This work is mainly an experimental study of work-hardening in age-hardenable Al-Zn-Mg(-Cu) alloy... more This work is mainly an experimental study of work-hardening in age-hardenable Al-Zn-Mg(-Cu) alloys. The main objective has been to characterize the microstructure and mechanical properties and evaluate the data with respect to already existing work-hardening models. The thesis is organized into a synopsis, providing information about the background for the study, the objectives, the experimental techniques and a summary of the work. The synopsis is followed by four papers and a report. The first two papers consider a commercial AA7108 aluminium alloy in the as-cast and homogenized state, heat treated to different tempers and deformed in uniaxial tension and compression, respectively. An extended Voce equation has been useful for analyzing the work-hardening behaviour in tension and in compression up to larger strains. In both papers two tempers have been selected for further investigations of the precipitate size and distribution, and the influence of the microstructure on the work-...
Three-point bending tests of extruded aluminium alloys have shown lower bendability when the bend... more Three-point bending tests of extruded aluminium alloys have shown lower bendability when the bending axis is aligned with the extrusion direction compared to the transverse direction. In an attempt to explain the observed anisotropy in the mechanical behaviour, the microstructure of underaged and overaged fibrous extruded 7xxx aluminium alloys has been studied with respect to distribution of primary particles and fracture propagation. The microstructure of the deformed materials reveals different fracture mechanisms depending on the direction of the bending axis. These mechanisms are independent of the ageing condition, indicating that the grain morphology and alignment of primary particles give rise to the observed anisotropy.
In traditional reinforced concrete, the alkaline pore solution which passivates the steel rebars ... more In traditional reinforced concrete, the alkaline pore solution which passivates the steel rebars will get neutralized with time in an exposed environment. Therefore, to prevent corrosion initiation, the permeability of the concrete is reduced and extra-thick concrete covers the steel rebars. Aluminum is passive in the neutralized environment, but the calcium hydroxide formed during the cement hydration will dissolve the aluminum. By substituting 55% of the cement in traditional cement paste with fast reactive supplementary cementitious material (SCM), aluminum will be compatible over time. In the initial state however, before the SCM consumes the hydroxide formed during the rapid cement hydration by the pozzolanic reaction, aluminum may corrode. Hydrogen gas then develops, resulting in a porous cement region enclosing the rebars with potentially reduced bond strength. In the present work, the chemical stability of a sand-cast aluminum lattice embedded in a paste where cement is repl...
In tri-layered Cold Roll Bonded (CRB) composite sheets of dissimilar metals, uneven thickness red... more In tri-layered Cold Roll Bonded (CRB) composite sheets of dissimilar metals, uneven thickness reduction of the different layers have been observed. This has been explained by the difference in yield strength/flow stress of the metals. The aim of this research was to study if these observations also depend on different parameters such as stacking sequence and initial sheet thickness, as opposed to only material properties. Hence, tri-layered CRB composite sheets consisting of AA6082 and IF-steel were produced with two different stacking sequences, St/Al/St vs Al/St/Al. Two different layer thicknesses of the intermediate layer were used. Rolling was performed in a single pass achieving approximately 55-65% total thickness reduction. Comparisons between the samples are given, focusing on the reduction of thickness achieved in each layer, the roll bonded interface characteristics and the bond strength of the joints. The overall thickness reduction achieved in each layer was found to be ...
The application of gold speckles on a polished surface is a successful technique for improving di... more The application of gold speckles on a polished surface is a successful technique for improving digital image correlation (DIC) contrast in scanning electron microscope (SEM) images. In the process of creating the gold speckles, the material is subjected to elevated temperatures for prolonged times. As a consequence, not all materials are suitable for the gold speckled method to improve the contrast for DIC measurements during an in-situ SEM tensile test. In this letter, the effect of gold remodeling on two different steels is investigated. These steels are a dual-phase (DP) steel and a ferrite–pearlite steel (NVE36). The results demonstrate that the temperature these steels are subjected to during gold remodeling will influence the tensile behavior of the DP steel while the NVE36 steel is unaffected by the heat treatment. As a result, we can conclude that the gold remodeling method for creating contrast in SEM images may affect the microstructure. However, the effect of these change...
