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Intermetallic formation and its relation to interface mass loss and tribology in die casting dies
Wear 256 (2004) 1232–1235
Intermetallic formation and its relation to interface mass loss and tribology in die casting dies V. Joshi, A. Srivastava, R. Shivpuri∗ Manufacturing Research Group, Department of Industrial, Welding and System Engineering, The Ohio State University, 210 Baker System Engineering Building, 1971 Neil Avenue, Columbus, OH 43210, USA
Abstract Due to high metal injection velocities typical of die casting process, oxide film on die surface breaks exposing virgin steel surface to liquid aluminum at 680 ◦ C. This paper describes in detail the fundamental tribochemical basis for the soldering and mass loss phenomena related to this steel-cast metal interaction. Cylindrical plain H13 coupons are dipped in hot liquid aluminum, kept for predetermined times, cleaned and characterized for surface and substrate changes. The thermodynamics behind the formation of intermetallic compounds based on Gibb’s free energy calculations are presented. Adhesive strength of soldering layer is determined through pull tests on casting solidified around the test pin. From these results, a dissolution–adhesion wear model is proposed for the growth and dissolution of intermetallics and soldering. In addition, this paper includes the soldering and adhesion behavior of nitrided surfaces. It is seen that the diffusion barrier treatment prevents soldering formation and provides for reduced friction and adhesion at the interface. © 2003 Elsevier B.V. All rights reserved. Keywords: Die-casting; Soldering; Dissolution; Intermetallic formation; Nitriding
1. Introduction In the die casting process, molten aluminum alloy at temperatures ranging from 670–710 ◦ C is injected into the die cavity at high velocities of the order of 30–100 m/s. The injection pressures are of the order of 50–80 MPa [1]. These severe thermo-chemical–mechanical operating conditions place considerable stress on the steel surface of the die cavity. In die casting operation, hot working tool steel H13 (composition: 0.4% C, 1.0% Si, 0.4% Mn, 5.0% Cr, 1.5% Mo and 1.0% V [2]) is commonly used as die material and aluminum silicon alloy 380 (with nominal composition of 8.5–9.5% Si, 3% Cu, 1% Fe, 0.5% Mn, 0.5% Ni and maximum 3% Zn [4]) as the cast metal. The injected aluminum–silicon alloy melt has solid particles in it [3]. These particles may include solidified aluminum, inclusions, hypereutectic silicon (in Al–Si alloys) and other phases which precipitate out when the alloy solidifies. The solid particles cause erosion of die surface and any oxides that might form at the surface when they impact the die surface at high injection velocities. This erosive wear depends on gate velocity, angle of attack and amount of hard particles. To resist erosion, die surfaces are hardened and
tempered to retain high hardness at die casting temperatures. But evidence of high erosion has been observed in many situations especially close to the gate area. Due to the high temperatures involved (around 800 ◦ C), chemical reaction and diffusion play an important role at the cast metal–die steel interface. Die steel dissolves in the aluminum alloy melt forming complex aluminum–iron–silicon intermetallics [1,4]. The thermodynamics and kinetics of this reaction is the primary mechanism for the initiation and build up of soldering on the die steel surface. The purpose of this work is to understand the physicochemical drivers that govern the interaction between die steel and liquid aluminum. This understanding is developed by studying the thermodynamics of the intermetallic formation and the mass loss due to dissolution of this intermetallic layer. A diffusion barrier treatment, nitriding, is applied at the interface and its effect in retarding the thermodynamics is studied by investigating the formation of intermetallics and the adhesive strength of the intermetallics (soldered material) to the die steel surface.
