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Jens Bergstrom - One of the best experts on this subject based on the ideXlab platform.

  • simulation and evaluation of thermal fatigue cracking of hot work tool steels
    International Journal of Fatigue, 2004
    Co-Authors: Anders Persson, Sture Hogmark, Jens Bergstrom
    Abstract:

    Abstract Die casting is a very cost efficient method to manufacture near net-shaped and complex cast products. One limitation for further cost reduction is fatigue cracking of the tool due to thermal cycling, which is observed as a crack network on the tool surface. Hot work tool steels are commonly used as die material. In this study, an experimental test machine for simulation of thermal fatigue is described. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded during the thermal cycling through a non-contact laser speckle technique. The applicability of the test is demonstrated on two hot work tool steel grades, hardened and tempered to different conditions, and heat cycled between Tmin 170 °C and Tmax 600–850 °C. It is shown that the test method can simulate surface cracking of tools exposed to thermal fatigue. The surface strain recordings proved to give sufficient information to successfully deduce the strains and stresses behind the mechanism of thermal fatigue surface cracking, without knowledge of the temperature distribution below the surface. It was also found that low-cycle fatigue occurs for the tests with Tmax 600 and 700 °C, although the estimated tensile stress after cooling does not exceed the Initial Yield Strength of the steel. Most probably, the reason is the gradual softening of the tool steels during the thermal cycling. Additionally, the presence of stress concentrators play a critical role during these conditions.

  • simulation and evaluation of thermal fatigue cracking of hot work tool steels
    International Journal of Fatigue, 2004
    Co-Authors: Anders Persson, Sture Hogmark, Jens Bergstrom
    Abstract:

    Die casting is a very cost-efficient method of forming thin-walled and complex near net-shaped products with close geometric tolerances and good surface finish. A permanent die tool is used to make large quantities of identical products. The performance and tool life are limited by several mechanisms, e.g. thermal fatigue cracking, erosion, and corrosion. To develop new and more resistant tool materials for die casting detailed knowledge of the actual casting conditions and the tool failure mechanisms are essential. This thesis contributes to an increased knowledge of tool failure in die casting by investigating and simulating actual casting conditions and tool failure mechanisms.A method to record the temperature fluctuations in a cavity insert during actual brass die casting was developed, and details of the temperature conditions were obtained. Also, a test method based on cyclic induction heating and internal cooling of hollow cylindrical test rods was developed, where the surface strain during thermal cycling could be measured. This method reproduced the characteristic type of surface cracking observed on die casting tools, and proved to give information of the strains and stresses behind the fatigue failure.In actual die casting, the dominant tool failure mechanism is thermal fatigue cracking. The formation of the cracks is associated to accumulation of the local plastic strain that occurs during each casting cycle. Initial crack growth is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by this oxidation in combination with crack filling of cast material, and by softening of the tool material. In addition, local enrichment of Pb at the crack front from the cast alloy melt was also observed to promote the crack growth in die casting of brass.In an investigation of thermal fatigue of two hot work tool steels, quenched and tempered to different conditions, it was found that low-cycle fatigue occurs, although the estimated tensile stress never exceed the Initial Yield Strength of the steel. The reason is a gradual softening of the steel during the thermal cycling, and the presence of stress raising defects. The resistance against thermal cracking improves with Initial tool steel hardness, because any Initial ranking in hardness among the steels is unaffected by the thermal cycling.Another investigation on a selection of surface engineered tool steels, including common diffusion treatments, PVD coatings and combinations of these, showed that surface engineering generally reduce the resistance against thermal cracking as compared to untreated references, since the engineering processes influence negatively on the mechanical properties of the hot work tool steels.Finally, corrosion tests of CrN PVD-coated tool steels by exposing them to molten aluminium revealed the mechanisms of initiation and progress of liquid metal corrosion of this material combination, and that the corrosion resistance improves with the CrN coating thickness.

