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Due to finite fossil fuel resources and the impact on our environment of burning fossil fuels, the automotive industry has been investigating ways to reduce the overall weight of automotive vehicles. This has led to increased interest in ways that light weight alloys such as magnesium can be used in fabrication of automotive parts and manufacturing processes such as welding that would enable increased use of magnesium. The objectives of this project were to characterize and determine the weldability of 2 mm thick AZ31B-H24 magnesium alloy by three different laser beam welding processes: a 4 kW Nuvonyx ISL-4000L high power diode laser, a 5 kW Trump TLC-1005 CO2 laser, and a 10 kW YLR-10000-WC fibre laser. The diode laser operated with a 0.9 by 12 mm spot size and with a maximum power density of 37 MW/m2. Due to its low power density, the diode laser was restricted to conduction-mode welding which produced wide fusion zones. The AZ31B magnesium laser welds exhibited a number of defects including hydrogen porosity, solidification cracking, liquation cracking, high vaporization rates, molten expulsions, and poor weld bead quality due to low surface tension. It was found that the majority of these defects could be controlled through the proper use of clamping and shielding of the weld pool and joint preparation and surface cleaning prior to welding. The as-received base material was delivered with a dark grey hydrated oxide layer. This surface condition was found to increase the overall diode laser beam absorption but was detrimental to the welding process when disrupted. When incorporated into the weld pool, the oxide created weak facets where solidification cracks would initiate or acted to localize strain during tensile testing. Proper joint preparation was required to produce a high quality diode laser weld: machining of the joint interface to remove interfacial gaps, chemical cleaning with acetone and ethanol to remove residual oils or grease, and stainless steel wire brushing to remove the oxide. Diode laser welds made using 3 kW power and 0.75 m/min welding speed achieved approximately 60% of the base metal’s ultimate strength and less than 15% of the base metal ductility. The reduced strength and ductility were attributed primarily to the weld defects which acted as strain localizers during plastic deformation and the lack of strain hardening in the weld metal. Both the CO2 and fibre lasers beams had focal spot sizes of 300 μm diameters and maximum power densities of 70 and 140 GW/m2, respectively. At these power densities, the CO2 and fibre lasers operated in keyhole-mode and produced welds which had narrower columnated fusion zones. The CO2 laser keyhole-mode welds exhibited keyhole instability and bulk material loss through vaporization that resulted in macro-porosity, under-fill, and generally poor weld bead quality. Welds produced using 5 kW power and 8 m/min welding speeds achieved approximately 70% of the base metal’s ultimate strength. The highest quality fibre laser welds were produced at 2 kW power and 100 mm/s welding speeds. These defect-free welds achieved transverse tensile strengths that were 86% of the base metal’s ultimate strength. The 14% loss of strength was attributed to the difference in temper of the base metal and the weld metal. The base material was received in a half-hard H24 temper and the as solidified weld metal is naturally in the softer F temper. This also resulted in a corresponding 15% reduction in hardness. Failure always occurred in the softened fusion zones of the welded samples where the measured hardness was reduced to an average 60 VHN25 from the base metal’s 75 HVN25. The fibre laser weld samples also experienced the greatest extension of any of the tested welds with a cross head displacement of 30% of the base metal. The extreme reduction in overall cross head displacement was attributed to the lower strength of the fusion zone. This led to strain localization in the transverse tensile specimens and premature failure that occurred prior to plastic deformation of the surrounding base material. Proper joint preparation was found to be critical when laser welding AZ31B magnesium sheet. Machined interfaces were required to minimize the gap and degreasing and stainless steel wire brushing were required for removal of the pre-existing hydrated oxide in order to produce sound laser welds. Helium shielding gas was found to improve the weld bead surface quality compared to argon. The keyhole-mode welds produced with the CO2 and fibre lasers were superior compared to the conduction-mode welds produced with the diode laser. This was due to the narrower fusion zone and reduced bulk material loss. Of the three laser welding processes examined in this study, the fibre laser produced the highest quality, strongest, and most ductile welds when analyzed in transverse tensile testing. However, direct comparisons between the CO2 and fibre laser welds could not be made because they were made using different joint preparations and welding conditions.

