Fig. 5). Figs. 6 and 7 reveal that their surfaces mainly consist of iron and silicon oxides, and that the interior is filled with residuals from solidified brass. Also observed were relatively short cracks (Fig. 8), filled with a mixture of iron and zinc oxides. In the space in the cracks between these oxides and the visually unaffected die steel itself, we further observed a diffuse Cr, Si and O rich layer. Cu was not detected in these cracks (Fig. 8). These results were in good accordance with those reported by Sjöström and Bergström [14]. The initial growth of the thermal fatigue cracks is facilitated by an oxidation attack on the crack surface, which forms the Cr, Si and O rich layer discussed above (Fig. 8). This result is in good agreement with those found by Kovrigin et al. [15] and
Fig. 7. EDS maps of the area indicated by frame EC in Figs. 6 and 7. The element concentration is proportional to the brightness.
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
Fig. 8. EDS maps of a short crack area. The element concentration is proportional to the brightness.
143
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D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
Nehrenberg [16]. The presence of brass and oxides, in the cracks increases the compressive stresses and plastic yield (Rp0.2) during the heating phase of the thermal cycle [17]. As a result, the tensile stress is further increased during the cooling phase, which in turn contributes to the continuing growth of the cracks. The thermal cracks favorite considerably the crack length while they are typically filled with a mixture of oxides [15], and residuals from the brass alloy (Figs. 6 and 7) [16]. The oxides filling the cracks consist mainly of iron oxides [16], but also include zinc and lead oxides [15], as shown in Figs. 6–8. Their structures in between the steel walls are layered or inhomogeneous and contain cracks and voids (Fig. 5b) [15]. The filling material, essentially composed of stacked layers, indicates that the filling of the cracks increases during the casting process.
Fig. 9. Hardness profile showing hardness versus distance from the surface of the die casting tool.
50µm deep
200µm deep
550µm deep
1150µm deep
1400µm deep
2000µm deep
10 mm
2400µm deep
2800µm deep Fig. 10. The network propagation versus depth (zone B).
3250µm deep
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
145
Fig. 11. Crack density versus the depth.
The tensile stresses imposed on the die material surface layer during the cooling phase cause local failure of filled material and promote the open thermal cracks (Fig. 5b). These cracks in the filling material act as channels, allowing oxygen to penetrate down to the crack tip area and oxidize both the steel and the filling material (Figs. 5–8). As a result, the size of the crack openings in the filling material indicates the amount of brass that will fill the crack during the next cycle (Fig. 5b). After many thermal casting cycles, we observed not only a network of surface cracks but also more individual cracks, as well as increased crack filling (Fig. 4). Eventually, few surface cracks may grow together: we observed local detachment of surface material (Fig. 4). We furthermore observed that the worst surface cracking appeared in die areas exposed to the critical thermal and/or mechanical stress concentrations, such as corners, or parts of the die that had been more or less surrounded by the melt (Fig. 5), is confirmed by previous research [3,16]. Measurements of micro-hardness of the die casting die were conducted at the thickness from the superficial surface layer to the depth (Fig. 9). The original hardness of the die casting die prior to use was estimated to be 360 Hv. After thousands of thermal heating and cooling castings (30,000–35,000 cycles), the hardness of the superficial surface had been reduced to 300 Hv. By the end, the micro-hardness of the die had been reduced to approximately 60 Hv. The softened surface layer of the die casting die was limited to a depth of less than 1.6 mm (Fig. 9). Thermal fatigue caused a site of damage on the mold surface and subsequently a network of crazing was formed. The same phenomenon was observed by Sjöström and Bergström in the aluminum die casting-dies [18]. In order to characterize the growth of the networks on the die surface and in depth, the 3D characterization of the crack networks was determined, through a step-by-step removal of thin layers (Fig. 10) [19]. After that, the crack density was calculated by dividing the total length of all cracks by the area of this reference surface (Fig. 11). Because of the loading itself, there is a strong loading gradient in the specimen depth. Therefore, the cracks cannot propagate very deep into the material. Furthermore, there is also a shielding effect, between neighboring cracks, that slows down and then stops the propagation of smaller cracks. These effects appear clearly in Fig. 10. After a depth of 1150 lm, the total crack length and crack density quickly becomes smaller and most cracks cease to penetrate. The complexity of the network also decreases as the depth increases: only a few of the biggest cracks continue to propagate. Fig. 11 shows that the longest cracks do not exceed 3.5 mm in length. Experimental observations in depth indicate that only two or three cracks reach the maximum depth, all of which initiated from singularity [19]. Those results prove the important role of singularity on the stress concentration and its cause an early accumulation of plastic strain.
