Investigation on some factors affecting crack formation in high resistance aluminum alloys

Aluminum alloys having good mechanical properties are Al-Zn-Mg alloys (7xxx) and Al-Cu-Li alloys (Weldalite®). These alloys may be subjected to stress corrosion cracking. In order to overcome this problem the Al 7050 alloy has been developed and it is widely used for aerospace applications. Despite that, some components made of this alloy cracked during the manufacturing process including machining and chemical anodization. In a previous work cracked Al 7050 components have been analyzed in order to identify possible causes of crack formation. In this work the susceptibility of this alloy to intergranular corrosion has been analysed and compared with that of other high resistance aluminum alloys

Study of defect formation in Al 7050 alloys

The Al 7050 alloy is an Al-Zn-Mg-Cu-Zr alloy having good mechanical properties. This alloy has been developed in order to overcome stress corrosion cracking problems that characterise 7xxx Al alloys. Despite Al 7050 is widely used for aerospace applications, it can be subjected to crack initiation and propagation during the manufacturing process. In this work cracked Al 7050 components have been analysed in order to identify possible causes of crack formation such as coarse intermetallic phase presence, voids or wrong mechanical machining processes

Fracture behaviour of alloys for a new laser ranged satellite

A new laser-ranged satellite called LARES 2 (Laser Relativity Satellite 2) has been recently designed for accurate tests of Einsten's theory of General Relativity and space geodesy. Some high density alloys (8.6-9.3 g/dm3) have been studied and characterised for producing the LARES 2 passive satellite. The considered materials were Copper and Nickel based alloys that have been produced and characterised. Aim of this work was to analyse their fracture behaviour that is a requirement for materials to be used for space applications. Fracture tests have been carried out on several specimens and fracture surfaces have been analysed

High-temperature oxidation behaviour of a TiAl-based alloy subjected to aluminium hot-dipping

In this research the oxidation resistance at high temperature of a TiAl-based alloy has been improved by hot-dipping the alloy in molten aluminium and by performing an interdiffusion process. After selecting the best process parameters, a compact TiAl3 coating characterized by an almost constant thickness was formed on the surface. Isothermal oxidation tests, carried out at 900, 950 and 1000 °C, showed that the coated alloy is able to form a continuous and thin alumina layer that is very protective. Microstructural investigations highlighted that, above 900 °C, long residence times at high temperature determine the diffusion through the TiAl3 layer of Cr that favours migration toward the outer surface of Al and thus the formation of a self-healing alumina layer

Fracture toughness of TiAl-Cr-Nb-Mo alloys produced via centrifugal casting

Fracture toughness of a TiAl base intermetallic alloy has been investigated at room temperature.The Ti-48Al-2.5Cr-0.5Nb-2Mo (at. %) alloy produced via centrifugal casting exhibits fine nearly lamellar microstructures, consisting mainly of fine lamellar grains, together with a very small quantity of residual ? phases along lamellar colony boundaries. In order to determine the alloy fracture toughness compact tension specimens were tested and the results were compared with those available in literature

Effects of the manufacturing process on fracture behaviour of cast TiAl intermetallic alloys

The ? -TiAl based intermetallic alloys are interesting candidate materials for high-temperatureapplications with the efforts being directed toward the replacement of Ni-based superalloys. TiAl-based alloysare characterised by a density (3.5-4 g/cm3) which is less than half of that of Ni-based superalloys, and thereforethese alloys have attracted broad attention as potential candidate for high-temperature structural applications.Specific composition/microstructure combinations should be attained with the aim of obtaining goodmechanical properties while maintaining satisfactory oxidation resistance, creep resistance and high temperaturestrength for targeted applications.Different casting methods have been used for producing TiAl based alloys. In our experimental work,specimens were produced by means of centrifugal casting. Tests carried out on several samples characterised bydifferent alloy compositions highlighted that solidification shrinkage and solid metal contraction during coolingproduce the development of relevant residual stresses that are sufficient to fracture the castings during coolingor to produce a delayed fracture. In this work, crack initiation and growth have been analysed in order toidentify the factors causing the very high residual stresses that often produce explosive crack propagationthroughout the casting