2. Intermetallic layers ∗
Corresponding author. Tel.: +1-614-292-7874; fax: +.:+1-614-292-7852. E-mail address: [email protected] (R. Shivpuri). 0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2003.08.001
Intermetallic layers (Alx Fey Siz ) form at the cast metal–die steel interface when the liquid metal is brought in intimate
V. Joshi et al. / Wear 256 (2004) 1232–1235
1233
45
Thickness (10-6m)
40 35 30 25 20 15
T = 710C
10
T = 740C
5 0 0
Fig. 1. Intermetallic layers formed after dipping for 2 h in liquid aluminum at 680 ◦ C.
contact with the die steel. Their morphology depends on the composition of the cast metal and die steel, the temperature of the cast metal and the time of contact. In Fig. 1 is included a photomicrograph of intermetallic layers formed after H13 steel coupon is immersed in A380 alloy melt for 2 h at 680 ◦ C [4]. 2.1. Thermodynamic criteria for intermetallic formation According to the Fe–Al phase diagram, the formation of four intermetallic phases at 680 ◦ C is possible [4]. The free energy of formation of these four phases, FeAl, FeAl2 , Fe2 Al5 and FeAl3 are as follows: Fe(a) + Al(a) ⇒ FeAl(s),
G◦ = −490.6 kcal/mol
FeAl(a) + Al ⇒ FeAl2 (s),
G◦ = −140.3 kcal/mol
FeAl2 (a) + Al ⇒ Fe2 Al5 (s),
G◦ = − 84.83 kcal/mol
Fe2 Al5 (a) + Al ⇒ FeAl3 (s),
G◦ = −120.65 kcal/mol
There are two possible ways an intermetallic layer can be formed at the interface, either by solid-state diffusion or by reaction and diffusion into the melt. The solid-state diffusion will occur when the aluminum from the supersaturated melt deposits on the die steel substrate. This solid-state aluminum–iron diffusion is driven by temperature and concentration gradients. It is a slow process and hence unlikely in die casting where a typical injection cycle is in milliseconds. On the other hand, chemical reactions and diffusion into the melt have much smaller time scales, and are highly likely the dominant mechanism behind intermetallic formation and soldering. The movement of the liquid–solid diffusion front results in mass loss or dissolution. Fig. 2 [5] includes a graph of intermetallic thickness as a function of time at two different immersion temperatures. The bell shaped curve results from the presence of two competing mechanisms: intermetallic growth and dissolution. When the slope of the curve is positive, the growth of the intermetallic layer is primarily due to the physiochemical reaction at the interface between the aluminum in the cast
500
1000
1500
2000
2500
3000
Time (seconds)
Fig. 2. Growth and dissolution of the intermetallic layers.
metal and the iron from the die steel. It is driven by the reaction kinetics. The negative slope (reduction in intermetallic thickness) is due to dissolution of the intermetallic layer in the hot cast metal that is lower in iron content. This dissolution is driven by solubility of iron in Al–Si melt. At the peak of the curve, the rate of diffusion is equal to the rate of dissolution. This dynamic relationship between dissolution and diffusion of iron and aluminum atoms results in weight loss and volumetric loss of the substrate [5]. 2.2. Model for dissolution and mass loss of die steel in liquid aluminum The formation and growth of intermetallic layer in die casting in the absence of a diffusion barrier can be explained as follows: • Stage 1: During melt injection and solidification, diffusion of aluminum and iron atoms across the interface occurs to form Fex Aly Siz intermetallic at the interface. Silicon changes the rate kinetics and the solubility of iron in aluminum. • Stage 2: A new die casting cycle begins and fresh melt enters the die cavity. The driving force for diffusion is slightly lower due to the presence of previous intermetallic layer. There is enough driving force for the intermetallic layer to continue to grow. The driving force for dissolution in this stage is high but still lower than that for diffusion. • Stage 3: In the next cycle, the thickness of the Fex Aly Siz intermetallic layer reaches a critical limit, the driving force for diffusion has decreased to become negligible and the driving force for dissolution becomes the dominating force. The soldering dissolves in the melt. • Stage 4: Mass loss has taken place on the die surface but is free of intermetallic layer. Driving force for diffusion increases due to loss of Fex Aly to the melt but is still negligible compared to the driving force for dissolution. The driving force for dissolution decreases with increasing intermetallic thickness. • Stage 5: The cyclic process of soldering growth and dissolution continues while the die surface continuously loses iron to the melt.