4. Conclusion In this study, the crack initiation and propagation in die casting die were investigated. The following conclusions can be drawn: Thermal fatigue was found to be the first damage mechanism in the die-casting die. The crack initiation is due to an accumulation of local plastic strain in the die surface, which is typical of a low-cycle fatigue process. The initial growth of the thermal fatigue cracks is proved by an oxidation of the crack surfaces. Crack growth is facilitated by filling of cracks with brass, additional oxidation, and softening of the die material. The oxides consist mainly of iron, silicon and chromium oxides, but also of zinc and lead oxides. The stability of the fatigue network is related to the depth of softened material. The singularity plays an important role on the stress concentration and cause an early accumulation of plastic strain.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Danzer R, Sturm F, Schindler A, Zleppnig W. Thermal fatigue cracks in pressure die casting dies. Gisserei-Praxis 1983;1920:287. Kosec B, Kosec L, Kopac J. Eng Failure Anal 2001;8:355–9. Neu RW, Sehitoglu H. Thermomechanical fatigue, oxidation, and creep. Part 1: Damage mechanisms. Metal Trans A 1989;20A. Davis JR, editor. Tool materials, ASM speciality handbook. Materials Park, OH: ASM International; 1995. p. 251. Roberts G, Kraus G, Kennedy R. Tool steels. 5th ed. Materials Park, OH: ASM International; 1998. Shivpuri R, Semiatin SL. Friction lubrication and wear technology. In: Olsen D, editor. ASM handbook, vol. 18. Materials Park, OH: ASM; 1992. p. 621–48. Lugscheider E, Barimani C, Guerreiro S, Bobzin K. Corrosion tests of PVD coatings with die lubricant used for Al high-pressure die-casting dies. Surface Coat Technol 1998;108–109:109–408. Heim D, Holler F, Mitterer C. Hard coatings produced by PACVD applied to aluminium die casting. Surface Coat Technol 1999;497:116–9. Russell S. Experiments with a new machine for testing materials by impact. Trans Am Soc Civil Eng 1898;39(826):237–50. Charpy G. Note sur l’essai des metaux a la flexion par choc de barreaux entailles. Mém Comp Rend Soc Ingnr Civil France 1901:848–77. Breitler R. Recent developments in die steels for pressure die casting. In: Transactions of the driving die casting into the 21st century, Detroit 1991, September–October 1991. p. 231–5. Sjöström J, Bergström J. On the influence of austenitizing treatment on isothermal and thermal fatigue lives hotwork tool steels. In: Proceedings of the transactions of the 22nd international die casting congress and exposition. Indiana: North American Die Casting Association; 2003. p. T03–T053. Torkar M, Godec M, Lamut M. Investigation of die for pressure die-casting of Mg-alloy. Eng Failure Anal 2009;16:176–81. Sjöström J, Bergström J. Thermal fatigue in hot-working tools. Scand J Matall 2005;34:221–31. Kovrigin VA, Starokozhev BS, Yurasov SA. Effect of oxidizing processes on crazing of die-casting moulds. Met Sci Heat Treat 1980;22:688. Nehrenberg AE. Metallurgical aspects of heat checking in brass die casting dies. Die Cast 1949:30. Klobcar D, Tusek J, Taljat B. Thermal fatigue of materials for die-casting tooling. Mater Sci Eng A 2008;472:198–207. Sjöström J, Bergström J. Thermal fatigue testing of chromium martensitic hot-work tool steel after different austenitizing treatments. J Mater Process Technol 2004;153–154:1089–96. Maillot V et al. Thermal fatigue crack networks parameters and stability: an experimental study. Int J Solid Struct 2004.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Thermal fatigue failure of brass die-casting dies Dhouha Mellouli a,⇑, Nader Haddar a, Alain Köster b, Hassine Ferid Ayedi a a b
Laboratory of Industrial Chemistry and Materials, National Engineering School of Sfax, Box 1173, W3038 Sfax, Tunisia Centre des Matériaux Pierre Marie Fourt, Ecole des Mines de Paris, France
a r t i c l e
i n f o
Article history: Received 17 June 2011 Received in revised form 27 October 2011 Accepted 7 November 2011 Available online 12 November 2011 Keywords: Hot-working tool steel Thermal fatigue Failure mechanisms Die casting die Brass
