Leandro Bolzoni

Evaluation of the mechanical properties of powder metallurgy Ti-6Al-7Nb alloy

Titanium and its alloys are common biomedical materials owing to their combination of mechanical properties, corrosion resistance and biocompatibility. Powder metallurgy (PM) techniques can be used to fabricate biomaterials with tailored properties because changing the processing parameters, such as the sintering temperature, products with different level of porosity and mechanical performances can be obtained. This study addresses the production of the biomedical Ti-6Al-7Nb alloy by means of the master alloy addition variant of the PM blending elemental approach. The sintering parameters investigated guarantee that the complete diffusion of the alloying elements and the homogenization of the microstructure is achieved. The sintering of the Ti-6Al-7Nb alloy induces a total shrinkage between 7.4% and 10.7% and the level of porosity decreases from 6.2% to 4.7% with the increment of the sintering temperature. Vickers hardness (280-300 HV30) and tensile properties (different combination of strength and elongation around 900MPa and 3%) are achieved.

Formation of equiaxed crystal structures in directionally solidified Al-Si alloys using Nb-based heterogeneous nuclei

The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.

Grain Refiner Development for Al Containing Mg Alloys

We have found a chemical compound which can refine the grain structure of both commercially used Al-free and Al-containing magnesium alloys. In this work, the addition of novel grain refiner (NGR) on microstructural evolution of two magnesium alloys (AZ91D and AM50) solidified under various cooling rates is presented. A wedge-shaped copper mould was used to achieve continuous variation in cooling rate for both alloys. The influence of addition of the grain refiner for AM50 alloy is investigated for high pressure die casting (HPDC) process. A series of tensile samples were produced to inspect microstructural and mechanical properties. The observed improvement in elongation for grain refiner added samples is correlated with the grain refinement in early solidified crystals that are commonly observed in HPDC products.

Influence of a Novel Master Alloy Addition on the Grain Refinement of Al-Si Cast Alloys

The grain refinement practice using Ti based chemical additions is well established for wrought Al alloys, especially in the last few decades. In the case of Al-Si casting alloys the practice of adding grain refiners and the impact on castability is not well established in industries. The main reason is the chemical instability of conventionally known Ti based grain refiner which reacts with silicon forming intermetallic phases. Recently, researchers at Brunel University have identified a novel chemical composition that can refine the grain structure of Al-Si alloys in an effective way. Over the last year, this novel grain refiner in the form of master alloy was developed and tested in various Al-Si cast alloys that are commonly used in industry. Significant grain refinement is obtained when the master alloy is added to the liquid metal prior to casting. Moreover, the grain size of the Al-Si cast alloys is observed to be less sensitive to cooling rate when the master alloy is added. In this work, the influence of addition of the master alloy on microstructural evolution of various Al-Si alloys cast under various cooling rates is presented.

Tailoring Mechanical Properties of Extruded Ti-6Al-4V Alloy from the Blended Elemental Route via Microstructure Control

Vacuum-sintered billets of Ti-6Al-4V alloy from powder blend were extruded at two different temperatures: 1150 °C and 950 °C. The extruded material at 1150 °C was subjected to various heat treatments to obtain different microstructures: annealing in the β phase, β solution treatment and aging and α+β solution treatment and aging. The materials processed were characterised using scanning electron microscopy, X-ray diffraction and the mechanical properties were measured by tensile test. The microstructure of both extrusions are fairly similar, consisting of lamellar colonies, and the mechanical properties are also comparable, with yield strengths of about 1000 MPa, ultimate tensile strength of about 1100 MPa and elongation at fracture of 8-9%. The β annealing treatment, through coarsening the lamellar microstructure, reduces the strength of the alloy while keeping a high ductility. Both solution treatments and aging, which produces aged martensite and aged martensite and primary alpha, respectively, increases the strength and reduces the ductility. There is a trade-off between ductility and strength when it comes to tailoring the microstructure, and the as-extruded Ti-6Al-4V condition is the one with the best balance between strength and ductility.

The Effect of the Green Density on the Properties of Powder Metallurgy Ti Sintered by High Frequency Induction Heating

Sintering is a vital technology used for consolidation of metal and ceramic powders. The process is generally long and energy consuming because of the way in which heat transfer happens in electrical and gas furnaces. This study focuses on optimizing the sintering process of metallic powders, in particular titanium, using high frequency induction heating as alternative sintering method. Using electromagnetic induction and the associated Eddy current effect, the heat is generated directly into the electrically conductive object. Consequently, faster heating rates and lower heat loses are achieved. The purpose of this study is to understand the effect of process parameters, such as the powder compact density, on the efficiency of the induction heating and the properties of the sintered materials. The average heating rates recorded while heating to 1300oC are in the range of 3.5o to 15.3o C per second. Significant densification and consolidation, evident by the amount of closed porosity and increase in tensile strength was found in spite of the short heating time. The results show that the powder compact density plays a crucial role on the heating efficiency as well on the properties of the sintered material such as final density, porosity distribution and tensile properties. The samples with higher initial density showed tensile strength and ductility values comparable to those of high vacuum sintered and those specified by international standards for powder metallurgy Ti products.