Due to finite fossil fuel resources and the impact on our environment of burning fossil fuels, the automotive industry has been investigating ways to reduce the overall weight of automotive vehicles. This has led to increased interest in ways that light weight alloys such as magnesium can be used in fabrication of automotive parts and manufacturing processes such as welding that would enable increased use of magnesium. The objectives of this project were to characterize and determine the weldability of 2 mm thick AZ31B-H24 magnesium alloy by three different laser beam welding processes: a 4 kW Nuvonyx ISL-4000L high power diode laser, a 5 kW Trump TLC-1005 CO2 laser, and a 10 kW YLR-10000-WC fibre laser. The diode laser operated with a 0.9 by 12 mm spot size and with a maximum power density of 37 MW/m2. Due to its low power density, the diode laser was restricted to conduction-mode welding which produced wide fusion zones. The AZ31B magnesium laser welds exhibited a number of defects including hydrogen porosity, solidification cracking, liquation cracking, high vaporization rates, molten expulsions, and poor weld bead quality due to low surface tension. It was found that the majority of these defects could be controlled through the proper use of clamping and shielding of the weld pool and joint preparation and surface cleaning prior to welding. The as-received base material was delivered with a dark grey hydrated oxide layer. This surface condition was found to increase the overall diode laser beam absorption but was detrimental to the welding process when disrupted. When incorporated into the weld pool, the oxide created weak facets where solidification cracks would initiate or acted to localize strain during tensile testing. Proper joint preparation was required to produce a high quality diode laser weld: machining of the joint interface to remove interfacial gaps, chemical cleaning with acetone and ethanol to remove residual oils or grease, and stainless steel wire brushing to remove the oxide. Diode laser welds made using 3 kW power and 0.75 m/min welding speed achieved approximately 60% of the base metal’s ultimate strength and less than 15% of the base metal ductility. The reduced strength and ductility were attributed primarily to the weld defects which acted as strain localizers during plastic deformation and the lack of strain hardening in the weld metal. Both the CO2 and fibre lasers beams had focal spot sizes of 300 μm diameters and maximum power densities of 70 and 140 GW/m2, respectively. At these power densities, the CO2 and fibre lasers operated in keyhole-mode and produced welds which had narrower columnated fusion zones. The CO2 laser keyhole-mode welds exhibited keyhole instability and bulk material loss through vaporization that resulted in macro-porosity, under-fill, and generally poor weld bead quality. Welds produced using 5 kW power and 8 m/min welding speeds achieved approximately 70% of the base metal’s ultimate strength. The highest quality fibre laser welds were produced at 2 kW power and 100 mm/s welding speeds. These defect-free welds achieved transverse tensile strengths that were 86% of the base metal’s ultimate strength. The 14% loss of strength was attributed to the difference in temper of the base metal and the weld metal. The base material was received in a half-hard H24 temper and the as solidified weld metal is naturally in the softer F temper. This also resulted in a corresponding 15% reduction in hardness. Failure always occurred in the softened fusion zones of the welded samples where the measured hardness was reduced to an average 60 VHN25 from the base metal’s 75 HVN25. The fibre laser weld samples also experienced the greatest extension of any of the tested welds with a cross head displacement of 30% of the base metal. The extreme reduction in overall cross head displacement was attributed to the lower strength of the fusion zone. This led to strain localization in the transverse tensile specimens and premature failure that occurred prior to plastic deformation of the surrounding base material. Proper joint preparation was found to be critical when laser welding AZ31B magnesium sheet. Machined interfaces were required to minimize the gap and degreasing and stainless steel wire brushing were required for removal of the pre-existing hydrated oxide in order to produce sound laser welds. Helium shielding gas was found to improve the weld bead surface quality compared to argon. The keyhole-mode welds produced with the CO2 and fibre lasers were superior compared to the conduction-mode welds produced with the diode laser. This was due to the narrower fusion zone and reduced bulk material loss. Of the three laser welding processes examined in this study, the fibre laser produced the highest quality, strongest, and most ductile welds when analyzed in transverse tensile testing. However, direct comparisons between the CO2 and fibre laser welds could not be made because they were made using different joint preparations and welding conditions.