a b s t r a c t This research has been conducted to elucidate the mechanisms of brass die casting failure. A die was examined and we have evaluated the causes of crack failure mechanisms after use in brass die casting. The dominating failure mechanism in the investigated die was thermal fatigue cracking. Crack initiation is associated to accumulation of the local plastic strain that occurs during each casting cycle. The crack growth is facilitated by a number of elements: oxidation of the cracks’ surfaces, filling of brass and softening of the die material. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Die-casting dies offer an interesting medium for the investigation of thermal fatigue failure. In fact, thermal fatigue cracking, due to thermal cycling, may significantly reduce the lifetime of the die. Cracks degrade not only the surface quality of dies but also consequently the casting surfaces. Thermal fatigue cracking is often observed on the tool surface as a network of fine cracks, called heat checking. Stress cracking, another variant of thermal fatigue cracks, may be observed as individual and clearly pronounced cracks in areas exposed to local stress concentrations in excess of die material yield strength [1]. Subsequently, the crack growth was pronounced by thermal fatigue, erosion of the melt flow, corrosion, oxidation, soldering of metal to the die surface, deformation of die contact surfaces and gross fracture [2,3]. The formation of thermal fatigue cracks may lead to a loss of surface material as small fragments splinter off from the surface. To endure these severe conditions the tools are made of hot-work tool steel, designed to have an adequate combination of hot strength, toughness and ductility, as well as thermal conductivity and thermal expansion [4,5]. Die maintenance may be done by grinding or welding if the surface quality or dimensions of the castings are no longer sufficient. However, the tool and service costs constitute a remarkable part of the production costs in die casting and there are numerous approaches to optimize the lifetime of the dies. In general, die life may be enhanced by geometric factors in die design (governing stresses and thermal gradients), die material considerations (e.g. machinability, heat treatment, toughness, resistance to wear and heat checking), processing conditions (e.g. preheating, heating and cooling cycles, machine closing force, lubricants, service intervals), and die surface considerations [6]. Surface treatments, such as nitriding, are often applied for casting dies to reduce abrasive wear and improve thermal fatigue resistance. In recent years, the application of hard coatings deposited by physical vapor deposition (PVD) techniques has been reported in the literature [7,8]. ⇑ Corresponding author. E-mail address: [email protected] (D. Mellouli). 1350-6307/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2011.11.006
138
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
The objective of this paper is to investigate and characterize the thermal fatigue failure of a prematurely failed brass diecasting dies. We measured and investigated the mechanical and chemical properties of the dies materials. Then, we inspected the die-casting dies cracks on the surface. Finally, we evaluated both the hardness changes as well as the growth and density of cracks in order to elucidate and explain the life-limiting failure mechanisms in field-tested brass die casting dies. 2. Materials and methods 2.1. Experimental techniques Identification of the failure mechanisms was realized by macroscopic examinations. The surface and fractured samples were examined using Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Mechanical properties were identified using monotonic uniaxial tensile tests at different temperatures with standard flat tensile specimens (5 2 mm2 as section). In order to investigate the fragilisation of the steel tool, we employed Charpy tests [9,10]. Standard Charpy V-notch specimens with a (10 10 mm2) section, a central 45° V-notch of 2 mm depth and a 0.25 mm notch root radius, were used. Hardness profiles were measured using Vickers micro-hardness machine on polished cross-sections with a load of 1 kg. 2.2. Experimental material 2.2.1. Chemical investigation and microstructure The chemical analysis of the steel die and H13 were carried out using an X-ray fluorescence spectrometer (Table 1). These results are similar to previous research analysis [11]. Fig. 1 shows the structure of the steel die, indicating that the steel mold has a martensitic structure. 2.2.2. Mechanicals properties Fig. 2 shows the evolution of mechanicals properties versus the temperature of the mold steel samples. The investigation demonstrates that the tensile strength (Rm), tensile yield strength (Rp0.2) (Fig. 2a) and Young modulus (Fig. 2b) decrease when the temperature of the steel increases. However, the reduction of the area at tensile fracture increases as the temperature increases (Fig. 2a). It is desirable that the modulus of elasticity of the die material be as small as possible. In fact, the higher modulus of the steel involved the higher thermo-mechanical stresses [12]. The Charpy-V fracture toughness was determined on three CVN samples at room temperature. The measured values were 14 ± 2 J, which is lower than normal (28–35 J) for this type of steel [13]. Fig. 3 shows examples of fracture surfaces from Charpy-V tests, clearly indicating the brittle nature of the specimen material.