Production issues in the manufacturing of TiAl turbine blades by investment casting

γ -TiAl based intermetallic alloys have attracted broad attention as a potential candidate for high-temperature structural applications. A careful selection of their composition and microstructure allows to obtain an interesting combination of oxidation resistance, creep resistance and high temperature strength for specific applications. Over the years, several complex manufacturing methods have been used for producing TiAl parts. In this work TiAl blades have been produced by means of investment casting. The work analyses the different production steps: design of a blade prototype, production of an ABS model by additive manufacturing that is used to make the wax model, preparation of the ceramic mold and centrifugal casting of the component. All the production issues related to TiAl alloy investment casting have been analyzed with the aim of setting up a tailored process able to produce sound components

Additively manufactured CuCrZr alloy: improvement of mechanical properties by heat treatment

CuCrZr alloy plays a fundamental role for the production of critical components because it is characterized by good thermal and electrical conductivity and by high mechanical strength after precipitation hardening treatment. In the framework of a wider research on the mechanical behaviour of additively manufactured CuCrZr alloy, this study focuses on the effects of heat treatment parameters on the alloy strength. The additive manufacturing process, characterized by very high cooling rates, determines the formation, in the as-built condition, of a supersaturated solid solution. The results obtained reveal that aging temperature and time are critical parameters for improving the mechanical behaviour of CuCrZr alloy which behaves differently than the alloy produced through the use of traditional techniques

Effect of composition and heat treatment on the mechanical properties of Fe Mn Al steels

Starting from the research aimed at the development of substitute alloys for stainless steels, with the aim of replacing strategic metals such as chromium and nickel with the more available manganese, FeMnAlC alloys have been studied and developed for several years. These alloys exhibit an attractive strength/ductility combination, low density, and some of them show good oxidation behaviour at high temperatures. In this work we studied alloys with novel compositions to highlight the effect of heat treatments on their mechanical behaviour, in order to tailor alloy composition and process parameters for a specific application. After a preliminary study, in this paper the effect of a solubilization treatment followed by aging in the temperature range 550 - 750 degrees C has been evaluated. The results of the investigation revealed that the steel characterized by the higher amount of Mn and Al shows, after heat treatment, the formation of phases that make the alloy very brittle. Considering the obtained results, it is evident that optimizing the alloy chemical composition is of paramount importance to guarantee a high fracture toughness if the steel works for limited time intervals at high temperature

Lead electrorefining process from exhausted lead acid batteries by using acidic and alkaline electrolytes

The recycling of lead acid batteries (LABs) comprises relevant concerns on the suitable methodologies to recover lead. In this investigation, two electrorefining processes, by using acidic and alkaline electrolytes, have been compared to determine the most significant results of both methodologies. Acidic electrolytes used 200 g/L HBF4, 1.2 g/L H3PO4, 10 g/L H3BO3, and 100 g/L PbO, while the alkaline ones employed 120 g/L NaOH, 75 g/L PbO, 50-92 g/L glycerol, and 2 g/L gelatin. All the solutions were studied by varying temperature and current density (CD) to determine remarkable changes on current efficiency (CE), cell voltage (CV), specific energy consumption (SEC), and on the deposit quality. The results highlighted that by using the acidic electrolyte containing dextrin it is possible to obtain a compact deposit even using high CDs. Acidic solution without dextrin addition allows to obtain good quality deposits by working at 40 degrees C and 100 A/m(2) CD. After testing the behavior of different alkaline electrolytes, it is possible to observe that the solution containing the highest glycerol concentration allows to obtain, for intermediate valued of CD, CE higher than 97% with a SEC of about 0.37 kWh/kg. By comparing the results it is evident that the acidic electrolyte is the one that allows to reach higher productivity with lower SEC. In the selected conditions, lead deposits appear pure and compact.[GRAPHICS]