Al–Mg–Si alloys are usually formed into their final shape by rolling or extrusion. After extrusio... more Al–Mg–Si alloys are usually formed into their final shape by rolling or extrusion. After extrusion, the aluminium profiles are usually straightened, causing the material to be subjected to a small plastic deformation. This study demonstrates the positive effect on strength that can be obtained from such small deformation levels or from only elastically straining the material. Elastic straining of a lean Al–Mg–Si alloy, when performed immediately after solution heat treatment, enhances the material yield strength after artificial ageing to T6. Transmission electron microscopy shows that this effect can be attributed to a higher number density and finer dispersion of the age-hardening precipitate needles. Furthermore, introducing a small plastic deformation of 1% after solution heat treatment results in a comparable strength increase to elastically straining the material. In this case, however, the strength increase is due to the increased dislocation density, which compensates for a ...
A ferrite-pearlite two-phase steel was investigated using in situ scanning electron microscope (S... more A ferrite-pearlite two-phase steel was investigated using in situ scanning electron microscope (SEM) tensile testing combined with digital image correlation (DIC). Two different speckled patterns were used and compared. The first pattern was achieved by etching a polished surface in order to reveal the microstructural features. Second, a gold speckled pattern was obtained. Here, a continuous layer of gold was applied to a polished surface. This continuous layer was remodeled into gold nanoparticles by keeping the specimen at 180 °C for 96 h with an Ar/Styrene mixture flowing across the specimen surface. The result is randomly distributed gold nanoparticles on the surface. These particles and the etched microstructure were then used by the DIC software to correlate an image series to obtain the local strain field of the material. The differences between the two techniques are numerous. Considering the etched surface, most microstructural features were grain boundaries and pearlite la...
An in situ scanning electron microscope (SEM) study was conducted on a super duplex stainless ste... more An in situ scanning electron microscope (SEM) study was conducted on a super duplex stainless steel (SDSS) containing 0%, 5% and 10% σ-phase. The material was heat treated at 850°C for 12 min and 15 min, respectively, to achieve the different amounts of σ-phase. The specimens were investigated at room temperature and at −40°C. The microstructure evolution during the deformation process was recorded using electron backscatter diffraction (EBSD) at different strain levels. Both σ-phase and χ-phase were observed along the grain boundaries in the microstructure in all heat treated specimens. Cracks started to form after 3-4% strain and were always oriented perpendicular to the tensile direction. After the cracks formed, they were initially arrested by the matrix. At later stages of the deformation process, cracks in larger σ-phase constituents started to coalesce. When the tensile test was conducted at −40°C, the ductility increased for the specimen without σ-phase, but with σ-phase present, the ductility was slightly reduced. With larger amounts of σ-phase present, however, an increase in tensile strength was also observed. With χ-phase present along the grain boundaries, a reduction of tensile strength was observed. This reduction seems to be related to χ-phase precipitating at the grain boundaries, creating imperfections, but not contributing towards the increase in strength. Compared to the effect of σ-phase, the low temperature is not as influential on the materials performance.
PROCEEDINGS OF THE 22ND INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2019
Metallurgical joining of steel and aluminum has become a field of interest in research in the las... more Metallurgical joining of steel and aluminum has become a field of interest in research in the last years, as it will allow the production of load adapted lightweight components in the future. The production process of cold extrusion enables both, forming and joining at the same time obtaining high bond strengths in the final component. However, the reachable relative bond strength varies highly between different material combinations. This presented paper focuses on the influence of the silicon amount in EN AW 6082 and thus the influence on the reachable bond strength. The silicon content is varied between 0.7 and 1.3 wt%, while the magnesium content is kept constant at 0.6 wt %, which is within the range of the specifications of the commercially available alloy. Before joining the materials were aged to their T6 state, as well as a state of similar hardness despite the different silicon contents. The results show a significant influence of the silicon, as only joints produced with the material with higher silicon content reach bond strengths of 150 MPa. Differences of the material flow and hardness increase due to the deformation of the joined components were also observed. To further understand the mechanism behind these observations, FE-simulations were conducted to show the material flow and therefore, the reasons for the different strengths and geometries.
Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, whi... more Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, which may act as nucleation sites for Mg2Si particles after homogenization. The purpose of adding Mn and Cr is to ensure a high density of uniformly distributed small β’-particles, which can be dissolved during further processing prior to the final age hardening step. However, their density and spatial distribution are critically dependent on the homogenization procedure. It is therefore important to have a robust and reliable method for assessing their spatial distribution. In the present work, an existing methodology for assessing spatial uniformity, the Global Shannon Entropy (GSE), was implemented and evaluated for different dispersoid structures characterized by scanning electron microscopy. This metric is highly dependent on the parameters used, but by careful selection of adequate parameters, it can be effective in detecting non-uniformity. An important weakness with the GSE was iden...
Under the concept of "Industry 4.0", production processes will be pushed to be increasingly inter... more Under the concept of "Industry 4.0", production processes will be pushed to be increasingly interconnected, information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization goes beyond the traditional aim of capacity maximization, contributing also for organization's profitability and value. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of maximization. The study of capacity optimization and costing models is an important research topic that deserves contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical model for capacity management based on different costing models (ABC and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization's value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity optimization might hide operational inefficiency.
Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, whi... more Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, which may act as nucleation sites for Mg 2 Si particles after homogenization. The purpose of adding Mn and Cr is to ensure a high density of uniformly distributed small β'-particles, which can be dissolved during further processing prior to the final age hardening step. However, their density and spatial distribution are critically dependent on the homogenization procedure. It is therefore important to have a robust and reliable method for assessing their spatial distribution. In the present work, an existing methodology for assessing spatial uniformity, the Global Shannon Entropy (GSE), was implemented and evaluated for different dispersoid structures characterized by scanning electron microscopy. This metric is highly dependent on the parameters used, but by careful selection of adequate parameters, it can be effective in detecting non-uniformity. An important weakness with the GSE was identified, and a modification to improve on the ability to differentiate degrees of non-uniformity is suggested. To evaluate the proposed methodology, the effect of heating rate on dispersoid precipitation behaviour during homogenisation of four Al-Mg-Si alloys with different Mn/Cr-content was investigated. In general, the dispersoid density increased with decreasing heating rates as longer times in the furnace resulted in particle coarsening. Slower heating rates were found to promote a more uniform dispersoid structure for most alloys investigated in this study. The metric with the new term demonstrated promising results, and improved the ability to differentiate degrees of spatial uniformity.
A constitutive model with microstructure, strength, work hardening submodels is developed for art... more A constitutive model with microstructure, strength, work hardening submodels is developed for artificial ageing of AA2050. • The model has for the first time successfully predicted ageing behaviour covering under-ageing to over-ageing. • Yield strength and work hardening behaviour from under-aged to over-aged conditions has been characterised and modelled. • Shearing-to-bypassing strengthening transition does not occur immediately when reaching peak-ageing state.
ABSTRACT In the 7xxx aluminium alloying system several mechanisms influence the hardening behavio... more ABSTRACT In the 7xxx aluminium alloying system several mechanisms influence the hardening behaviour of the alloys, e.g. particle size and distribution, dislocation density and alloying elements in solid solution. This work is an experimental study of ageing and work-hardening considering a commercial AA7108 alloy in the as-cast and homogenized condition. Tensile specimens have been exposed to a solution heat treatment and a two-step age-hardening treatment with varying time at the final temperature. The tensile data for the different tempers have been evaluated in elucidation of already existing models based on a one-parameter framework. The precipitate size and distribution have been further investigated in the transmission electron microscope for a selection of tempers, and the influence of these parameters on the work-hardening behaviour has been discussed.
Al-Mg-Si alloys often add small additions of e.g. Mn or Cr, to form dispersoids, which may act as... more Al-Mg-Si alloys often add small additions of e.g. Mn or Cr, to form dispersoids, which may act as nucleation sites for Mg2Si particles after homogenization. The purpose is to ensure a high density of uniformly distributed small ’-particles, which can be dissolved during further processing prior to the final age hardening step. However, their density and spatial distribution are critically dependent on the homogenization procedure. It is therefore important to have a robust and reliable method for assessing their spatial distribution. In the present work an existing methodology for assessing spatial uniformity, the Global Shannon Entropy (GSE), has been implemented and evaluated for different dispersoid structures characterized by scanning electron microscopy. This metric is highly dependent on the parameters used, but by careful selection of adequate parameters, it can be effective in detecting non-uniformity. An important weakness with the GSE was identified, and a modification to...