Table 1 Chemical composition (wt.%) of the die and the H13.
Die H13
C
Si
Mn
P
S
Cr
Mo
Al
V
0.34 0.35
1.21 1.32
0.364 0.288
0.013 0.012
0.0024 0.0027
5.5 5.6
1.18 1.47
0.022 0.0
1.16 1.25
Fig. 1. The SEM microanalysis of the steel mold (Nital 3% etched).
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
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Fig. 2. Mechanical properties of mold steel showing (a) tensile strength (Rm), tensile yield strength (Rp0.2), hold yield strength (Re), reduction of area at tensile fracture (Z) versus temperature and (b) modulus of elasticity versus temperature.
Fig. 3. SEM fractographic of fracture surface of the Charpy-V sample test. (a) 20; (b) 500.
2.3. Die-casting conditions for investigative tools The die casting dies had been used for the production of brass window handles. The geometry of the studied mold is shown in Fig. 4. The field tests were made in a cold chamber machine. During brass die-casting, molten brass at a temperature of about 950 °C is injected into the mold. The cycle time for one casting was 20 s. The maximum metal pressure during injection was approximately 180 MPa. The total shot weight of each casting was 1.4 kg. Water was continuously circulated through cooling channels in the dies (20 °C < Twater < 50 °C). After the ejection of each casting, the tool surfaces were lubricated. The die lifetime was estimated to be 30,000–35,000 castings.
140
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
(a)
(b)
(c)
50 mm
5 mm
10 mm
10 mm
(a)
(b)
(c)
Fig. 4. Typical macroscopic surface damages observed on the different zones (a), (b) and (c) on the casting die failures. (a) Zone A; (b) zone B; (c) zone C.
3. Results and discussion The macroscopic surface damage on the die casting was severe and thermal fatigue cracking, found at almost all sharp corners, was the most prevalent type of damage (Fig. 4). Thermal fatigue cracking, usually referred to as heat checking, often appears on the tool surface as a network of fine cracks (Fig. 4b). Another variant of thermal fatigue cracks (stress cracks) may be shown as individual and clearly pronounced cracks in areas exposed to local stress concentrations (Fig. 4a and c), the same phenomena was observed by Neu and Sehitoglu [3]. Macroscopic inspections revealed local filling of solidified brass in the cracks on the surface die (Fig. 5). Studies of polished cross-sections elucidate the morphology of the crack pattern (Fig. 5). The cracks tend to grow perpendicular to
BC
(a)
(b)
EC
Fig. 5. Polished cross-section of a steel mold (a) overview revealing typical thermal fatigue cracks. (b) Close-up of the crack indicated by the arrow in (a).
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
141
the die surface and reach several millimeters (2.5 mm) in depth (Fig. 5a). All of these cracks are filled with brass that has a layered structure next to the steel walls and a more inhomogeneous structure in the center where it contains voids and cracks (Fig. 5b). Further analysis was conducted in order to investigate the nucleation and growth of the cracks on the surface of the die (Beginning (BC)–End (EC) of crack).
Fig. 6. EDS maps of the area indicated by frame BC in Figs. 6 and 7. The element concentration is proportional to the brightness.
142
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
At an early stage of development the cracks are typically filled already, and especially after further development (Fig. 5). Figs. 6 and 7 reveal that their surfaces mainly consist of iron and silicon oxides, and that the interior is filled with residuals from solidified brass. Also observed were relatively short cracks (Fig. 8), filled with a mixture of iron and zinc oxides. In the space in the cracks between these oxides and the visually unaffected die steel itself, we further observed a diffuse Cr, Si and O rich layer. Cu was not detected in these cracks (Fig. 8). These results were in good accordance with those reported by Sjöström and Bergström [14]. The initial growth of the thermal fatigue cracks is facilitated by an oxidation attack on the crack surface, which forms the Cr, Si and O rich layer discussed above (Fig. 8). This result is in good agreement with those found by Kovrigin et al. [15] and
Fig. 7. EDS maps of the area indicated by frame EC in Figs. 6 and 7. The element concentration is proportional to the brightness.
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
Fig. 8. EDS maps of a short crack area. The element concentration is proportional to the brightness.