  • Thermal fatigue of a tool steel : experiment and numerical simulation
    2002
    Co-Authors: Vincent Velay, Anders Persson, Gérard Bernhart, Luc Penazzi, Jens Bergstrom
    Abstract:

    Die casting is a very cost-efficient method of forming thin-walled and complex near net-shaped products with close geometric tolerances and good surface finish. A permanent die tool is used to make large quantities of identical products. The performance and tool life are limited by several mechanisms, e.g. thermal fatigue cracking, erosion, and corrosion. To develop new and more resistant tool materials for die casting detailed knowledge of the actual casting conditions and the tool failure mechanisms are essential. This thesis contributes to an increased knowledge of tool failure in die casting by investigating and simulating actual casting conditions and tool failure mechanisms.A method to record the temperature fluctuations in a cavity insert during actual brass die casting was developed, and details of the temperature conditions were obtained. Also, a test method based on cyclic induction heating and internal cooling of hollow cylindrical test rods was developed, where the surface strain during thermal cycling could be measured. This method reproduced the characteristic type of surface cracking observed on die casting tools, and proved to give information of the strains and stresses behind the fatigue failure.In actual die casting, the dominant tool failure mechanism is thermal fatigue cracking. The formation of the cracks is associated to accumulation of the local plastic strain that occurs during each casting cycle. Initial crack growth is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by this oxidation in combination with crack filling of cast material, and by softening of the tool material. In addition, local enrichment of Pb at the crack front from the cast alloy melt was also observed to promote the crack growth in die casting of brass.In an investigation of thermal fatigue of two hot work tool steels, quenched and tempered to different conditions, it was found that low-cycle fatigue occurs, although the estimated tensile stress never exceed the Initial Yield Strength of the steel. The reason is a gradual softening of the steel during the thermal cycling, and the presence of stress raising defects. The resistance against thermal cracking improves with Initial tool steel hardness, because any Initial ranking in hardness among the steels is unaffected by the thermal cycling.Another investigation on a selection of surface engineered tool steels, including common diffusion treatments, PVD coatings and combinations of these, showed that surface engineering generally reduce the resistance against thermal cracking as compared to untreated references, since the engineering processes influence negatively on the mechanical properties of the hot work tool steels.Finally, corrosion tests of CrN PVD-coated tool steels by exposing them to molten aluminium revealed the mechanisms of initiation and progress of liquid metal corrosion of this material combination, and that the corrosion resistance improves with the CrN coating thickness.

Anders Persson - One of the best experts on this subject based on the ideXlab platform.

  • simulation and evaluation of thermal fatigue cracking of hot work tool steels
    International Journal of Fatigue, 2004
    Co-Authors: Anders Persson, Sture Hogmark, Jens Bergstrom
    Abstract:

    Abstract Die casting is a very cost efficient method to manufacture near net-shaped and complex cast products. One limitation for further cost reduction is fatigue cracking of the tool due to thermal cycling, which is observed as a crack network on the tool surface. Hot work tool steels are commonly used as die material. In this study, an experimental test machine for simulation of thermal fatigue is described. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded during the thermal cycling through a non-contact laser speckle technique. The applicability of the test is demonstrated on two hot work tool steel grades, hardened and tempered to different conditions, and heat cycled between Tmin 170 °C and Tmax 600–850 °C. It is shown that the test method can simulate surface cracking of tools exposed to thermal fatigue. The surface strain recordings proved to give sufficient information to successfully deduce the strains and stresses behind the mechanism of thermal fatigue surface cracking, without knowledge of the temperature distribution below the surface. It was also found that low-cycle fatigue occurs for the tests with Tmax 600 and 700 °C, although the estimated tensile stress after cooling does not exceed the Initial Yield Strength of the steel. Most probably, the reason is the gradual softening of the tool steels during the thermal cycling. Additionally, the presence of stress concentrators play a critical role during these conditions.