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The proposed model of competing mechanisms reasonably explains the higher thickness of intermetallic layer at 710 ◦ C. As the temperature is raised to 740 ◦ C (higher superheat), the dissolution mechanism dominates resulting in lower layer thickness and the maximum thickness (peak) occurring earlier. Similar reductions in intermetallic layers are obtained when the coupons are rotated (increase in melt velocity) and mass transport increases [4].
3. Test results and discussions The test coupons were standard core pins manufactured by DME Inc, USA. These were heat treated as follows: stress relieved for 0.5 h at 537 ◦ C; vacuum hardened for 90 min at 1024 ◦ C; quenched; tempered for 3 h at 537 ◦ C, for 3 h at 1100 ◦ F and for 2 h at 1000 ◦ F. The final hardness was measured as 46–48 HRc. The aluminum–silicon alloy A380 was heated to the melt temperature 680 ◦ C in an alumina crucible. The test coupons were immersed in the molten aluminum alloy. The temperature of the bath was automatically controlled by a temperature controller. After dipping for a predetermined time, the pins were extracted, cooled and analyzed [6]. To understand the diffusion and dissolution-based model further, some of the coupons were ion-nitrided to see if, by providing a diffusion-barrier nitriding layer, whether the intermetallic formation can be prevented. Details of ion-nitriding treatment and nitrided layers are explained elsewhere [6]. Fig. 3 shows the microstructure of the soldered region in the nitrided and un-nitrided H13 steel surfaces after dipping for 5 min in liquid Al 380 at 680 ◦ C. From these microstructures, it can easily be seen that, if the multi-elemental diffusion is prevented by surface treatment, there is no intermetallic formation as seen in the case of nitrided steel.
4. Chemiabsorption and adhesion Molten aluminum has affinity for the H13 die steel surface. This affinity results in chemiabsorption and adhesion of aluminum and formation of intermetallics on the pin surface. This adhered surface substantially increases the ejection force required to separate the casting from the die surface. A tribologically sound surface (well lubricated with no adhesion) will permit clean/ low force ejection of the casting. Adhesive strength between a die casting alloy and die steel is a measure of the soldering strength of the two materials. This adhesive strength was measured by a specially developed test consisting of solidifying a casting around pin samples, and then pulling the casting from the pin using a fixture mounted on an MTS machine [6]. This force of separation is a measure of the adhesive tendency of the casting to the pin surface. A measured amount of A380 was melted in a small crucible and the nitrided pin dipped to a constant depth. After around 15–20 s, the melt solidified onto the dipped pin. This solidified cylinder (casting) was then pulled (ejected) from the pin using a specially designed clamping mechanism. The force of ejection was measured as a function of time. As the ejection speed was held constant, “time” is a measure of the ejection “distance” or displacement of the pin relative to the casting. Fig. 4 shows the plots for the ejection force as a function of time for nitrided pins and the plain H13 pins. It can be seen that the pulling force reaches a value of 200 MPa for the plain H13 pin before the pin is detached from the casting in 14 s. This large value of ejection force is due to the strong bond between the cast metal and the plain H13 pin surface because of the formation of intermettalics. Once this bond is broken (in 10 s), the ejection force quickly drops to 0 and the pin is free to slide. The sliding force is proportional to friction force that depends of roughness of the pin surface as no lubricant is used. For the
Fig. 3. Photomicrographs (200×) of the soldered interface after 5 min of dip in Al 380 at 680 ◦ C. (a) Plain H13 surface has intermetallic layer of approximately 20 m. (b) Nitrided H13 surface (dark band) has no discernable intermetallic layer.
V. Joshi et al. / Wear 256 (2004) 1232–1235
220 200 180 160 140 120 100 80 60 40 20 0
Plain H13 H13 + Nitriding
0 2 3 3 4 5 6 7 8 9 10 11 12 13 14
Ejection Force in MPa
Ejection force test results
1235
• The intermetallic layers are formed based on the thermodynamics and the Gibb’s free energy minimization concepts. • A model has been proposed that is based on the diffusion and dissolution of the die-steel in liquid aluminum to form the intermetallic layers and mass loss. • It was confirmed in the microstructural analysis and ejection tests that, the tribological conditions of the die steel worsen because of the intermetallic layer. • Nitriding prevents formation of intermetallics that results in solder free surface and low forces during ejection.