The demand for lightweight materials in automobiles has been motivated by two factors: fuel economy and air pollution reduction. One of the first steps taken in automotive vehicle weight reduction was the use of aluminum alloys for both structural and non-structural parts. Although magnesium alloys, that have one fourth the density of steel and one third that of aluminum, have also been used in automobiles, however, their applications were limited to non-structural parts. Recently, interest has been focused on using magnesium alloys as structural materials for automotive load-bearing components. Load-bearing components in automobiles are usually subjected to multiaxial cyclic loading. Fatigue is considered to be a significant cause of ground vehicle component failure. Therefore, for magnesium alloys to be used for these components, an understanding of their fatigue behaviour is necessary. In this study, series of monotonic and cyclic tests were conducted on smooth specimens machined from AZ31B magnesium extrusion section. Two loading modes were considered in this investigation, axial and torsional. Monotonic tensile and compressive tests were performed at three different orientations, longitudinal (LD), i.e., parallel to the extrusion direction, 45° and transverse (TD) directions. Monotonic torsion tests were performed on specimens that were machined along the LD. Similarly, cyclic axial and torsional as well as multiaxial axial-torsional tests were performed on specimens that that were machined along the LD. Three different phase angles were considered for multiaxial tests: in-phase, and 45° and 90° out-of-phase. It was found that monotonic axial stress-strain behaviour is direction dependent due to the different deformation mechanisms involved. Significant yield anisotropy and sigmoidal-type hardening were observed. Twinning-detwinning deformation was considered as the major cause of these behaviours. On the other hand, monotonic torsional stress-strain curve had a linear hardening behaviour. Cyclic axial behaviour was found to be affected by twinning-detwinning deformation. Its most significant characteristics are: yield asymmetry, power-like hardening in compressive reversal and sigmoidal-type hardening in tensile reversal. This unusual behaviour was attributed to the contribution of three different deformation mechanisms: slip, twinning and detwinning. Due to yield asymmetry, significant positive mean stress was observed especially at LCF. Cyclic hardening was also observed and it was found to be associated with a substantial decrease in plastic strain energy density. Cyclic shear behaviour was symmetric and did not exhibit any of the aforementioned behaviours in cyclic axial loading. Two major observations were made from multiaxial tests. First, additional hardening due to nonproportionality was observed. Second, phase angle has no effect on fatigue life. Three fatigue life models were considered for multiaxial fatigue life prediction: Smith-Watson-Topper, Fatemi-Socie and Jahed-Varvani. The first two models are based on strain and are evaluated on specific critical planes. The third model is based on energy densities calculated from hysteresis loops. Strain- and energy-life curves had knees and pronounced plateaus. Therefore, it was not possible to model the entire fatigue life using Coffin-Manson-Type equations. Low cycle fatigue lives were predicted within ۬x scatter bounds using the Fatemi-Socie and the Jahed-Varvani models for all loading conditions which was not the case with Smith-Watson-Topper model. Total energy, the sum of plastic and positive elastic strain energy densities, was found to correlate fatigue lives for several wrought Mg-alloys under different loading conditions.