  • simulation and evaluation of thermal fatigue cracking of hot work tool steels
    International Journal of Fatigue, 2004
    Co-Authors: Anders Persson, Sture Hogmark, Jens Bergstrom
    Abstract:

    Die casting is a very cost-efficient method of forming thin-walled and complex near net-shaped products with close geometric tolerances and good surface finish. A permanent die tool is used to make large quantities of identical products. The performance and tool life are limited by several mechanisms, e.g. thermal fatigue cracking, erosion, and corrosion. To develop new and more resistant tool materials for die casting detailed knowledge of the actual casting conditions and the tool failure mechanisms are essential. This thesis contributes to an increased knowledge of tool failure in die casting by investigating and simulating actual casting conditions and tool failure mechanisms.A method to record the temperature fluctuations in a cavity insert during actual brass die casting was developed, and details of the temperature conditions were obtained. Also, a test method based on cyclic induction heating and internal cooling of hollow cylindrical test rods was developed, where the surface strain during thermal cycling could be measured. This method reproduced the characteristic type of surface cracking observed on die casting tools, and proved to give information of the strains and stresses behind the fatigue failure.In actual die casting, the dominant tool failure mechanism is thermal fatigue cracking. The formation of the cracks is associated to accumulation of the local plastic strain that occurs during each casting cycle. Initial crack growth is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by this oxidation in combination with crack filling of cast material, and by softening of the tool material. In addition, local enrichment of Pb at the crack front from the cast alloy melt was also observed to promote the crack growth in die casting of brass.In an investigation of thermal fatigue of two hot work tool steels, quenched and tempered to different conditions, it was found that low-cycle fatigue occurs, although the estimated tensile stress never exceed the Initial Yield Strength of the steel. The reason is a gradual softening of the steel during the thermal cycling, and the presence of stress raising defects. The resistance against thermal cracking improves with Initial tool steel hardness, because any Initial ranking in hardness among the steels is unaffected by the thermal cycling.Another investigation on a selection of surface engineered tool steels, including common diffusion treatments, PVD coatings and combinations of these, showed that surface engineering generally reduce the resistance against thermal cracking as compared to untreated references, since the engineering processes influence negatively on the mechanical properties of the hot work tool steels.Finally, corrosion tests of CrN PVD-coated tool steels by exposing them to molten aluminium revealed the mechanisms of initiation and progress of liquid metal corrosion of this material combination, and that the corrosion resistance improves with the CrN coating thickness.

  • Thermal fatigue of a tool steel : experiment and numerical simulation
    2002
    Co-Authors: Vincent Velay, Anders Persson, Gérard Bernhart, Luc Penazzi, Jens Bergstrom
    Abstract:

    Die casting is a very cost-efficient method of forming thin-walled and complex near net-shaped products with close geometric tolerances and good surface finish. A permanent die tool is used to make large quantities of identical products. The performance and tool life are limited by several mechanisms, e.g. thermal fatigue cracking, erosion, and corrosion. To develop new and more resistant tool materials for die casting detailed knowledge of the actual casting conditions and the tool failure mechanisms are essential. This thesis contributes to an increased knowledge of tool failure in die casting by investigating and simulating actual casting conditions and tool failure mechanisms.A method to record the temperature fluctuations in a cavity insert during actual brass die casting was developed, and details of the temperature conditions were obtained. Also, a test method based on cyclic induction heating and internal cooling of hollow cylindrical test rods was developed, where the surface strain during thermal cycling could be measured. This method reproduced the characteristic type of surface cracking observed on die casting tools, and proved to give information of the strains and stresses behind the fatigue failure.In actual die casting, the dominant tool failure mechanism is thermal fatigue cracking. The formation of the cracks is associated to accumulation of the local plastic strain that occurs during each casting cycle. Initial crack growth is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by this oxidation in combination with crack filling of cast material, and by softening of the tool material. In addition, local enrichment of Pb at the crack front from the cast alloy melt was also observed to promote the crack growth in die casting of brass.In an investigation of thermal fatigue of two hot work tool steels, quenched and tempered to different conditions, it was found that low-cycle fatigue occurs, although the estimated tensile stress never exceed the Initial Yield Strength of the steel. The reason is a gradual softening of the steel during the thermal cycling, and the presence of stress raising defects. The resistance against thermal cracking improves with Initial tool steel hardness, because any Initial ranking in hardness among the steels is unaffected by the thermal cycling.Another investigation on a selection of surface engineered tool steels, including common diffusion treatments, PVD coatings and combinations of these, showed that surface engineering generally reduce the resistance against thermal cracking as compared to untreated references, since the engineering processes influence negatively on the mechanical properties of the hot work tool steels.Finally, corrosion tests of CrN PVD-coated tool steels by exposing them to molten aluminium revealed the mechanisms of initiation and progress of liquid metal corrosion of this material combination, and that the corrosion resistance improves with the CrN coating thickness.