Time in Seconds
Fig. 4. Ejection force as a function of time for the H13 and nitrided samples as the soldered pins are extracted from the casting.
nitrided pin, no intermetallics form and the bond is weak. Consequently, the ejection force is proportional to the sliding friction between the nitrided surface and the casting. The ejection force reaches a steady state value between 60 and 80 MPa and drops to 0.
5. Conclusions This paper presents a model for soldering growth and dissolution in aluminum die casting using die steel molds. This model is based on the following observations: • The physicochemical interactions between die steel and liquid aluminum lead to intermetallic formation and the mass loss of the die steel by dissolution of the intermetallic layer.
Acknowledgements The authors acknowledge the support received from the office of Industrial Technologies, Department of Energy for the SBIR grant (DE-FG02-98ER82702), Advanced Heat Treat Corp (Dr. Rolinski) and to UES, Inc., Dayton, OH (Drs. Bhattacharya and Dixit). References [1] P. Hairy, M. Richard, Transactions, in: Proceedings of the 19th International Die-Casting Congress and Exposition, NADCA, 1997. [2] S. Chellapilla, MS Thesis, The Ohio State University, Ohio, 1997. [3] M. Sundqvist, J. Bergström, T. Björk, R. Westergård, Transactions, in: Proceedings of the 19th International Die-Casting Congress and Exposition, NADCA, 1997. [4] M. Yu, PhD Dissertation, The Ohio State University, Ohio, 1994. [5] A. Lakare, S. Gopal, R. Shivpuri, T99-111, in: Proceedings of the Transactions of 20th International Die-Casting Congress, NADCA, 1999. [6] V. Joshi, R. Shivpuri, A. Srivastava, R. Bhattacharya, S. Dixit, D. Bhat, Surface Coating and Technology, vol. 146/147, September–October 2001, pp. 338–343.
Intermetallic formation and its relation to interface mass loss and tribology in die casting dies V. Joshi, A. Srivastava, R. Shivpuri∗ Manufacturing Research Group, Department of Industrial, Welding and System Engineering, The Ohio State University, 210 Baker System Engineering Building, 1971 Neil Avenue, Columbus, OH 43210, USA
Abstract Due to high metal injection velocities typical of die casting process, oxide film on die surface breaks exposing virgin steel surface to liquid aluminum at 680 ◦ C. This paper describes in detail the fundamental tribochemical basis for the soldering and mass loss phenomena related to this steel-cast metal interaction. Cylindrical plain H13 coupons are dipped in hot liquid aluminum, kept for predetermined times, cleaned and characterized for surface and substrate changes. The thermodynamics behind the formation of intermetallic compounds based on Gibb’s free energy calculations are presented. Adhesive strength of soldering layer is determined through pull tests on casting solidified around the test pin. From these results, a dissolution–adhesion wear model is proposed for the growth and dissolution of intermetallics and soldering. In addition, this paper includes the soldering and adhesion behavior of nitrided surfaces. It is seen that the diffusion barrier treatment prevents soldering formation and provides for reduced friction and adhesion at the interface. © 2003 Elsevier B.V. All rights reserved. Keywords: Die-casting; Soldering; Dissolution; Intermetallic formation; Nitriding
1. Introduction In the die casting process, molten aluminum alloy at temperatures ranging from 670–710 ◦ C is injected into the die cavity at high velocities of the order of 30–100 m/s. The injection pressures are of the order of 50–80 MPa [1]. These severe thermo-chemical–mechanical operating conditions place considerable stress on the steel surface of the die cavity. In die casting operation, hot working tool steel H13 (composition: 0.4% C, 1.0% Si, 0.4% Mn, 5.0% Cr, 1.5% Mo and 1.0% V [2]) is commonly used as die material and aluminum silicon alloy 380 (with nominal composition of 8.5–9.5% Si, 3% Cu, 1% Fe, 0.5% Mn, 0.5% Ni and maximum 3% Zn [4]) as the cast metal. The injected aluminum–silicon alloy melt has solid particles in it [3]. These particles may include solidified aluminum, inclusions, hypereutectic silicon (in Al–Si alloys) and other phases which precipitate out when the alloy solidifies. The solid particles cause erosion of die surface and any oxides that might form at the surface when they impact the die surface at high injection velocities. This erosive wear depends on gate velocity, angle of attack and amount of hard particles. To resist erosion, die surfaces are hardened and
