Lorella Ceschini

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 ...

Microstructure and mechanical properties of heavy section ductile iron castings: experimental and numerical evaluation of effects of cooling rates

This paper focuses on the development of experimental relationships between the microstructure and mechanical properties of a sand cast ductile iron and on the design of a direct ‘in mould’ thermal analysis system for heavy section castings. The casting system was developed in order to obtain different thermal modules, and it has been studied by numerical simulation. The microstructure was estimated through numerical models implemented in a commercial casting simulation code and validated by comparing simulated and experimental data obtained from more than 2000 optical micrographs considering nodularity, density and size of nodules, fractions of graphite, ferrite and pearlite. Empirical relationships between the microstructure and mechanical properties were developed and then implemented in the simulation software in order to evaluate the local mechanical properties. The results of hardness and tensile tests carried out on samples extracted from the castings showed good accordance between predicted and measured mechanical properties.

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.

Fatigue Life Improvement of Holed Plates Made of an Innovative Medium C Micro-Alloyed Steel by Local Plastic Deformation

This paper deals with the influence of local plastic deformation on the fatigue strength of holed plates manufactured with an innovative medium-carbon micro-alloyed steel with high silicon content (hi-Si MCM). Local deformation around the hole is achieved by means of an interference fitted pin. The effect was investigated both experimentally and numerically. Microstructural characterization, hardness, and tensile tests were carried out first. Tension–tension fatigue tests were performed under two different conditions: open-hole (OH) specimens and specimens with a press fitted pin with 0.6% nominal specific interference. A 2D elastic–plastic finite element analyses (FEAs) investigation was done as well, in order to analyze the stress field in the vicinity of the hole. The stress history and distribution in the neighborhood of the hole indicate a significant reduction of the stress amplitude produced by the external loading (remote stress) when a residual stress field is generated by the pin insertion. In fact, experimental stress-life (SN) curves pointed out increased fatigue strength of the interference fit specimens, compared with the OH ones. Finally, scanning electron microscope (SEM) analyses of the fractured fatigue specimens were carried out, in order to investigate the mechanisms of failure and to relate them to the peculiar microstructural features that characterize this innovative steel.

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.

Microstructure, Hardness and Impact Toughness of Heat-Treated Nanodispersed Surface and Friction Stir-Processed Aluminum Alloy AA7075

Friction stir processing (FSP) is a recent surface engineering processing technique that is gaining wide recognition for manufacturing nanodispersed surface composites, which are of high specific strength, hardness and resistance to wear and corrosion. Herein, four-pass FSP was applied on aluminum alloy 7075 (AA7075-O) with and without the addition of alumina nanoparticles (Al2O3) of average size ~40xa0nm. All FSP parameters were constant at 40xa0mm/min transverse speed, 500xa0rpm and tilt angle of 3°. FSP rotation direction was reversed every other pass. The friction stir-processed materials were sectioned and solution treated at 515xa0°C for 1.5xa0h, followed by age hardening at 120xa0°C for 12, 24, 36, 48 and 60xa0h. The effect of heat treatment regimes on microstructure, hardness and toughness was examined, as well as the fracture mode. The new friction stir-processed surfaces without and with nanodispersion showed enhancement in the hardness of the surface of the AA7075-O material (65xa0HV) to almost a double (100 and 140xa0HV) after four-pass FSP (before heat treatment) without and with incorporating nanoalumina particles, respectively. After 48-h aging at 120xa0°C, a significant enhancement in impact toughness was achieved for both the friction stir-processed without and with nanodispersion (181 and 134xa0J, respectively), compared to the reference material AA7075 in T6 condition (104xa0J).

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.