Abstract
Combining efficiency and safety in machines and mechanical devices is not always an easy task, since efficiency implies lightness, and hence material usage reduction, while safety normally requires larger and bulkier parts, instead. The additive manufacturing (AM) technologies seem to be promising in reconciling these two conflicting requirements, being capable of realizing tailored and more effectively shaped components. The main goal of this thesis is to provide useful advanced tools to be used in designing components made of AM ductile alloys, to maximize material exploitation and to grant their structural integrity. To this purpose, a thorough knowledge of the mechanical behavior of these materials up to failure is firstly acquired, by means of a broad experimental campaign on different structural AM alloys. Secondly, proper numerical models for material failure prediction under complex stress states are selected, calibrated and validated.
More in detail, a static mechanical characterization based on standard uniaxial tests and nonconventional multiaxial tests was carried out on AlSi10Mg, Ti6Al4V, and 17-4PH. These alloys were investigated both in as-built and machined surface finish conditions. The same three alloys obtained through standard technologies, i.e. not AM, were also tested for reference. From the experiments, the material elastoplastic behavior and the ultimate fracture limits were identified. Some microstructural observations, hardness and roughness measurements were done, in the attempt to seek possible links between the microstructure and the observed macroscopic material behavior.
Then, four ductile damage models were chosen from the literature, assessing their effectiveness when applied to AM alloys. The damage models were calibrated by means of properly devised strategies using the results of the experiments, and their prediction capabilities were subsequently accurately evaluated and presented.
Furthermore, advanced multiaxial tests, namely shear – tension and tension – torsion, were executed on AlSi10Mg and 17-4PH using dedicated equipment, to validate both the plasticity behavior and check the transferability of the damage models to different stress states.
A more focused investigation on the effects of some heat treatments and surface finishing on the static mechanical performance of SLM Ti6Al4V was also performed, highlighting how the post-processing affects the overall behavior of the investigated materials. The types of post-processing were chosen among actual industrial applications affordable choices.
Eventually, as a test case, the better understanding of AM materials behavior was employed to redesign a hip prosthesis made of Ti6Al4V. To reduce the stress shielding phenomenon, a reticular structure was introduced into the stem to increase the overall compliance, having the desirable side effect of reducing the weight, also. The most effective of the investigated damage models can be used concurrently in the redesign of the prosthesis, to prevent undesired failures under critical loading conditions.Combining efficiency and safety in machines and mechanical devices is not always an easy task, since efficiency implies lightness, and hence material usage reduction, while safety normally requires larger and bulkier parts, instead. The additive manufacturing (AM) technologies seem to be promising in reconciling these two conflicting requirements, being capable of realizing tailored and more effectively shaped components. The main goal of this thesis is to provide useful advanced tools to be used in designing components made of AM ductile alloys, to maximize material exploitation and to grant their structural integrity. To this purpose, a thorough knowledge of the mechanical behavior of these materials up to failure is firstly acquired, by means of a broad experimental campaign on different structural AM alloys. Secondly, proper numerical models for material failure prediction under complex stress states are selected, calibrated and validated.
More in detail, a static mechanical characterization based on standard uniaxial tests and nonconventional multiaxial tests was carried out on AlSi10Mg, Ti6Al4V, and 17-4PH. These alloys were investigated both in as-built and machined surface finish conditions. The same three alloys obtained through standard technologies, i.e. not AM, were also tested for reference. From the experiments, the material elastoplastic behavior and the ultimate fracture limits were identified. Some microstructural observations, hardness and roughness measurements were done, in the attempt to seek possible links between the microstructure and the observed macroscopic material behavior.
Then, four ductile damage models were chosen from the literature, assessing their effectiveness when applied to AM alloys. The damage models were calibrated by means of properly devised strategies using the results of the experiments, and their prediction capabilities were subsequently accurately evaluated and presented.
Furthermore, advanced multiaxial tests, namely shear – tension and tension – torsion, were executed on AlSi10Mg and 17-4PH using dedicated equipment, to validate both the plasticity behavior and check the transferability of the damage models to different stress states.
A more focused investigation on the effects of some heat treatments and surface finishing on the static mechanical performance of SLM Ti6Al4V was also performed, highlighting how the post-processing affects the overall behavior of the investigated materials. The types of post-processing were chosen among actual industrial applications affordable choices.
Eventually, as a test case, the better understanding of AM materials behavior was employed to redesign a hip prosthesis made of Ti6Al4V. To reduce the stress shielding phenomenon, a reticular structure was introduced into the stem to increase the overall compliance, having the desirable side effect of reducing the weight, also. The most effective of the investigated damage models can be used concurrently in the redesign of the prosthesis, to prevent undesired failures under critical loading conditions.