tempered to retain high hardness at die casting temperatures. But evidence of high erosion has been observed in many situations especially close to the gate area. Due to the high temperatures involved (around 800 ◦ C), chemical reaction and diffusion play an important role at the cast metal–die steel interface. Die steel dissolves in the aluminum alloy melt forming complex aluminum–iron–silicon intermetallics [1,4]. The thermodynamics and kinetics of this reaction is the primary mechanism for the initiation and build up of soldering on the die steel surface. The purpose of this work is to understand the physicochemical drivers that govern the interaction between die steel and liquid aluminum. This understanding is developed by studying the thermodynamics of the intermetallic formation and the mass loss due to dissolution of this intermetallic layer. A diffusion barrier treatment, nitriding, is applied at the interface and its effect in retarding the thermodynamics is studied by investigating the formation of intermetallics and the adhesive strength of the intermetallics (soldered material) to the die steel surface.
2. Intermetallic layers ∗
Corresponding author. Tel.: +1-614-292-7874; fax: +.:+1-614-292-7852. E-mail address: [email protected] (R. Shivpuri). 0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2003.08.001
Intermetallic layers (Alx Fey Siz ) form at the cast metal–die steel interface when the liquid metal is brought in intimate
V. Joshi et al. / Wear 256 (2004) 1232–1235
1233
45
Thickness (10-6m)
40 35 30 25 20 15
T = 710C
10
T = 740C
5 0 0
Fig. 1. Intermetallic layers formed after dipping for 2 h in liquid aluminum at 680 ◦ C.
contact with the die steel. Their morphology depends on the composition of the cast metal and die steel, the temperature of the cast metal and the time of contact. In Fig. 1 is included a photomicrograph of intermetallic layers formed after H13 steel coupon is immersed in A380 alloy melt for 2 h at 680 ◦ C [4]. 2.1. Thermodynamic criteria for intermetallic formation According to the Fe–Al phase diagram, the formation of four intermetallic phases at 680 ◦ C is possible [4]. The free energy of formation of these four phases, FeAl, FeAl2 , Fe2 Al5 and FeAl3 are as follows: Fe(a) + Al(a) ⇒ FeAl(s),
G◦ = −490.6 kcal/mol
FeAl(a) + Al ⇒ FeAl2 (s),
G◦ = −140.3 kcal/mol
FeAl2 (a) + Al ⇒ Fe2 Al5 (s),
G◦ = − 84.83 kcal/mol
Fe2 Al5 (a) + Al ⇒ FeAl3 (s),
G◦ = −120.65 kcal/mol
There are two possible ways an intermetallic layer can be formed at the interface, either by solid-state diffusion or by reaction and diffusion into the melt. The solid-state diffusion will occur when the aluminum from the supersaturated melt deposits on the die steel substrate. This solid-state aluminum–iron diffusion is driven by temperature and concentration gradients. It is a slow process and hence unlikely in die casting where a typical injection cycle is in milliseconds. On the other hand, chemical reactions and diffusion into the melt have much smaller time scales, and are highly likely the dominant mechanism behind intermetallic formation and soldering. The movement of the liquid–solid diffusion front results in mass loss or dissolution. Fig. 2 [5] includes a graph of intermetallic thickness as a function of time at two different immersion temperatures. The bell shaped curve results from the presence of two competing mechanisms: intermetallic growth and dissolution. When the slope of the curve is positive, the growth of the intermetallic layer is primarily due to the physiochemical reaction at the interface between the aluminum in the cast
500
1000
1500
2000
2500
3000
Time (seconds)
Fig. 2. Growth and dissolution of the intermetallic layers.