Sture Hogmark - One of the best experts on this subject based on the ideXlab platform.

  • simulation and evaluation of thermal fatigue cracking of hot work tool steels
    International Journal of Fatigue, 2004
    Co-Authors: Anders Persson, Sture Hogmark, Jens Bergstrom
    Abstract:

    Abstract Die casting is a very cost efficient method to manufacture near net-shaped and complex cast products. One limitation for further cost reduction is fatigue cracking of the tool due to thermal cycling, which is observed as a crack network on the tool surface. Hot work tool steels are commonly used as die material. In this study, an experimental test machine for simulation of thermal fatigue is described. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded during the thermal cycling through a non-contact laser speckle technique. The applicability of the test is demonstrated on two hot work tool steel grades, hardened and tempered to different conditions, and heat cycled between Tmin 170 °C and Tmax 600–850 °C. It is shown that the test method can simulate surface cracking of tools exposed to thermal fatigue. The surface strain recordings proved to give sufficient information to successfully deduce the strains and stresses behind the mechanism of thermal fatigue surface cracking, without knowledge of the temperature distribution below the surface. It was also found that low-cycle fatigue occurs for the tests with Tmax 600 and 700 °C, although the estimated tensile stress after cooling does not exceed the Initial Yield Strength of the steel. Most probably, the reason is the gradual softening of the tool steels during the thermal cycling. Additionally, the presence of stress concentrators play a critical role during these conditions.

  • simulation and evaluation of thermal fatigue cracking of hot work tool steels
    International Journal of Fatigue, 2004
    Co-Authors: Anders Persson, Sture Hogmark, Jens Bergstrom
    Abstract:

    Die casting is a very cost-efficient method of forming thin-walled and complex near net-shaped products with close geometric tolerances and good surface finish. A permanent die tool is used to make large quantities of identical products. The performance and tool life are limited by several mechanisms, e.g. thermal fatigue cracking, erosion, and corrosion. To develop new and more resistant tool materials for die casting detailed knowledge of the actual casting conditions and the tool failure mechanisms are essential. This thesis contributes to an increased knowledge of tool failure in die casting by investigating and simulating actual casting conditions and tool failure mechanisms.A method to record the temperature fluctuations in a cavity insert during actual brass die casting was developed, and details of the temperature conditions were obtained. Also, a test method based on cyclic induction heating and internal cooling of hollow cylindrical test rods was developed, where the surface strain during thermal cycling could be measured. This method reproduced the characteristic type of surface cracking observed on die casting tools, and proved to give information of the strains and stresses behind the fatigue failure.In actual die casting, the dominant tool failure mechanism is thermal fatigue cracking. The formation of the cracks is associated to accumulation of the local plastic strain that occurs during each casting cycle. Initial crack growth is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by this oxidation in combination with crack filling of cast material, and by softening of the tool material. In addition, local enrichment of Pb at the crack front from the cast alloy melt was also observed to promote the crack growth in die casting of brass.In an investigation of thermal fatigue of two hot work tool steels, quenched and tempered to different conditions, it was found that low-cycle fatigue occurs, although the estimated tensile stress never exceed the Initial Yield Strength of the steel. The reason is a gradual softening of the steel during the thermal cycling, and the presence of stress raising defects. The resistance against thermal cracking improves with Initial tool steel hardness, because any Initial ranking in hardness among the steels is unaffected by the thermal cycling.Another investigation on a selection of surface engineered tool steels, including common diffusion treatments, PVD coatings and combinations of these, showed that surface engineering generally reduce the resistance against thermal cracking as compared to untreated references, since the engineering processes influence negatively on the mechanical properties of the hot work tool steels.Finally, corrosion tests of CrN PVD-coated tool steels by exposing them to molten aluminium revealed the mechanisms of initiation and progress of liquid metal corrosion of this material combination, and that the corrosion resistance improves with the CrN coating thickness.

