The focus of this investigation was to examine the influence of dissolved hydrogen on the fatigue... more The focus of this investigation was to examine the influence of dissolved hydrogen on the fatigue crack growth behavior of an austempered low-alloy AISI 4140 steel. The investigation also examined the influence of dissolved hydrogen on the fatigue threshold in this material. The material was tested in two conditions, as-received (cold rolled and annealed) and austempered (austenitized at 882 • C for 1 h and austempered at 332 • C for 1 h). The microstructure of the annealed specimens consisted of a mix of ferrite and fine pearlite; the microstructure of the austempered specimens was lower bainite. Tensile and Compact Tension specimens were prepared. To examine the influence of dissolved hydrogen, two subsets of the CT specimens were charged with hydrogen for three different time periods between 150 and 250 h. All of the CT samples were then subjected to fatigue crack growth tests in the threshold and linear regions at room temperature. The test results indicate that austempering resulted in significant improvement in the yield and tensile strength as well as the fracture toughness of the material. The test results also show that, in the absence of dissolved hydrogen, the crack growth rate in the threshold and linear regions was lower in austempered samples compared to the as-received (annealed) samples. The fatigue threshold was also slightly greater in the austempered samples. In presence of dissolved hydrogen, the crack growth rate was dependent upon the ∆K value. In the low ∆K region (<30 MPa √ m), the presence of dissolved hydrogen caused the crack growth rate to be higher in the austempered samples as compared to annealed samples. Above this value, the crack growth rate was increasingly greater in the annealed specimens when compared to the austempered specimens in presence of dissolved hydrogen. It is concluded that austempering of 4140 steel appears to provide a processing route by which the strength, hardness, and fracture toughness of the material can be increased with little or no degradation in the ductility and fatigue crack growth behavior.
This paper presents the results of a study to evaluate stainless steel thermal spray coatings on ... more This paper presents the results of a study to evaluate stainless steel thermal spray coatings on aluminum-based substrates. Coating wear, corrosion, and thermal cycling resistance were evaluated using pin-ondisk, cyclic corrosion, and a custom laser-based heating system, respectively, for 300-series stainless steel coatings deposited by arc-sprayed and cold-sprayed systems. Arc spray coatings were found to have equivalent wear and frictional performance as compared to gray cast iron (wear rates of 0.730-1.524 versus 1.294 9 10-5 mm 3 /m, COF of 0.35-0.36 versus 0.36). However, arc spray coating exhibited low adhesion (* 30 MPa) and early spalling under thermal cycling conditions, i.e., \ 600 cycles, as well as corrosion and delamination. The cold-sprayed coatings were found to have high corrosion and thermal cycling resistance, i.e., above 10,000 cycles. However, cold-sprayed coatings exhibited wear rates higher than those of the cast iron (cold spray wear rates of 4.774 9 10-5 mm 3 /m). A duplex coating composed of a cold-sprayed bond coat and an arc-sprayed top coat was created and showed good wear properties under simulated extreme braking conditions. The results from this study show that stainless steel coatings on aluminum substrates could be viable substitute for the cast iron in future brake rotors.
This paper presents an alternative method of material damage evaluation based on the X-ray comput... more This paper presents an alternative method of material damage evaluation based on the X-ray computer tomography-detected microdefects and multiscale computer simulation. This is achieved by developing a method of the digital diagnosis and full-field numerical calculation of material degradation in macroscopic material test specimens. The method comprised three basic components: (a) digital detection and processing of micro/mesoscale material defects of macroscopic material test specimens; (b) multilevel meshing and multilevel finite element analysis for evaluating local/global material degradation; and (c) synchronized experimental and numerical determination of material damage. The unique contributions of the proposed approach include (a) a multilevel finite element meshing and analysis scheme that makes the full-field estimation of material degradation in macroscopic test specimens computationally tractable on regular workstations, (b) full-field exploration of mesoscale material d...
The focus of this investigation was to examine the influence of intercritical austempering proces... more The focus of this investigation was to examine the influence of intercritical austempering process on the microstructure and mechanical properties of low-alloyed austempered ductile cast iron (ADI). The investigation also examined the influence of intercritical austempering process on the plane strain fracture toughness of the material. The effect of both austenitization and austempering temperature on the microstructure and mechanical properties was examined. The microstructural analysis was carried out using optical microscopy, scanning electron microscopy and X-ray diffraction. The test results indicate that by intercritical austempering it is possible to produce proeutectoid ferrite in the matrix microstructure. Furthermore, the yield, tensile strength 2 and the fracture toughness of the ADI decreases with decrease in austenitizing temperature. Lower austenitizing temperature produces more proeutectoid ferrite in the matrix. A considerable increase in ductility was observed in the samples with higher proeutectoid ferrite content. The fracture surfaces of the ADI samples revealed that dimple ductile fracture produced higher fracture toughness of 60±5 MPa√m in this intercritically austempered ADI.