metal and the iron from the die steel. It is driven by the reaction kinetics. The negative slope (reduction in intermetallic thickness) is due to dissolution of the intermetallic layer in the hot cast metal that is lower in iron content. This dissolution is driven by solubility of iron in Al–Si melt. At the peak of the curve, the rate of diffusion is equal to the rate of dissolution. This dynamic relationship between dissolution and diffusion of iron and aluminum atoms results in weight loss and volumetric loss of the substrate [5]. 2.2. Model for dissolution and mass loss of die steel in liquid aluminum The formation and growth of intermetallic layer in die casting in the absence of a diffusion barrier can be explained as follows: • Stage 1: During melt injection and solidification, diffusion of aluminum and iron atoms across the interface occurs to form Fex Aly Siz intermetallic at the interface. Silicon changes the rate kinetics and the solubility of iron in aluminum. • Stage 2: A new die casting cycle begins and fresh melt enters the die cavity. The driving force for diffusion is slightly lower due to the presence of previous intermetallic layer. There is enough driving force for the intermetallic layer to continue to grow. The driving force for dissolution in this stage is high but still lower than that for diffusion. • Stage 3: In the next cycle, the thickness of the Fex Aly Siz intermetallic layer reaches a critical limit, the driving force for diffusion has decreased to become negligible and the driving force for dissolution becomes the dominating force. The soldering dissolves in the melt. • Stage 4: Mass loss has taken place on the die surface but is free of intermetallic layer. Driving force for diffusion increases due to loss of Fex Aly to the melt but is still negligible compared to the driving force for dissolution. The driving force for dissolution decreases with increasing intermetallic thickness. • Stage 5: The cyclic process of soldering growth and dissolution continues while the die surface continuously loses iron to the melt.
1234
V. Joshi et al. / Wear 256 (2004) 1232–1235
The proposed model of competing mechanisms reasonably explains the higher thickness of intermetallic layer at 710 ◦ C. As the temperature is raised to 740 ◦ C (higher superheat), the dissolution mechanism dominates resulting in lower layer thickness and the maximum thickness (peak) occurring earlier. Similar reductions in intermetallic layers are obtained when the coupons are rotated (increase in melt velocity) and mass transport increases [4].
3. Test results and discussions The test coupons were standard core pins manufactured by DME Inc, USA. These were heat treated as follows: stress relieved for 0.5 h at 537 ◦ C; vacuum hardened for 90 min at 1024 ◦ C; quenched; tempered for 3 h at 537 ◦ C, for 3 h at 1100 ◦ F and for 2 h at 1000 ◦ F. The final hardness was measured as 46–48 HRc. The aluminum–silicon alloy A380 was heated to the melt temperature 680 ◦ C in an alumina crucible. The test coupons were immersed in the molten aluminum alloy. The temperature of the bath was automatically controlled by a temperature controller. After dipping for a predetermined time, the pins were extracted, cooled and analyzed [6]. To understand the diffusion and dissolution-based model further, some of the coupons were ion-nitrided to see if, by providing a diffusion-barrier nitriding layer, whether the intermetallic formation can be prevented. Details of ion-nitriding treatment and nitrided layers are explained elsewhere [6]. Fig. 3 shows the microstructure of the soldered region in the nitrided and un-nitrided H13 steel surfaces after dipping for 5 min in liquid Al 380 at 680 ◦ C. From these microstructures, it can easily be seen that, if the multi-elemental diffusion is prevented by surface treatment, there is no intermetallic formation as seen in the case of nitrided steel.