The current research addresses the formability of two magnesium sheet alloys, a conventional AZ31B and a rare earth alloyed ZEK100. Both alloys had a nominal thickness of 1.6 mm. Both Limiting Dome Height (LDH) and Cylindrical Cup Draw experiments were performed between room temperature and 350°C. To examine the effect of sheet directionality and anisotropy, LDH experiments were performed in both the sheet rolling and transverse directions. In addition, strain measurements were performed along both sheet orientations of the cylindrical cup and LDH specimens for which the geometry is symmetric. The LDH tests were used to study the formability of ZEK100 and AZ31B (O and H24 tempers) magnesium alloy sheet between room temperature and 350°C. At room temperature, AZ31B-O and AZ31B-H24 exhibit limited formability, with dome heights of only 11-12 mm prior to the onset of necking. In contrast, the dome heights of ZEK100 at room temperature reached 29 mm (a 140% improvement over AZ31B). Increasing the temperature above 200°C did not affect the relative ranking of the three sheet samples, however it did reduce the magnitude of the difference in dome heights. The rare earth alloyed ZEK100 had pronounced benefits at intermediate temperatures, achieving an LDH of 37 mm at 150°C; this dome height was only reached by AZ31B at a much higher temperature of 250°C. To further characterize the formability of ZEK100, forming limit curves (FLCs) were developed from the LDH tests in both the rolling and transverse directions. Comparisons to AZ31B were made at selected temperatures. Surface strain data was collected with an in situ digital image correlation (DIC) system incorporating two cameras for stereo observation. Results from these experiments further highlighted the enhanced formability relative to AZ31B over the entire temperature range between room temperature and 350°C, with the most dramatic improvements between room temperature and 150°C. The plane strain forming limit (FLC0) for ZEK100 at 150°C was 0.4 which equals that of AZ31B at 250°C. At higher temperatures (300°C), the two alloys exhibited similar performance with both achieving similar dome heights at necking of 37 mm (AZ31B) and 41 mm (ZEK100). To round out the investigation of ZEK100 for industrial applications, cylindrical cup deep drawing experiments were performed on ZEK100 sheet between 25°C and 250°C under isothermal and non-isothermal conditions. Draw ratios of 1.75, 2.00 and 2.25 were considered to examine the effects of draw ratio on draw depth. The effect of sheet anisotropy during deep drawing was investigated by measuring the earring profiles, sheet thickness and strain distribution along both the rolling and transverse directions. Isothermal test results showed enhanced warm temperature drawing performance of ZEK100 over AZ31B sheet; for example, a full draw of 203.2 mm (8”) blanks of ZEK100 was achieved with a tool temperature of 150°C, whereas a tool temperature of 225°C was needed to fully draw AZ31B-O blanks of this diameter. Non-isothermal deep draw experiments showed further improvement in drawability with significantly lower tooling temperatures required for a full cup draw using ZEK100. ZEK100 achieved a full draw of 228.6 mm (9") blanks with a die and blank holder temperature of 150°C and a cooled punch (25°C) while the same size blank of AZ31B required a die and blank holder temperature 225°C and a cooled punch (150°C). Temperature process windows were developed from the isothermal and non-isothermal results to show a direct comparison of drawing behaviour between ZEK100 and AZ31B. Overall, ZEK100 offers significantly improved forming performance compared to AZ31B, particularly at temperatures below 200°C. This lower temperature enhanced formability is attractive since it is less demanding in terms of lubricant requirements and reduces the need for higher temperature tooling.

The demand for lightweight materials in automobiles has been motivated by two factors: fuel economy and air pollution reduction. One of the first steps taken in automotive vehicle weight reduction was the use of aluminum alloys for both structural and non-structural parts. Although magnesium alloys, that have one fourth the density of steel and one third that of aluminum, have also been used in automobiles, however, their applications were limited to non-structural parts. Recently, interest has been focused on using magnesium alloys as structural materials for automotive load-bearing components. Load-bearing components in automobiles are usually subjected to multiaxial cyclic loading. Fatigue is considered to be a significant cause of ground vehicle component failure. Therefore, for magnesium alloys to be used for these components, an understanding of their fatigue behaviour is necessary. In this study, series of monotonic and cyclic tests were conducted on smooth specimens machined from AZ31B magnesium extrusion section. Two loading modes were considered in this investigation, axial and torsional. Monotonic tensile and compressive tests were performed at three different orientations, longitudinal (LD), i.e., parallel to the extrusion direction, 45° and transverse (TD) directions. Monotonic torsion tests were performed on specimens that were machined along the LD. Similarly, cyclic axial and torsional as well as multiaxial axial-torsional tests were performed on specimens that that were machined along the LD. Three different phase angles were considered for multiaxial tests: in-phase, and 45° and 90° out-of-phase. It was found that monotonic axial stress-strain behaviour is direction dependent due to the different deformation mechanisms involved. Significant yield anisotropy and sigmoidal-type hardening were observed. Twinning-detwinning deformation was considered as the major cause of these behaviours. On the other hand, monotonic torsional stress-strain curve had a linear hardening behaviour. Cyclic axial behaviour was found to be affected by twinning-detwinning deformation. Its most significant