Porous constructs of calcium polyphosphate (CPP) are under investigation as a substrate for tissue engineering of cartilage for repair of osteochondral defects. Previous studies have shown that CPP has the required features to satisfy these requirements. However, its degradation rate is lower than desired. This study investigated the effect of doping with MgCO3, MgCl2, K2CO3 or KCl at a molar ratio of M/Ca = 0.02 on sintering and in vitro degradation behavior of CPP. Doping with magnesium or potassium improved the tensile and compressive strengths of CPP at similar porosities. After 15 days of aging in phosphate buffer saline, the rate of tensile strength loss was faster for the doped CPP groups than undoped CPP. The chemical degradation rate of Mg-doped CPP groups was the fastest among CPP groups. The chemical degradation rate of K-doped CPP groups was slower than undoped CPP.

A scanning reference electrode technique (SRET) apparatus has been designed and commissioned to investigate the corrosion of friction stir spot welds (FSSW) made in AZ31 magnesium alloys. The operational parameters of the apparatus have been calibrated to give good spatial resolution. By combining the SRET data with material flow data and immersion test data it was found that the FSSW process caused the formation of distinct noble and active regimes within the weld area. The noble region was aligned with the stir zone (SZ) and was caused by a dynamically recrystallized grain structure which is void of dislocations / twins, and β Mg17Al12. Localized corrosion attack was observed in both SRET and immersion testing along the thermo-mechanically affected zone (TMAZ). The same effect was consistently observed with a flat versus concave shoulder tool, and dwell times of 1s and 4s.

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.

The need for lighter weight vehicles to improve fuel efficiency is becoming increasingly imperative. Sheet magnesium alloys offer the potential as a light weight material for use in the transportation industry due to their high specific strength and stiffness. In fact, magnesium alloys have the highest strength-to-weight ratio of all the common structural metals. Though the demand for light weight materials is present and sheet magnesium is available, the use of these materials in automotive applications has been rather limited due to high production costs and poor corrosion performance and formability. A promising process to produce wrought magnesium sheet in a more cost effective manner is Twin Roll Casting (TRC). In addition, enhanced corrosion resistance and ductility may be realized in these sheet alloys with the possible introduction of a clad layer during the TRC process thereby producing a laminate sheet where the surface properties are different from the core. The focus of this research was to investigate the potential of cladding magnesium alloy AZ31 material during the TRC process. As part of this research, a thermal fluid mathematical model of the TRC process was developed, which was then further refined to include the addition of a clad layer during the process. The TRC model was validated through experimental work conducted at the Pohang Institute of Science and Technology (POSTECH University), where TRC experiments of AZ31 were conducted under various casting conditions. The as-cast microstructure of the AZ31 sheets were characterized and measurements of the secondary dendrite arm spacing (SDAS) made at the mid-region were compared to predicted microstructures from the TRC model based on solidification history. The predicted SDAS matched with the measured values, thus, validating the model. Using the validated TRC model the feasibility of adding a clad layer was assessed and various simulations were conducted to observe the effects of cast speed, cast thickness, and clad material on the thermal history and temperature profile in both the clad and core domains. The material properties and clad thickness did not seem to impact the temperature profiles significantly, while the cast speed and initial temperature dictated whether or not the cast would be successful. Using these operational parameters a process window was created (based on the CANMET facility) to illustrate the feasibility of casting and cladding during TRC. This window is beneficial for future experimentation and understanding the effects of these casting parameters.