Manufacturing Engineering and Materials Handling, Parts A and B, 2005
In Part 1 of this paper, a machining force model was developed based on an enhanced version of Zh... more In Part 1 of this paper, a machining force model was developed based on an enhanced version of Zheng’s Continuum Mechanics model that incorporates microstructural effects. Machining experiments identified Secondary Dendrite Arm Spacing (SDAS) as a significant microstructure feature of 319 aluminum in terms of machinability. A new material constitutive relationship that incorporates SDAS microstructure effects on the flow stress was proposed. In this part of the paper, disk turning tests are performed to simulate the orthogonal cutting process. The cutting forces obtained from some of these tests are used in concert with an inverse form of the continuum mechanics machining model to estimate the parameters in the material constitutive equation. The enhanced continuum mechanics orthogonal cutting model is then applied to predict cutting forces when machining Al319. Comparison of the model predicted and experimentally acquired cutting forces is demonstrated to show good agreement.
The automotive use of cast aluminum has greatly increased during the past decade, especially for ... more The automotive use of cast aluminum has greatly increased during the past decade, especially for engine blocks and cylinder heads. One physical property that is important in elevated-temperature applications is longterm dimensional stability of the cast aluminum component. Certain cast aluminum alloys (like 319) can undergo dimensional changes when exposed to engine operating temperatures over long periods of time; when these changes occur, the shape of the casting is distorted and the performance of the component may be diminished. Thus, a study was conducted to measure dimensional growth changes in a cast 319-type aluminum alloy as a function of heat-treatment, exposure temperature, and exposure time at the given temperature. The results show that all three factors have a significant effect upon the dimensional stability.
The high temperature fatigue behaviors of three cast aluminum alloys used for cylinder head fabri... more The high temperature fatigue behaviors of three cast aluminum alloys used for cylinder head fabrication – 319, A356 and AS7GU – are compared under isothermal fatigue at room temperature and elevated temperatures. The thermo-mechanical fatigue behavior for both out-ofphase and in-phase loading conditions (100–300°C) has also been investigated. It has been observed that all three of these alloys present a very similar behavior under both isothermal and thermo-mechanical low-cycle fatigue. Under high-cycle fatigue, however, the alloys A356 and AS7GU exhibit superior performance.
Abstract The ability to quantify the material damage at different length scales is critical in th... more Abstract The ability to quantify the material damage at different length scales is critical in the multiscale analysis of material behavior from nanoscale to macroscale. In this article, on the basis of the equivalence of complementary elastic energy we propose a multiresolution ...
Austempered Ductile Cast Iron (ADI) has emerged as an important commercial engineering material i... more Austempered Ductile Cast Iron (ADI) has emerged as an important commercial engineering material in recent years because of excellent properties such as high strength with good ductility as well as good fatigue strength and fracture toughness together with excellent wear resistance. These properties are as a result of a microstructure composed primarily of acicular ferrite (α) and high carbon austenite (γ HC). The role of the austenite in ADI is a topic that has been discussed extensively in the literature. Several investigators in the past have reported that the austenite in ADI is both thermally and mechanically stable; while others have reported it is neither thermally nor mechanically stable. Moreover, it is not clear whether the austenite formed at lower austempering (or bainitic) temperatures is more mechanically and thermally stable than that produced in the upper austempering (or bainitic) ranges. In this investigation, a comprehensive study was carried out to examine both the thermal and mechanical stability of austenite in the upper and lower austempering (or bainitic) temperature ranges. Compact tension and cylindrical tensile specimens were prepared and austempered at five different temperatures ranging from lower to upper bainitic temperatures. A select number of samples were then cryogenically treated to examine the thermal stability of the high carbon austenite. In order to examine the mechanical stability of austenite the microstructure and mechanical properties of these materials were evaluated as a function of austempering temperature, cryogenic treatment, and mechanical testing. The results of this study show that the austenite formed at lower austempering temperatures (260 and 288 1C) was neither thermally nor mechanically stable; while the austenite formed in the upper bainitic temperature range (371-399 1C) was partially stable under both thermal and mechanical conditions. Both stress-and strain-induced austenite-to-martensite transformations were observed in the ADI samples during mechanical tests. The test results also showed that cryogenic treatment can improve the mechanical properties without compromising the fracture resistance of the ADI. This is hypothesized to be due to a threshold level for the amount of austenite transformed to martensite; below this level, the amount transformed is not sufficient to produce an observable change in the fracture toughness.