Erik Van Der Giessen - One of the best experts on this subject based on the ideXlab platform.

  • distribution of dislocation source length and the size dependent Yield Strength in freestanding thin films
    Journal of The Mechanics and Physics of Solids, 2010
    Co-Authors: Siamak Soleymani Shishvan, Erik Van Der Giessen
    Abstract:

    Abstract A method is proposed to estimate the size-dependent Yield Strength of columnar-grained freestanding thin films. The estimate relies on assuming a distribution of the size of Frank–Read sources, which is then translated into a log-normal distribution of the source Strength, depending on film thickness, grain size and theoretical Strength of the material, augmented with a single fit parameter. Two-dimensional discrete dislocation plasticity (DDP) simulations are carried out for two sets of Cu films and the fit parameter is determined from independent experiments. Subsequent DDP predictions of the stress–strain curves in comparison with the corresponding experimental data show excellent agreement of Initial Yield Strength and hardening rate for films of varying film thickness and grain size. Having thus demonstrated the power of the proposed Strength distribution, it is shown that the mode of this distribution governs the most effective source Strength. This is then used to suggest a method to estimate the Yield Strength of thin films as a function of film thickness and grain size. Simple maps are presented that are in very good agreement with recent experimental results for Cu thin films.

David J. Dunstan - One of the best experts on this subject based on the ideXlab platform.

  • Indentation size effect at the initiation of plasticity for ceramics and metals
    Journal of Physics D: Applied Physics, 2008
    Co-Authors: T. T. Zhu, Xiaodong Hou, Andrew J. Bushby, David J. Dunstan
    Abstract:

    In nanoindentation, the plasticity size effect has been observed for several years, where a higher hardness is measured as contact size decreases. For spherical indenters, Lim and Chaudhri (1999 Phil. Mag. 79 2979) first showed that the entire flow curve appears at higher contact pressures for smaller radius indenters in copper. However, few papers have reported the Initial Yield size effect due to the difficulty in defining the Yield point. Recently, Spary et al (2006 Phil. Mag. 86 5581) demonstrated that the Initial Yield Strength of metals increases linearly with inverse cube root of indenter radius, by nanoindentation together with finite elemental modelling. Here, we use a clear method to determine the onset of plasticity in spherical nanoindentation without the uncertainties of modelling. This enables us to measure the Yield pressure of tungsten metal and a series of ceramics with a high degree of accuracy and over a large range of indenter radii (hundreds of nanometres to several tens of micrometres). Our data of all ceramics and metals show clearly that there is a significant Yield Strength enhancement, which is inversely proportional to the cube root of the indenter radius. Normalizing the data for each material by its Yield pressure for an infinite radius indenter, we find that the data for metals and ceramics fall on lines of different slope, indicating that material parameters influence the indentation Yield Strength size effect as well as the geometrical size effect.