Fatigue of Forged AZ80 Magnesium Alloy

The majority of research surrounding the fatigue of Mg alloys generally exhibits a rigid dichotomy between theoretical and applied contributions. This research work addresses both of these domains of the field from a more holistic sense, yet still remains highly detail oriented. Automotive suspension components generally have complex geometries and undergo highly multiaxial loading. This is partly due to the packaging constraints imposed by the many dynamic systems within a vehicle, and the impetuous towards lightweighting to improve efficiency and reduce greenhouse gas emissions. As such, the current optimal solution for such a component typically a complex shape made from a material with high specific strength. Both forging and casting lend themselves to facilitating large scale production of such components using industrially compatible processes. Forging however produces a product with attributes which are more optimally suited for advanced vehicle lightweighting applications. Of the commercially available Mg alloys, the AZ80 alloy is a Mg alloy with good forgeability, a high aluminium content, and superior strength. However the fatigue properties of this alloy are largely unknown, especially in complex multiaxial loading paths such as which automotive suspension components undergo. This thesis acts to fill this gap in knowledge, by providing the foundation for the understanding of the complex cyclic behavior of forged AZ80 Mg, as well as predicting its fatigue life to ensure the satisfaction and safety of the end consumer. Various small scale forging methods were investigated and characterized in such a way that it one can connect them to the larger scale component in the engineering application. Two varieties of base material were selected to be forged into these small scale forgings, cast and extruded. Furthermore, an understanding was developed on the influence of material texture on the cyclic deformation mechanism and resulting fatigue life. The implications of multiaxial loading on the fatigue behaviour was also characterized as well as the effect of nonproportional loading. A variety of different models were utilized to reliably predict the fatigue life of forged AZ80 Mg in both simple uniaxial and complex non-proportional bi-axial loading paths. The culmination of all of these research objectives enabled effective utilization of forged AZ80 Mg as a lightweight material for a variety of different fatigue critical engineering applications. It was concluded that the thermomechanical history imparted to the material via forging resulted in a texture intensification and a rotation of the crystallographic cells to align with the loading direction during forging. Secondly, following forging, both the cast-forged and extruded-forged material exhibited an significant increase in fatigue life. It was also discovered that the style of closed-die forging being investigated had spatially varying properties with texture orientations which varied based on the local forging directions and intensities which were dependent on the starting texture as well as thevi thermomechanical history. Furthermore, following characterization of the materials behaviour over a variety of different loading paths, the biaxial fatigue response is somewhat dominated by the axial component and the non-proportional effect to be detrimental to the fatigue life. Finally, it was concluded that the optimal forging condition tends towards the coldest temperature and fastest strain rate which are pragmatically possible (within the context of warm forging) that produce a forging free of defects and of high quality. This optimal condition corresponded to extruded AZ80 Mg forged at a temperature of 250°C and 20 mm/sec

Improvement of Fatigue Properties of AZ31B Extruded Magnesium Alloy through Forging

Axial monotonic and load-controlled fatigue tests were performed to investigate the influence of forging at various temperatures and different deformation rates, on both the microstructural and mechanical behaviour of extruded AZ31B magnesium alloy. The obtained microstructural analysis showed that the extruded AZ31B magnesium alloy possesses a bimodal grain structure with strong basal texture. In contrast, once forged, the material showed refined grains and a modified texture. A monotonic yield and ultimate tensile strength of about 223 MPa and 278 MPa were observed for the forged samples showing an increase of 18%, from the as-extruded material. The optimum forging condition was determined to be the coldest of the investigated temperatures, based on the improvement in both monotonic and cyclic properties vs. the as-extruded material. The fractographic analysis of the failure surfaces showed that ductile type fractures occurred in both as-extruded and forged samples. However, more dimples and plastic deformation were identified in the fracture surfaces of the forged specimens. A significant improvement of fatigue life was also observed for all of the forged samples, in particular those forged at 400°C and 39 mm/min. Forging improved the fatigue life via a combination of grain refinement and texture modification resulting in improved strength and ductility

Effect of thermomechanical processing defects on fatigue and fracture behaviour of forged magnesium