characteristics are: yield asymmetry, power-like hardening in compressive reversal and sigmoidal-type hardening in tensile reversal. This unusual behaviour was attributed to the contribution of three different deformation mechanisms: slip, twinning and detwinning. Due to yield asymmetry, significant positive mean stress was observed especially at LCF. Cyclic hardening was also observed and it was found to be associated with a substantial decrease in plastic strain energy density. Cyclic shear behaviour was symmetric and did not exhibit any of the aforementioned behaviours in cyclic axial loading. Two major observations were made from multiaxial tests. First, additional hardening due to nonproportionality was observed. Second, phase angle has no effect on fatigue life. Three fatigue life models were considered for multiaxial fatigue life prediction: Smith-Watson-Topper, Fatemi-Socie and Jahed-Varvani. The first two models are based on strain and are evaluated on specific critical planes. The third model is based on energy densities calculated from hysteresis loops. Strain- and energy-life curves had knees and pronounced plateaus. Therefore, it was not possible to model the entire fatigue life using Coffin-Manson-Type equations. Low cycle fatigue lives were predicted within ۬x scatter bounds using the Fatemi-Socie and the Jahed-Varvani models for all loading conditions which was not the case with Smith-Watson-Topper model. Total energy, the sum of plastic and positive elastic strain energy densities, was found to correlate fatigue lives for several wrought Mg-alloys under different loading conditions.

Magnesium alloys are of interest to the automotive industry because of their high specific strength and potential to reduce vehicle weight and fuel consumption. In order to incorporate more magnesium components into automotive structures, efficient welding and joining techniques must be developed. Specifically, a method of making butt-joint welds must be found in order to use sheet magnesium alloys in the form of tailor-welded blanks for structural applications. The existing welding processes each have disadvantages when applied to magnesium alloy sheet. The double-sided arc welding (DSAW) process has been shown to produce high quality welds in aluminum alloy sheet, for tailor-welded blank applications. The DSAW process has not yet been applied to AZ31B magnesium alloy, which has thermo-physical and oxide forming properties similar to those of aluminum alloys. Therefore, this research explores the weldability of AZ31B magnesium alloy, using the DSAW process. Experimental, butt-joint configuration welds were made in 2 mm thick AZ31B-H42 magnesium alloy sheet. Acceptable welds have been produced using welding speeds ranging from 12 mm/s to 100 mm/s and welding powers from 1.6 kW to 8.7 kW. The influence of these parameters on the appearance, geometry, mechanical properties and microstructure of the resulting welds was investigated. Optimal appearance, geometric profile and mechanical properties were obtained at the lowest welding speeds and powers. Under these conditions, mechanical properties of the weld metal were equivalent to those of the fully annealed (0-temper) base metal. However, progressive deterioration in appearance, geometry and mechanical properties occurred at higher welding speeds. The deterioration in mechanical properties was associated with 2 microstructural defects that were observed at higher welding speeds: 1) the formation of larger amounts of Mg17Al12 -phase particles, at the grain boundaries, and 2) the formation of solidification shrinkage micro-porosity at these same inter-granular locations. This research demonstrates that the DSAW process is capable of producing acceptable quality, butt-joint welds in AZ31B magnesium alloy sheet at welding speeds up to 100 mm/s. However, in order to achieve the highest quality welds, low welding power, and, low welding speed, should be used. The highest quality welds were produced at welding speeds of 12 mm/s.

Wrought magnesium alloys are considered attractive candidates for structural members in automotive and aerospace industries due to their high specific strength. Although new processes have helped to produce high purity magnesium alloys with higher resistance to corrosion, these alloys still need protection against corrosion when they are used in aggressive environments. Cold spray coating is one of the protective methods that are employed for this purpose. The similarity between cold spray coating and shot peening process poses the question whether cold spray coating can improve the fatigue strength in addition to providing corrosion protection. The objective of this research is to answer this question for the specific case of the coating of wrought magnesium alloy AZ31B with aluminum powder. This study comprises two parts. The first part characterises the residual stress induced by cold spray coating. This investigation employs both numerical and experimental methods. For the numerical study, the cold spray coating process has been simulated via ANSYS software classic package. The numerical results have been compared to experimental results from X-Ray Diffraction (XRD) stress measurement of a coated sample. For the second part of this research, the fatigue strength of as received, stress relieved, and stress relieved/coated specimens have been compared. Three groups of AZ31B specimens have been prepared and tested by rotating bending machine and their S-N curves have been prepared. Comparison of the results reveals that there is a considerable loss in fatigue strength of as received specimens after stress relief. This is due to the removal of compressive residual stress in the raw material induced by the extrusion process. Also, comparison of S-N curves of stress relieved and stress relieved/coated specimens shows fatigue life improvement after cold spray coating. The maximum improvement is 49 percent in the load of 120 MPa and the endurance limit has improved 9 percent.