Pure magnesium (Mg) and its alloys with calcium (Ca) and both Ca and zinc (Zn) have potential as bioresorbable bone implant materials provided the corrosion rate can be controlled. Thus, corrosion behaviour was investigated for pure Mg, Mg-2Ca, and Mg-2Ca-1Zn cast alloys subjected to either no heat treatment or to solutionizing and aging heat treatment. In addition, corrosion behaviour was investigated for surface modifications involving the deposition of silver (Ag) nanoparticles. These materials and constructs were all nominally biocompatible in that they would not elicit a strong and immediate adverse tissue reaction when implanted in bone. Static immersion tests in Hanks’ balanced salt solution were performed to evaluate the corrosion behaviour. The Mg-2Ca alloy exhibited the highest corrosion rate when compared with pure Mg and Mg-2Ca-1Zn for any length of immersion time. For short immersion times (48 hours), solutionizing followed by natural aging reduced the corrosion rate of Mg-2Ca alloy, but this heat treatment did not seem to have an effect on the corrosion rate of Mg-2Ca-1Zn alloy. As well, for short immersion times (48 hours), solutionizing and artificial aging also did not seem to have a large effect on corrosion rates for either Mg-2Ca or Mg-2Ca-1Zn, when compared to solutionizing and natural aging. Corrosion behaviour of surface-modified samples was sensitive to certain features of the Ag depositions. It was found that when the deposited Ag tracks were thick and wide, the corrosion rate of Ag-deposited samples increased significantly when compared to samples without any Ag deposition. However, when the Ag tracks were thinner and somewhat narrower, the corrosion rate did not appear to be much higher than that of samples without Ag deposition. Therefore, controlled Ag deposition may not be too detrimental to the corrosion behaviour of Mg and Mg alloys. The corrosion product morphology appeared to be similar for both the samples deposited with Ag and samples without any Ag. Needle-like formations were observed in small areas on the corroded surfaces. X-ray diffraction revealed Mg(OH)₂ as the main corrosion product. Because energy dispersive X-ray analysis consistently revealed multiple elements in the corrosion products (such as Mg, O, Ca, P, small amounts of C, and sometimes Cl), it was concluded that other compounds (possibly hydroxyapatite, magnesium chloride, and/or magnesium- and calcium-containing phosphates) may have formed in addition to the Mg(OH)₂.

Although Twin Roll Casting (TRC) process has been used for almost 60 years in the aluminum industry, TRC of magnesium is relatively new. In TRC, molten metal is fed onto water-cooled rolls, where it solidifies and is then rolled. Solidification of the molten metal starts at the point of first metal-roll contact and is completed before the kissing point (point of least roll separation) of the two rolls. The unique thermo-physical properties inherent to magnesium and its alloys, such as lower specific heat and latent heat of fusion and larger freezing ranges (in comparison with aluminum and steel) make it challenging for TRC of this alloy. Therefore, a comprehensive understanding of the process and the interaction between the casting conditions and strip final quality is imperative to guarantee high quality twin roll cast strip production. A powerful tool to achieve such knowledge is to develop a mathematical model of the process. In this thesis, a 2D mathematical model for TRC of AZ31 magnesium alloy has been developed and validated based on the TRC facility located at the Natural Resources Canada Government Materials Laboratory (CanmetMATERIALS) in Hamilton, ON, Canada. The validation was performed by comparing the predicted exit strip temperature and secondary dendrite arm spacing (SDAS) through the strip thickness with those measured and obtained by experiments. The model was developed in two stages, first a thermal-fluid model was developed followed by validation and then a thermal-fluid-stress model was developed. This is the first time a comprehensive thermal-fluid-stress model has been developed to simulate the TRC process for magnesium alloys. The work has led to new knowledge about the TRC process and its effects on magnesium strip quality including the following: 1) Using ALSIM and ANSYS® CFX® commercial packages a 2D mathematical model of thermal-fluid-stress behavior of the magnesium sheet during TRC was successfully developed and validated. 2) An average value of 11 kW/m2°C for the Heat Transfer Coefficient (HTC) was found to best represent the heat transfer between the roll and the strip during TRC casting of AZ31 using the CanmetMATERIALS facility. 3) Modeling results showed that increasing casting speed, casting thicker strips and applying higher HTCs led to less uniform microstructure through thickness in terms of SDAS. 4) Simulations showed the importance of casting parameters such as casting speed and set-back distance on the thermal history and stress development in the sheet during TRC; higher casting speeds led to deeper sumps and higher exit temperatures as well as lower overall rolling loads and lower total strains experienced during TRC. 5) The effect of roll diameter on the thermal history and stress development in the strip was also studied and indicated how larger roll diameters increased the surface normal stress and rolling loads but had little effect on the mushy zone thickness. 6) The correlation between the mechanisms of center-line and inverse segregation formation and thermo-mechanical behavior of the strip was performed. The modeling results suggested that increasing the set-back distance decreases the risk of both defects. Moreover, increasing the roll diameter reduces the propensity to inverse segregation but has a minor effect for center-line segregation formation.

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.

