Alessandro Morri

Aluminum and Magnesium Metal Matrix Nanocomposites

The book looks into the recent advances in the ex-situ production routes and properties of aluminum and magnesium based metal matrix nanocomposites (MMNCs), produced either by liquid or semi-solid ...

Influence of Heat Treatment on Microstructure and Mechanical Properties of Rare Earth-Rich Magnesium Alloy

The experimental activity was aimed at optimizing the T6 heat treatment conditions of the innovative rare earth-rich magnesium alloy EV31A (Elektron 21®). The investigated alloy (Mg–Nd2.8–Gd1.5–Zr0.5–Zn0.2) contains Nd and Gd in proper amounts to maximize both castability and high-temperature performance. The effect of treatment parameters on microstructure, hardness and tensile properties was evaluated. Mg12NdxGd(1−x) ternary eutectic, Zr clusters and Mg/Nd–Gd intermetallic compounds were found in the as-cast alloy. A remarkable microstructural evolution, involving Mg/Nd–Gd compounds dissolution, was observed in the aged alloy; the precipitation sequence was identified as βIIxa0→xa0βIxa0→xa0β. DTA analyses confirmed that 793xa0K (520xa0°C/968xa0°F) is the optimum solutionizing temperature in order to avoid incipient melting; on the contrary, the solutionizing time, should be reduced with respect to the standard conditions without loss in the final alloy hardness and tensile properties. The effect of aging parameters was also investigated at fixed solutionizing and quenching conditions; different aging temperatures [463, 473, 483xa0K (190, 200, 210xa0°C/374, 392, 410xa0°F)] and aging times (up to 48xa0h) were studied. The aging curves at 463 K (190xa0°C/374xa0°F) and 473 K (200xa0°C/392xa0°F) showed a similar trend, with a large peak-aging plateau; on the contrary, the peak-aging time was significantly reduced at 483 K (210xa0°C/410xa0°F). Tensile tests, performed on the most promising heat treatment conditions, did not show remarkable differences in terms of yield, ultimate tensile strength and elongation to failure with respect to the standard heat treatments conditions. This study therefore suggests that standard solutionizing and aging times can be considerably shortened, ensuring excellent mechanical properties, compared to standard T6 heat treatment, leading to commercial implications with regard to operational costs and energy saving.

The Influence of Cooling Rate on Microstructure, Tensile and Fatigue Behavior of Heat-Treated Al-Si-Cu-Mg Alloys

Al-Si-Mg alloys are commonly employed for the production of automotive castings. In view of the recent stringent emissions standards and consequent engine downsizing, these components must withstand higher temperatures and stresses than in the past. In this regard, the heat treatable quaternary Al-Si-Cu-Mg alloys gained particular interest in recent years, due to their superior mechanical properties and higher thermal stability. The present research activity was addressed to evaluate the influence of cooling rate on microstructure and consequently on room temperature tensile and fatigue behaviour of the A354 and C355 alloys. Samples for mechanical tests were produced under controlled cooling rates, in order to induce different secondary dendrite arm spacing (SDAS) values, classified as fine (20-25μm) and coarse (50-70μm). The experimental results showed that the cooling rate strongly influences the type, size and morphology of intermetallic particles. The presence of coarse intermetallic phases, mostly Fe-based, observed in coarse SDAS specimens, was reported to strongly affect ultimate tensile strength (UTS), elongation to failure and fatigue strength of both the investigated alloys. A correlation between UTS and fatigue resistance was found, independent of microstructural coarseness.

EN AW-4032 T6 Piston Alloy After High-Temperature Exposure: Residual Strength and Microstructural Features

Abstract This study aims to evaluate the effects of prolonged thermal exposure on both microstructural evolution and mechanical properties of the EN AW-4032 T6 piston alloy. For the purpose, the experimental activities have been carried out on samples machined from forged and heat-treated automotive pistons. The effects of overaging have been investigated in the temperature range of 140-290xa0°C, firstly by evaluating the time-temperature-hardness curves and then by carrying out room-temperature tensile tests on overaged samples. The material softening was substantial and extremely rapid when the soaking temperature exceeded 250xa0°C. During overaging, both the tensile strength and the residual hardness considerably decreased, and a relationship between these parameters has been established. The alloy behavior in the plastic field has been modeled according to the Hollomon’s equation, showing that both the strain hardening exponent and the strength coefficient are a function of the residual hardness. The results were finally related to the corresponding microstructural changes: OM and FEG-SEM metallographic and fractographic analyses on overaged samples gave evidence of coarsened precipitates along the grain boundaries.

Fatigue limits of titanium-bar joints made with the laser and the electric resistance welding techniques: microstructural characterization and hardness properties

Fatigue behavior of the titanium bars is of utmost importance for the safe and reliable operation of dental implants and prosthetic constructions based on these implants. To date, however, only few data are available on the fatigue strength of dental prostheses made with electric resistance welding and laser welding techniques. This in-vitro study highlighted that although the joints made with the laser welding approach are credited of a superior tensile strength, joints made with electric resistance welding exhibited double the minimum fatigue strength with respect to the joints made with laser welding (120 vs 60u2009N).

Effects of Casting Size on Microstructure and Mechanical Properties of Spheroidal and Compacted Graphite Cast Irons: Experimental Results and Comparison with International Standards

The aim of this research was to investigate the effects of casting size (10-210xa0mm) on the microstructure and mechanical properties of spheroidal (SGI) and compacted (CGI) graphite cast irons. A comparison of the experimental mechanical data with those specified by ISO standards is presented and discussed. The study highlighted that the microstructure and mechanical properties of SGI (also known as ductile or nodular cast iron) are more sensitive to casting size than CGI (also known as vermicular graphite cast irons). In particular, in both types of cast iron, hardness, yield strength and ultimate tensile strength decreased, with increasing casting size, by 27% in SGI and 17% in CGI. Elongation to failure showed, instead, an opposite trend, decreasing from 5 to 3% in CGI, while increasing from 5 to 11% in SGI. These results were related to different microstructures, the ferritic fraction being more sensitive to the casting size in SGI than CGI. Degeneration of spheroidal graphite was observed at casting size above 120xa0mm. The microstructural similarities between degenerated SGI and CGI suggested the proposal of a unified empirical constitutional law relating the most important microstructural parameters to the ultimate tensile strength. An outstanding result was also the finding that standard specifications underestimated the mechanical properties of both cast irons (in particular SGI) and, moreover, did not take into account their variation with casting size, at thicknesses over 60xa0mm.