The focus of this investigation was to examine the influence of dissolved hydrogen on the fatigue... more The focus of this investigation was to examine the influence of dissolved hydrogen on the fatigue crack growth behavior of an austempered low-alloy AISI 4140 steel. The investigation also examined the influence of dissolved hydrogen on the fatigue threshold in this material. The material was tested in two conditions, as-received (cold rolled and annealed) and austempered (austenitized at 882 • C for 1 h and austempered at 332 • C for 1 h). The microstructure of the annealed specimens consisted of a mix of ferrite and fine pearlite; the microstructure of the austempered specimens was lower bainite. Tensile and Compact Tension specimens were prepared. To examine the influence of dissolved hydrogen, two subsets of the CT specimens were charged with hydrogen for three different time periods between 150 and 250 h. All of the CT samples were then subjected to fatigue crack growth tests in the threshold and linear regions at room temperature. The test results indicate that austempering resulted in significant improvement in the yield and tensile strength as well as the fracture toughness of the material. The test results also show that, in the absence of dissolved hydrogen, the crack growth rate in the threshold and linear regions was lower in austempered samples compared to the as-received (annealed) samples. The fatigue threshold was also slightly greater in the austempered samples. In presence of dissolved hydrogen, the crack growth rate was dependent upon the ∆K value. In the low ∆K region (<30 MPa √ m), the presence of dissolved hydrogen caused the crack growth rate to be higher in the austempered samples as compared to annealed samples. Above this value, the crack growth rate was increasingly greater in the annealed specimens when compared to the austempered specimens in presence of dissolved hydrogen. It is concluded that austempering of 4140 steel appears to provide a processing route by which the strength, hardness, and fracture toughness of the material can be increased with little or no degradation in the ductility and fatigue crack growth behavior.
This paper presents the results of a study to evaluate stainless steel thermal spray coatings on ... more This paper presents the results of a study to evaluate stainless steel thermal spray coatings on aluminum-based substrates. Coating wear, corrosion, and thermal cycling resistance were evaluated using pin-ondisk, cyclic corrosion, and a custom laser-based heating system, respectively, for 300-series stainless steel coatings deposited by arc-sprayed and cold-sprayed systems. Arc spray coatings were found to have equivalent wear and frictional performance as compared to gray cast iron (wear rates of 0.730-1.524 versus 1.294 9 10-5 mm 3 /m, COF of 0.35-0.36 versus 0.36). However, arc spray coating exhibited low adhesion (* 30 MPa) and early spalling under thermal cycling conditions, i.e., \ 600 cycles, as well as corrosion and delamination. The cold-sprayed coatings were found to have high corrosion and thermal cycling resistance, i.e., above 10,000 cycles. However, cold-sprayed coatings exhibited wear rates higher than those of the cast iron (cold spray wear rates of 4.774 9 10-5 mm 3 /m). A duplex coating composed of a cold-sprayed bond coat and an arc-sprayed top coat was created and showed good wear properties under simulated extreme braking conditions. The results from this study show that stainless steel coatings on aluminum substrates could be viable substitute for the cast iron in future brake rotors.
This paper presents an alternative method of material damage evaluation based on the X-ray comput... more This paper presents an alternative method of material damage evaluation based on the X-ray computer tomography-detected microdefects and multiscale computer simulation. This is achieved by developing a method of the digital diagnosis and full-field numerical calculation of material degradation in macroscopic material test specimens. The method comprised three basic components: (a) digital detection and processing of micro/mesoscale material defects of macroscopic material test specimens; (b) multilevel meshing and multilevel finite element analysis for evaluating local/global material degradation; and (c) synchronized experimental and numerical determination of material damage. The unique contributions of the proposed approach include (a) a multilevel finite element meshing and analysis scheme that makes the full-field estimation of material degradation in macroscopic test specimens computationally tractable on regular workstations, (b) full-field exploration of mesoscale material d...
The focus of this investigation was to examine the influence of intercritical austempering proces... more The focus of this investigation was to examine the influence of intercritical austempering process on the microstructure and mechanical properties of low-alloyed austempered ductile cast iron (ADI). The investigation also examined the influence of intercritical austempering process on the plane strain fracture toughness of the material. The effect of both austenitization and austempering temperature on the microstructure and mechanical properties was examined. The microstructural analysis was carried out using optical microscopy, scanning electron microscopy and X-ray diffraction. The test results indicate that by intercritical austempering it is possible to produce proeutectoid ferrite in the matrix microstructure. Furthermore, the yield, tensile strength 2 and the fracture toughness of the ADI decreases with decrease in austenitizing temperature. Lower austenitizing temperature produces more proeutectoid ferrite in the matrix. A considerable increase in ductility was observed in the samples with higher proeutectoid ferrite content. The fracture surfaces of the ADI samples revealed that dimple ductile fracture produced higher fracture toughness of 60±5 MPa√m in this intercritically austempered ADI.