Cast Al-Si and Al-Si-Mg alloys containing up to 12.6 wt. pct. silicon and 1.0 wt. pct. magnesium were prepared. The changes in electrical conductivity/resistivity of these alloys due to strontium additions have been investigated and explained in terms of variations in microstructure. The conductivity behaviour of strontium-containing and strontium-free alloys was found to exhibit marked differences, depending on the silicon and magnesium contents and the rate of solidification. The electrical conductivity of single phase alloys containing less than 1.60 wt. pct. Si decreased with increasing silicon and magnesium levels. However, strontium had no effect on the conductivity of these solid solution alloys since it does not dissolve appreciably in the aluminum matrix or change the solid solubility of silicon and magnesium in aluminum. Silicon precipitation processes in the supersaturated solid solution alloys of Al-Si and Al-Si-Sr have been examined using the Johnson-Mehl-Avrami equation and found to be isokinetic. Strontium, however, retarded the growth rate of silicon precipitates. Strontium did not affect the kinetics of G.P. zone formation in Al-Si-Mg alloys but it suppressed the formation of stable Mg$ sb2$Si precipitates during subsequent aging at 175$ sp circ$C. Unlike the single phase alloys, two phase Al-Si and Al-Si-Sr alloys, in the range of 2.0 to 12.6 wt. pct. Si, exhibited different electrical conductivity behaviour. The strontium-containing alloys showed a higher conductivity than alloys with no strontium, and this conductivity difference increased as the silicon and magnesium contents were increased and the solidification rate was decreased. It has been demonstrated this difference is due to changes in the silicon morphology. Electron scattering at the interface between the aluminum matrix and the eutectic silicon phase contributes significantly more to the resistivity of unmodified alloys than that of modified alloys. In addition, the resistivity of

Although Twin Roll Casting (TRC) process has been used for almost 60 years in the aluminum industry, TRC of magnesium is relatively new. In TRC, molten metal is fed onto water-cooled rolls, where it solidifies and is then rolled. Solidification of the molten metal starts at the point of first metal-roll contact and is completed before the kissing point (point of least roll separation) of the two rolls. The unique thermo-physical properties inherent to magnesium and its alloys, such as lower specific heat and latent heat of fusion and larger freezing ranges (in comparison with aluminum and steel) make it challenging for TRC of this alloy. Therefore, a comprehensive understanding of the process and the interaction between the casting conditions and strip final quality is imperative to guarantee high quality twin roll cast strip production. A powerful tool to achieve such knowledge is to develop a mathematical model of the process. In this thesis, a 2D mathematical model for TRC of AZ31 magnesium alloy has been developed and validated based on the TRC facility located at the Natural Resources Canada Government Materials Laboratory (CanmetMATERIALS) in Hamilton, ON, Canada. The validation was performed by comparing the predicted exit strip temperature and secondary dendrite arm spacing (SDAS) through the strip thickness with those measured and obtained by experiments. The model was developed in two stages, first a thermal-fluid model was developed followed by validation and then a thermal-fluid-stress model was developed. This is the first time a comprehensive thermal-fluid-stress model has been developed to simulate the TRC process for magnesium alloys. The work has led to new knowledge about the TRC process and its effects on magnesium strip quality including the following: 1) Using ALSIM and ANSYS® CFX® commercial packages a 2D mathematical model of thermal-fluid-stress behavior of the magnesium sheet during TRC was successfully developed and validated. 2) An average value of 11 kW/m2°C for the Heat Transfer Coefficient (HTC) was found to best represent the heat transfer between the roll and the strip during TRC casting of AZ31 using the CanmetMATERIALS facility. 3) Modeling results showed that increasing casting speed, casting thicker strips and applying higher HTCs led to less uniform microstructure through thickness in terms of SDAS. 4) Simulations showed the importance of casting parameters such as casting speed and set-back distance on the thermal history and stress development in the sheet during TRC; higher casting speeds led to deeper sumps and higher exit temperatures as well as lower overall rolling loads and lower total strains experienced during TRC. 5) The effect of roll diameter on the thermal history and stress development in the strip was also studied and indicated how larger roll diameters increased the surface normal stress and rolling loads but had little effect on the mushy zone thickness. 6) The correlation between the mechanisms of center-line and inverse segregation formation and thermo-mechanical behavior of the strip was performed. The modeling results suggested that increasing the set-back distance decreases the risk of both defects. Moreover, increasing the roll diameter reduces the propensity to inverse segregation but has a minor effect for center-line segregation formation.

Thesis (Ph. D.)--West Virginia University, 2007. / Title from document title page. Document formatted into pages; contains xvi, 145 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 142-145).