Yield of Binary Ti-Cu and Ti-Mn Alloys Produced via Powder Metallurgy

High strength, low density, good corrosion resistance and biocompatibility is the combination of properties that Ti and its alloys can provide for engineering applications. Its costs are the most important limiting factor for the widespread use of Ti. Cost reduction for Ti alloys can derive from the use of cheaper alloying elements as well as the use of alternative manufacturing techniques. In this study binary Ti-X alloys (where X = Cu or Mn) were formulated and produced using the conventional powder metallurgy route of pressing and sintering. These chemical elements were selected because they are β stabilisers and can be used to create α+β Ti alloys. The study shows that with the techniques and processing parameters used handable products without delamination can be pressed. Moreover, chemically homogenous materials with density and mechanical property values comparable to those of other wrought-equivalent Ti alloys produced via powder metallurgy were achieved.

Low-Cost α+β PM Ti Alloys by Fe/Ni Addition to Pure Ti

Ti and its alloys can deliver a very interesting combination of properties such as low density, high strength, corrosion resistance and biocompatibility and, therefore, are very flexible materials which can be adapted to various applications. Nonetheless, Ti and Ti alloys are only employed in critical applications (i.e. aeronautical and aerospace, nautical, medical, etc.) or in products for leisure. In both of these cases the higher fabrication costs of Ti in comparison to its competitors (i.e. steel and aluminium) is not the limiting factor as it is for many structural applications, especially for mass production (i.e. automotive sector). The use of creative techniques and the decrement of the starting price of Ti have been identified as the two main routes to follow to decrease the fabrication costs. In this study, the production of low-cost α+β Ti alloys has been assessed by combining the addition of cheap alloying elements (in particular a Fe/Ni powder) with the classical powder metallurgy route (pressing and sintering). Physical and mechanical properties as well as microstructural analysis of these low-cost alloys were measured and correlated to the processing parameters used to sinter them. It is found that the low-cost Ti alloys show similar behaviour to conventional α+β Ti alloys and, thus, have the potential to be used for non-critical applications.

Development of low cost PM Ti alloys by thermomechanical processing of powder blends

This research focuses on the development of low cost powder metallurgy (PM) Ti alloys suitable for application in PM thermomechanical processing with mechanical properties comparable to those of wrought Ti6Al4V alloy. The alloy systems studied are Ti3Al2V, Ti5Fe and Ti3.2Fe1Cr0.6Ni0.1Mo (Ti5SS). The alloy mixtures were produced by blending Ti HDH powders with Al40V, 316SS master alloy powders or elemental Fe powder. The blended powders were further consolidated using various methods: high vacuum sintering (HVS), induction sintering (IS), powder compact forging (PCF) and powder compact extrusion (PCE). It is found that, PM Ti3Al2V and Ti5Fe alloy processed by PCE or PCF followed by recrystallization annealing (RA) achieved tensile properties comparable with wrought Ti6Al4V alloy. Tensile properties such as yield strength (YS) of 910MPa, UTS of 1010MPa and 15% elongation to fracture for Ti3Al2V alloy are reported. Ti5Fe alloy gives YS and UTS of 870MPa and 968MPa respectively, combined with 20.3% elongation to fracture. The tensile results are related to the microstructure developed during the consolidation processes. The oxygen contamination as a result of the high temperature processing is also reported.

Development of Low-Cost Powder Metallurgy Titanium Alloys by Addition of Commercial 430 Stainless Steel Powder

Titanium is characterised by an outstanding combination of properties (i.e. low density, high strength, corrosion resistance and biocompatibility) but its industrial applications are restricted due to its high extraction and fabrication costs. Near-net-shape or net-shape powder metallurgy techniques offer the possibility to reduce production costs due to intrinsic advantages (i.e. high yield of material, limited or no machining) avoiding the problems of working with molten titanium (reaction with the casting tools). Another reduction of the final costs can be achieved by using cheap alloying elements such as iron. In this study, the effect of the addition of commercial 430 stainless steel powder to elemental titanium to be processed by cold uniaxial pressing and sintering is studied. The properties of the sintered materials, relative density, mechanical properties and microstructural features of this low-cost titanium alloy are analysed and compared to those of the titanium workhorse alloy (Ti-6A1-4V).