4. Chemiabsorption and adhesion Molten aluminum has affinity for the H13 die steel surface. This affinity results in chemiabsorption and adhesion of aluminum and formation of intermetallics on the pin surface. This adhered surface substantially increases the ejection force required to separate the casting from the die surface. A tribologically sound surface (well lubricated with no adhesion) will permit clean/ low force ejection of the casting. Adhesive strength between a die casting alloy and die steel is a measure of the soldering strength of the two materials. This adhesive strength was measured by a specially developed test consisting of solidifying a casting around pin samples, and then pulling the casting from the pin using a fixture mounted on an MTS machine [6]. This force of separation is a measure of the adhesive tendency of the casting to the pin surface. A measured amount of A380 was melted in a small crucible and the nitrided pin dipped to a constant depth. After around 15–20 s, the melt solidified onto the dipped pin. This solidified cylinder (casting) was then pulled (ejected) from the pin using a specially designed clamping mechanism. The force of ejection was measured as a function of time. As the ejection speed was held constant, “time” is a measure of the ejection “distance” or displacement of the pin relative to the casting. Fig. 4 shows the plots for the ejection force as a function of time for nitrided pins and the plain H13 pins. It can be seen that the pulling force reaches a value of 200 MPa for the plain H13 pin before the pin is detached from the casting in 14 s. This large value of ejection force is due to the strong bond between the cast metal and the plain H13 pin surface because of the formation of intermettalics. Once this bond is broken (in 10 s), the ejection force quickly drops to 0 and the pin is free to slide. The sliding force is proportional to friction force that depends of roughness of the pin surface as no lubricant is used. For the
Fig. 3. Photomicrographs (200×) of the soldered interface after 5 min of dip in Al 380 at 680 ◦ C. (a) Plain H13 surface has intermetallic layer of approximately 20 m. (b) Nitrided H13 surface (dark band) has no discernable intermetallic layer.
V. Joshi et al. / Wear 256 (2004) 1232–1235
220 200 180 160 140 120 100 80 60 40 20 0
Plain H13 H13 + Nitriding
0 2 3 3 4 5 6 7 8 9 10 11 12 13 14
Ejection Force in MPa
Ejection force test results
1235
• The intermetallic layers are formed based on the thermodynamics and the Gibb’s free energy minimization concepts. • A model has been proposed that is based on the diffusion and dissolution of the die-steel in liquid aluminum to form the intermetallic layers and mass loss. • It was confirmed in the microstructural analysis and ejection tests that, the tribological conditions of the die steel worsen because of the intermetallic layer. • Nitriding prevents formation of intermetallics that results in solder free surface and low forces during ejection.
Time in Seconds
Fig. 4. Ejection force as a function of time for the H13 and nitrided samples as the soldered pins are extracted from the casting.
nitrided pin, no intermetallics form and the bond is weak. Consequently, the ejection force is proportional to the sliding friction between the nitrided surface and the casting. The ejection force reaches a steady state value between 60 and 80 MPa and drops to 0.
5. Conclusions This paper presents a model for soldering growth and dissolution in aluminum die casting using die steel molds. This model is based on the following observations: • The physicochemical interactions between die steel and liquid aluminum lead to intermetallic formation and the mass loss of the die steel by dissolution of the intermetallic layer.
Acknowledgements The authors acknowledge the support received from the office of Industrial Technologies, Department of Energy for the SBIR grant (DE-FG02-98ER82702), Advanced Heat Treat Corp (Dr. Rolinski) and to UES, Inc., Dayton, OH (Drs. Bhattacharya and Dixit). References [1] P. Hairy, M. Richard, Transactions, in: Proceedings of the 19th International Die-Casting Congress and Exposition, NADCA, 1997. [2] S. Chellapilla, MS Thesis, The Ohio State University, Ohio, 1997. [3] M. Sundqvist, J. Bergström, T. Björk, R. Westergård, Transactions, in: Proceedings of the 19th International Die-Casting Congress and Exposition, NADCA, 1997. [4] M. Yu, PhD Dissertation, The Ohio State University, Ohio, 1994. [5] A. Lakare, S. Gopal, R. Shivpuri, T99-111, in: Proceedings of the Transactions of 20th International Die-Casting Congress, NADCA, 1999. [6] V. Joshi, R. Shivpuri, A. Srivastava, R. Bhattacharya, S. Dixit, D. Bhat, Surface Coating and Technology, vol. 146/147, September–October 2001, pp. 338–343.