The microstructural origins of premature fatigue failures were investigated on a variety of forged components manufactured from AZ80 and ZK60 magnesium, both at the test specimen level and the full-scale component level. Both stress and strain-controlled approaches were used to characterize the macroscopically defect-free forged material behaviour as well as with varying levels of defect intensities. The effect of thermomechanical processing defects due to forging of a industrially relevant full-scale component were characterized and quantified using a variety of techniques. The fracture initiation and early crack growth behaviour was deterministically traced back to a combination of various effects having both geometric and microstructural origins, including poor fusion during forging, entrainment of contaminants sub-surface, as well as other inhomogeneities in the thermomechanical processing history.             At the test specimen level, the fracture behaviour under both stress and strain controlled uniaxial loading was characterized for forged AZ80 Mg and a structure-property relationship was developed. The fracture surface morphology was quantitatively assessed revealing key features which characterize the presence and severity of intrinsic forging defects.  A significant degradation in fatigue performance was observed as a result of forging defects accelerating fracture initiation and early crack growth, up to 6 times reduction in life (relative to the defect free material) under constant amplitude fully reversed fatigue loading.             At the full-scale component level, the fatigue and fracture behaviour under combined structural loading was also characterized for a number of ZK60 forged components with varying levels of intrinsic thermomechanical processing defects. A novel in-situ non-contact approach (utilizing Digital-Image Correlation) was used as a screening test to establish the presence of these intrinsic defects and reliably predict their effect on the final fracture behaviour in an accelerated manner compared to conventional methods

Low-cycle fatigue characterization and texture induced ratcheting behaviour of forged AZ80 Mg alloys

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.ijfatigue.2018.06.028 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Stress-controlled uniaxial “push-pull” fatigue testing was conducted on as-received (cast and extruded) and closed-die cast-forged and extruded-forged AZ80 Mg alloy. The as-cast material possessed random texture and somewhat symmetric cyclic responses. The extruded and forged materials possessed sharp basal texture and asymmetric cyclic responses. All materials exhibited tension/compression asymmetry in their cyclic response to varying degrees, depending on the thermomechanical processing conditions. It was discovered that the style of closed-die forging being investigated had spatially varying properties with texture orientations which varied based on the local forging directions and intensities which were dependent on the starting texture as well as the thermomechanical history. Under fatigue testing, the materials all developed some form of mean strain, with the nature and magnitude of this mean strain being dependent on primarily its texture intensity and propensity to twin in either tension or compression reversals. The type of mean strain (tensile or compressive) depends upon both the orientation and intensity of the starting texture of material. The texture induced ratcheting and resulting mean strain evolution was most pronounced in the as-cast material and had a significant impact on the fatigue life. Following forging, the material exhibited an increase in fatigue life of anywhere from 2 to 15 times for the cast then forged material and more modest yet still significant 8 times longer at stress amplitudes around 140 MPa for the extruded then forged material. The extruded forged material exhibited similar fatigue lives to that of the base material at stress amplitudes which approached the yield strength. The nature of the mean stress development and degree of fatigue life improvement depended on the processing conditions and the type of base material (cast or extruded) utilized to create the forging. Two energy based models were utilized to predict the life of the forged material, and gave a reliable life prediction for a variety of material conditions that were investigated.Natural Sciences and Engineering Research Council of Canad

Monotonic and cyclic behaviour of cast and cast-forged AZ80 Mg

The final publication is available at Elsevier via https://doi.org/10.1016/j.ijfatigue.2017.06.038 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Tensile and strain-controlled fatigue tests were performed to investigate the influence of forging on the performance of cast AZ80 magnesium alloy. The obtained microstructural analysis showed that the as-cast AZ80 magnesium alloy has dendritic α-Mg phase with eutectic Mg17Al12 morphology and a random texture. In contrast, the forged samples showed refined grains and a strong basal texture. During tensile testing, a maximum yield and ultimate tensile strength of 182 MPa and 312 MPa were obtained for the forged samples, representing increases of 121% and 33%, respectively, from the as-cast condition. At the same time, a significant improvement (73%) in ductility was obtained in forged samples. It was also observed that the forged samples achieved comparatively longer fatigue life under strain-controlled cyclic loading. Analysis of the fracture surfaces showed that a cleavage-type morphology was typical for the as-cast samples, while the occurrence of dimples and other evidence of plastic deformation were identified in the fracture surfaces of the forged specimens, indicating a more ductile response. Forging caused grain refinement and texture modification, both of which enhance alloy performance by improving strength and ductility, and leading to longer fatigue life. Strain and energy-based models were investigated for their suitability to predict the life of the forged material. Both the Smith-Watson Topper and the Jahed-Varvani energy-based models gave reliable life prediction.Natural Sciences and Engineering Research Council of Canada (NSERC) through the Automotive Partnership Canada (APC) under APCPJ 459269–13 grant with contributions from Multimatic Technical Centre, Ford Motor Company, and Centerline Windsor are acknowledged