Manufacturing Engineering and Materials Handling, Parts A and B, 2005
In Part 1 of this paper, a machining force model was developed based on an enhanced version of Zh... more In Part 1 of this paper, a machining force model was developed based on an enhanced version of Zheng’s Continuum Mechanics model that incorporates microstructural effects. Machining experiments identified Secondary Dendrite Arm Spacing (SDAS) as a significant microstructure feature of 319 aluminum in terms of machinability. A new material constitutive relationship that incorporates SDAS microstructure effects on the flow stress was proposed. In this part of the paper, disk turning tests are performed to simulate the orthogonal cutting process. The cutting forces obtained from some of these tests are used in concert with an inverse form of the continuum mechanics machining model to estimate the parameters in the material constitutive equation. The enhanced continuum mechanics orthogonal cutting model is then applied to predict cutting forces when machining Al319. Comparison of the model predicted and experimentally acquired cutting forces is demonstrated to show good agreement.
The automotive use of cast aluminum has greatly increased during the past decade, especially for ... more The automotive use of cast aluminum has greatly increased during the past decade, especially for engine blocks and cylinder heads. One physical property that is important in elevated-temperature applications is longterm dimensional stability of the cast aluminum component. Certain cast aluminum alloys (like 319) can undergo dimensional changes when exposed to engine operating temperatures over long periods of time; when these changes occur, the shape of the casting is distorted and the performance of the component may be diminished. Thus, a study was conducted to measure dimensional growth changes in a cast 319-type aluminum alloy as a function of heat-treatment, exposure temperature, and exposure time at the given temperature. The results show that all three factors have a significant effect upon the dimensional stability.
The high temperature fatigue behaviors of three cast aluminum alloys used for cylinder head fabri... more The high temperature fatigue behaviors of three cast aluminum alloys used for cylinder head fabrication – 319, A356 and AS7GU – are compared under isothermal fatigue at room temperature and elevated temperatures. The thermo-mechanical fatigue behavior for both out-ofphase and in-phase loading conditions (100–300°C) has also been investigated. It has been observed that all three of these alloys present a very similar behavior under both isothermal and thermo-mechanical low-cycle fatigue. Under high-cycle fatigue, however, the alloys A356 and AS7GU exhibit superior performance.
Abstract The ability to quantify the material damage at different length scales is critical in th... more Abstract The ability to quantify the material damage at different length scales is critical in the multiscale analysis of material behavior from nanoscale to macroscale. In this article, on the basis of the equivalence of complementary elastic energy we propose a multiresolution ...
Austempered Ductile Cast Iron (ADI) has emerged as an important commercial engineering material i... more Austempered Ductile Cast Iron (ADI) has emerged as an important commercial engineering material in recent years because of excellent properties such as high strength with good ductility as well as good fatigue strength and fracture toughness together with excellent wear resistance. These properties are as a result of a microstructure composed primarily of acicular ferrite (α) and high carbon austenite (γ HC). The role of the austenite in ADI is a topic that has been discussed extensively in the literature. Several investigators in the past have reported that the austenite in ADI is both thermally and mechanically stable; while others have reported it is neither thermally nor mechanically stable. Moreover, it is not clear whether the austenite formed at lower austempering (or bainitic) temperatures is more mechanically and thermally stable than that produced in the upper austempering (or bainitic) ranges. In this investigation, a comprehensive study was carried out to examine both the thermal and mechanical stability of austenite in the upper and lower austempering (or bainitic) temperature ranges. Compact tension and cylindrical tensile specimens were prepared and austempered at five different temperatures ranging from lower to upper bainitic temperatures. A select number of samples were then cryogenically treated to examine the thermal stability of the high carbon austenite. In order to examine the mechanical stability of austenite the microstructure and mechanical properties of these materials were evaluated as a function of austempering temperature, cryogenic treatment, and mechanical testing. The results of this study show that the austenite formed at lower austempering temperatures (260 and 288 1C) was neither thermally nor mechanically stable; while the austenite formed in the upper bainitic temperature range (371-399 1C) was partially stable under both thermal and mechanical conditions. Both stress-and strain-induced austenite-to-martensite transformations were observed in the ADI samples during mechanical tests. The test results also showed that cryogenic treatment can improve the mechanical properties without compromising the fracture resistance of the ADI. This is hypothesized to be due to a threshold level for the amount of austenite transformed to martensite; below this level, the amount transformed is not sufficient to produce an observable change in the fracture toughness.
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Papers by James Boileau