This work is mainly an experimental study of work-hardening in age-hardenable Al-Zn-Mg(-Cu) alloy... more This work is mainly an experimental study of work-hardening in age-hardenable Al-Zn-Mg(-Cu) alloys. The main objective has been to characterize the microstructure and mechanical properties and evaluate the data with respect to already existing work-hardening models. The thesis is organized into a synopsis, providing information about the background for the study, the objectives, the experimental techniques and a summary of the work. The synopsis is followed by four papers and a report. The first two papers consider a commercial AA7108 aluminium alloy in the as-cast and homogenized state, heat treated to different tempers and deformed in uniaxial tension and compression, respectively. An extended Voce equation has been useful for analyzing the work-hardening behaviour in tension and in compression up to larger strains. In both papers two tempers have been selected for further investigations of the precipitate size and distribution, and the influence of the microstructure on the work-...
Three-point bending tests of extruded aluminium alloys have shown lower bendability when the bend... more Three-point bending tests of extruded aluminium alloys have shown lower bendability when the bending axis is aligned with the extrusion direction compared to the transverse direction. In an attempt to explain the observed anisotropy in the mechanical behaviour, the microstructure of underaged and overaged fibrous extruded 7xxx aluminium alloys has been studied with respect to distribution of primary particles and fracture propagation. The microstructure of the deformed materials reveals different fracture mechanisms depending on the direction of the bending axis. These mechanisms are independent of the ageing condition, indicating that the grain morphology and alignment of primary particles give rise to the observed anisotropy.
In traditional reinforced concrete, the alkaline pore solution which passivates the steel rebars ... more In traditional reinforced concrete, the alkaline pore solution which passivates the steel rebars will get neutralized with time in an exposed environment. Therefore, to prevent corrosion initiation, the permeability of the concrete is reduced and extra-thick concrete covers the steel rebars. Aluminum is passive in the neutralized environment, but the calcium hydroxide formed during the cement hydration will dissolve the aluminum. By substituting 55% of the cement in traditional cement paste with fast reactive supplementary cementitious material (SCM), aluminum will be compatible over time. In the initial state however, before the SCM consumes the hydroxide formed during the rapid cement hydration by the pozzolanic reaction, aluminum may corrode. Hydrogen gas then develops, resulting in a porous cement region enclosing the rebars with potentially reduced bond strength. In the present work, the chemical stability of a sand-cast aluminum lattice embedded in a paste where cement is repl...
In tri-layered Cold Roll Bonded (CRB) composite sheets of dissimilar metals, uneven thickness red... more In tri-layered Cold Roll Bonded (CRB) composite sheets of dissimilar metals, uneven thickness reduction of the different layers have been observed. This has been explained by the difference in yield strength/flow stress of the metals. The aim of this research was to study if these observations also depend on different parameters such as stacking sequence and initial sheet thickness, as opposed to only material properties. Hence, tri-layered CRB composite sheets consisting of AA6082 and IF-steel were produced with two different stacking sequences, St/Al/St vs Al/St/Al. Two different layer thicknesses of the intermediate layer were used. Rolling was performed in a single pass achieving approximately 55-65% total thickness reduction. Comparisons between the samples are given, focusing on the reduction of thickness achieved in each layer, the roll bonded interface characteristics and the bond strength of the joints. The overall thickness reduction achieved in each layer was found to be ...
The application of gold speckles on a polished surface is a successful technique for improving di... more The application of gold speckles on a polished surface is a successful technique for improving digital image correlation (DIC) contrast in scanning electron microscope (SEM) images. In the process of creating the gold speckles, the material is subjected to elevated temperatures for prolonged times. As a consequence, not all materials are suitable for the gold speckled method to improve the contrast for DIC measurements during an in-situ SEM tensile test. In this letter, the effect of gold remodeling on two different steels is investigated. These steels are a dual-phase (DP) steel and a ferrite–pearlite steel (NVE36). The results demonstrate that the temperature these steels are subjected to during gold remodeling will influence the tensile behavior of the DP steel while the NVE36 steel is unaffected by the heat treatment. As a result, we can conclude that the gold remodeling method for creating contrast in SEM images may affect the microstructure. However, the effect of these change...