Wrought magnesium alloys are considered attractive candidates for structural members in automotive and aerospace industries due to their high specific strength. Although new processes have helped to produce high purity magnesium alloys with higher resistance to corrosion, these alloys still need protection against corrosion when they are used in aggressive environments. Cold spray coating is one of the protective methods that are employed for this purpose. The similarity between cold spray coating and shot peening process poses the question whether cold spray coating can improve the fatigue strength in addition to providing corrosion protection. The objective of this research is to answer this question for the specific case of the coating of wrought magnesium alloy AZ31B with aluminum powder. This study comprises two parts. The first part characterises the residual stress induced by cold spray coating. This investigation employs both numerical and experimental methods. For the numerical study, the cold spray coating process has been simulated via ANSYS software classic package. The numerical results have been compared to experimental results from X-Ray Diffraction (XRD) stress measurement of a coated sample. For the second part of this research, the fatigue strength of as received, stress relieved, and stress relieved/coated specimens have been compared. Three groups of AZ31B specimens have been prepared and tested by rotating bending machine and their S-N curves have been prepared. Comparison of the results reveals that there is a considerable loss in fatigue strength of as received specimens after stress relief. This is due to the removal of compressive residual stress in the raw material induced by the extrusion process. Also, comparison of S-N curves of stress relieved and stress relieved/coated specimens shows fatigue life improvement after cold spray coating. The maximum improvement is 49 percent in the load of 120 MPa and the endurance limit has improved 9 percent.

Magnesium alloys are of interest to the automotive industry because of their high specific strength and potential to reduce vehicle weight and fuel consumption. In order to incorporate more magnesium components into automotive structures, efficient welding and joining techniques must be developed. Specifically, a method of making butt-joint welds must be found in order to use sheet magnesium alloys in the form of tailor-welded blanks for structural applications. The existing welding processes each have disadvantages when applied to magnesium alloy sheet. The double-sided arc welding (DSAW) process has been shown to produce high quality welds in aluminum alloy sheet, for tailor-welded blank applications. The DSAW process has not yet been applied to AZ31B magnesium alloy, which has thermo-physical and oxide forming properties similar to those of aluminum alloys. Therefore, this research explores the weldability of AZ31B magnesium alloy, using the DSAW process. Experimental, butt-joint configuration welds were made in 2 mm thick AZ31B-H42 magnesium alloy sheet. Acceptable welds have been produced using welding speeds ranging from 12 mm/s to 100 mm/s and welding powers from 1.6 kW to 8.7 kW. The influence of these parameters on the appearance, geometry, mechanical properties and microstructure of the resulting welds was investigated. Optimal appearance, geometric profile and mechanical properties were obtained at the lowest welding speeds and powers. Under these conditions, mechanical properties of the weld metal were equivalent to those of the fully annealed (0-temper) base metal. However, progressive deterioration in appearance, geometry and mechanical properties occurred at higher welding speeds. The deterioration in mechanical properties was associated with 2 microstructural defects that were observed at higher welding speeds: 1) the formation of larger amounts of Mg17Al12 -phase particles, at the grain boundaries, and 2) the formation of solidification shrinkage micro-porosity at these same inter-granular locations. This research demonstrates that the DSAW process is capable of producing acceptable quality, butt-joint welds in AZ31B magnesium alloy sheet at welding speeds up to 100 mm/s. However, in order to achieve the highest quality welds, low welding power, and, low welding speed, should be used. The highest quality welds were produced at welding speeds of 12 mm/s.