143
144
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
Nehrenberg [16]. The presence of brass and oxides, in the cracks increases the compressive stresses and plastic yield (Rp0.2) during the heating phase of the thermal cycle [17]. As a result, the tensile stress is further increased during the cooling phase, which in turn contributes to the continuing growth of the cracks. The thermal cracks favorite considerably the crack length while they are typically filled with a mixture of oxides [15], and residuals from the brass alloy (Figs. 6 and 7) [16]. The oxides filling the cracks consist mainly of iron oxides [16], but also include zinc and lead oxides [15], as shown in Figs. 6–8. Their structures in between the steel walls are layered or inhomogeneous and contain cracks and voids (Fig. 5b) [15]. The filling material, essentially composed of stacked layers, indicates that the filling of the cracks increases during the casting process.
Fig. 9. Hardness profile showing hardness versus distance from the surface of the die casting tool.
50µm deep
200µm deep
550µm deep
1150µm deep
1400µm deep
2000µm deep
10 mm
2400µm deep
2800µm deep Fig. 10. The network propagation versus depth (zone B).
3250µm deep
D. Mellouli et al. / Engineering Failure Analysis 20 (2012) 137–146
145
Fig. 11. Crack density versus the depth.
The tensile stresses imposed on the die material surface layer during the cooling phase cause local failure of filled material and promote the open thermal cracks (Fig. 5b). These cracks in the filling material act as channels, allowing oxygen to penetrate down to the crack tip area and oxidize both the steel and the filling material (Figs. 5–8). As a result, the size of the crack openings in the filling material indicates the amount of brass that will fill the crack during the next cycle (Fig. 5b). After many thermal casting cycles, we observed not only a network of surface cracks but also more individual cracks, as well as increased crack filling (Fig. 4). Eventually, few surface cracks may grow together: we observed local detachment of surface material (Fig. 4). We furthermore observed that the worst surface cracking appeared in die areas exposed to the critical thermal and/or mechanical stress concentrations, such as corners, or parts of the die that had been more or less surrounded by the melt (Fig. 5), is confirmed by previous research [3,16]. Measurements of micro-hardness of the die casting die were conducted at the thickness from the superficial surface layer to the depth (Fig. 9). The original hardness of the die casting die prior to use was estimated to be 360 Hv. After thousands of thermal heating and cooling castings (30,000–35,000 cycles), the hardness of the superficial surface had been reduced to 300 Hv. By the end, the micro-hardness of the die had been reduced to approximately 60 Hv. The softened surface layer of the die casting die was limited to a depth of less than 1.6 mm (Fig. 9). Thermal fatigue caused a site of damage on the mold surface and subsequently a network of crazing was formed. The same phenomenon was observed by Sjöström and Bergström in the aluminum die casting-dies [18]. In order to characterize the growth of the networks on the die surface and in depth, the 3D characterization of the crack networks was determined, through a step-by-step removal of thin layers (Fig. 10) [19]. After that, the crack density was calculated by dividing the total length of all cracks by the area of this reference surface (Fig. 11). Because of the loading itself, there is a strong loading gradient in the specimen depth. Therefore, the cracks cannot propagate very deep into the material. Furthermore, there is also a shielding effect, between neighboring cracks, that slows down and then stops the propagation of smaller cracks. These effects appear clearly in Fig. 10. After a depth of 1150 lm, the total crack length and crack density quickly becomes smaller and most cracks cease to penetrate. The complexity of the network also decreases as the depth increases: only a few of the biggest cracks continue to propagate. Fig. 11 shows that the longest cracks do not exceed 3.5 mm in length. Experimental observations in depth indicate that only two or three cracks reach the maximum depth, all of which initiated from singularity [19]. Those results prove the important role of singularity on the stress concentration and its cause an early accumulation of plastic strain.
4. Conclusion In this study, the crack initiation and propagation in die casting die were investigated. The following conclusions can be drawn: Thermal fatigue was found to be the first damage mechanism in the die-casting die. The crack initiation is due to an accumulation of local plastic strain in the die surface, which is typical of a low-cycle fatigue process. The initial growth of the thermal fatigue cracks is proved by an oxidation of the crack surfaces. Crack growth is facilitated by filling of cracks with brass, additional oxidation, and softening of the die material. The oxides consist mainly of iron, silicon and chromium oxides, but also of zinc and lead oxides. The stability of the fatigue network is related to the depth of softened material. The singularity plays an important role on the stress concentration and cause an early accumulation of plastic strain.
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