Al–Mg–Si alloys are usually formed into their final shape by rolling or extrusion. After extrusio... more Al–Mg–Si alloys are usually formed into their final shape by rolling or extrusion. After extrusion, the aluminium profiles are usually straightened, causing the material to be subjected to a small plastic deformation. This study demonstrates the positive effect on strength that can be obtained from such small deformation levels or from only elastically straining the material. Elastic straining of a lean Al–Mg–Si alloy, when performed immediately after solution heat treatment, enhances the material yield strength after artificial ageing to T6. Transmission electron microscopy shows that this effect can be attributed to a higher number density and finer dispersion of the age-hardening precipitate needles. Furthermore, introducing a small plastic deformation of 1% after solution heat treatment results in a comparable strength increase to elastically straining the material. In this case, however, the strength increase is due to the increased dislocation density, which compensates for a ...
A ferrite-pearlite two-phase steel was investigated using in situ scanning electron microscope (S... more A ferrite-pearlite two-phase steel was investigated using in situ scanning electron microscope (SEM) tensile testing combined with digital image correlation (DIC). Two different speckled patterns were used and compared. The first pattern was achieved by etching a polished surface in order to reveal the microstructural features. Second, a gold speckled pattern was obtained. Here, a continuous layer of gold was applied to a polished surface. This continuous layer was remodeled into gold nanoparticles by keeping the specimen at 180 °C for 96 h with an Ar/Styrene mixture flowing across the specimen surface. The result is randomly distributed gold nanoparticles on the surface. These particles and the etched microstructure were then used by the DIC software to correlate an image series to obtain the local strain field of the material. The differences between the two techniques are numerous. Considering the etched surface, most microstructural features were grain boundaries and pearlite la...
An in situ scanning electron microscope (SEM) study was conducted on a super duplex stainless ste... more An in situ scanning electron microscope (SEM) study was conducted on a super duplex stainless steel (SDSS) containing 0%, 5% and 10% σ-phase. The material was heat treated at 850°C for 12 min and 15 min, respectively, to achieve the different amounts of σ-phase. The specimens were investigated at room temperature and at −40°C. The microstructure evolution during the deformation process was recorded using electron backscatter diffraction (EBSD) at different strain levels. Both σ-phase and χ-phase were observed along the grain boundaries in the microstructure in all heat treated specimens. Cracks started to form after 3-4% strain and were always oriented perpendicular to the tensile direction. After the cracks formed, they were initially arrested by the matrix. At later stages of the deformation process, cracks in larger σ-phase constituents started to coalesce. When the tensile test was conducted at −40°C, the ductility increased for the specimen without σ-phase, but with σ-phase present, the ductility was slightly reduced. With larger amounts of σ-phase present, however, an increase in tensile strength was also observed. With χ-phase present along the grain boundaries, a reduction of tensile strength was observed. This reduction seems to be related to χ-phase precipitating at the grain boundaries, creating imperfections, but not contributing towards the increase in strength. Compared to the effect of σ-phase, the low temperature is not as influential on the materials performance.
PROCEEDINGS OF THE 22ND INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2019
Metallurgical joining of steel and aluminum has become a field of interest in research in the las... more Metallurgical joining of steel and aluminum has become a field of interest in research in the last years, as it will allow the production of load adapted lightweight components in the future. The production process of cold extrusion enables both, forming and joining at the same time obtaining high bond strengths in the final component. However, the reachable relative bond strength varies highly between different material combinations. This presented paper focuses on the influence of the silicon amount in EN AW 6082 and thus the influence on the reachable bond strength. The silicon content is varied between 0.7 and 1.3 wt%, while the magnesium content is kept constant at 0.6 wt %, which is within the range of the specifications of the commercially available alloy. Before joining the materials were aged to their T6 state, as well as a state of similar hardness despite the different silicon contents. The results show a significant influence of the silicon, as only joints produced with the material with higher silicon content reach bond strengths of 150 MPa. Differences of the material flow and hardness increase due to the deformation of the joined components were also observed. To further understand the mechanism behind these observations, FE-simulations were conducted to show the material flow and therefore, the reasons for the different strengths and geometries.
Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, whi... more Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, which may act as nucleation sites for Mg2Si particles after homogenization. The purpose of adding Mn and Cr is to ensure a high density of uniformly distributed small β’-particles, which can be dissolved during further processing prior to the final age hardening step. However, their density and spatial distribution are critically dependent on the homogenization procedure. It is therefore important to have a robust and reliable method for assessing their spatial distribution. In the present work, an existing methodology for assessing spatial uniformity, the Global Shannon Entropy (GSE), was implemented and evaluated for different dispersoid structures characterized by scanning electron microscopy. This metric is highly dependent on the parameters used, but by careful selection of adequate parameters, it can be effective in detecting non-uniformity. An important weakness with the GSE was iden...
Under the concept of "Industry 4.0", production processes will be pushed to be increasingly inter... more Under the concept of "Industry 4.0", production processes will be pushed to be increasingly interconnected, information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization goes beyond the traditional aim of capacity maximization, contributing also for organization's profitability and value. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of maximization. The study of capacity optimization and costing models is an important research topic that deserves contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical model for capacity management based on different costing models (ABC and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization's value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity optimization might hide operational inefficiency.
Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, whi... more Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, which may act as nucleation sites for Mg 2 Si particles after homogenization. The purpose of adding Mn and Cr is to ensure a high density of uniformly distributed small β'-particles, which can be dissolved during further processing prior to the final age hardening step. However, their density and spatial distribution are critically dependent on the homogenization procedure. It is therefore important to have a robust and reliable method for assessing their spatial distribution. In the present work, an existing methodology for assessing spatial uniformity, the Global Shannon Entropy (GSE), was implemented and evaluated for different dispersoid structures characterized by scanning electron microscopy. This metric is highly dependent on the parameters used, but by careful selection of adequate parameters, it can be effective in detecting non-uniformity. An important weakness with the GSE was identified, and a modification to improve on the ability to differentiate degrees of non-uniformity is suggested. To evaluate the proposed methodology, the effect of heating rate on dispersoid precipitation behaviour during homogenisation of four Al-Mg-Si alloys with different Mn/Cr-content was investigated. In general, the dispersoid density increased with decreasing heating rates as longer times in the furnace resulted in particle coarsening. Slower heating rates were found to promote a more uniform dispersoid structure for most alloys investigated in this study. The metric with the new term demonstrated promising results, and improved the ability to differentiate degrees of spatial uniformity.
A constitutive model with microstructure, strength, work hardening submodels is developed for art... more A constitutive model with microstructure, strength, work hardening submodels is developed for artificial ageing of AA2050. • The model has for the first time successfully predicted ageing behaviour covering under-ageing to over-ageing. • Yield strength and work hardening behaviour from under-aged to over-aged conditions has been characterised and modelled. • Shearing-to-bypassing strengthening transition does not occur immediately when reaching peak-ageing state.
ABSTRACT In the 7xxx aluminium alloying system several mechanisms influence the hardening behavio... more ABSTRACT In the 7xxx aluminium alloying system several mechanisms influence the hardening behaviour of the alloys, e.g. particle size and distribution, dislocation density and alloying elements in solid solution. This work is an experimental study of ageing and work-hardening considering a commercial AA7108 alloy in the as-cast and homogenized condition. Tensile specimens have been exposed to a solution heat treatment and a two-step age-hardening treatment with varying time at the final temperature. The tensile data for the different tempers have been evaluated in elucidation of already existing models based on a one-parameter framework. The precipitate size and distribution have been further investigated in the transmission electron microscope for a selection of tempers, and the influence of these parameters on the work-hardening behaviour has been discussed.
Al-Mg-Si alloys often add small additions of e.g. Mn or Cr, to form dispersoids, which may act as... more Al-Mg-Si alloys often add small additions of e.g. Mn or Cr, to form dispersoids, which may act as nucleation sites for Mg2Si particles after homogenization. The purpose is to ensure a high density of uniformly distributed small ’-particles, which can be dissolved during further processing prior to the final age hardening step. However, their density and spatial distribution are critically dependent on the homogenization procedure. It is therefore important to have a robust and reliable method for assessing their spatial distribution. In the present work an existing methodology for assessing spatial uniformity, the Global Shannon Entropy (GSE), has been implemented and evaluated for different dispersoid structures characterized by scanning electron microscopy. This metric is highly dependent on the parameters used, but by careful selection of adequate parameters, it can be effective in detecting non-uniformity. An important weakness with the GSE was identified, and a modification to...
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