Stefania Toschi

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 ...

Ex Situ Production Routes for Metal Matrix Nanocomposites

Among different production routes hitherto developed for the manufacturing of metal matrix nanocomposites, a distinction can be done depending upon the matrix state during the production process, which can be molten, solid or semi-solid. In this Chapter, an overview of ex situ production routes is given, highlighting their general potential and shortcomings. Relevant case studies on the most promising and widespread casting production routes will be discussed more in detail in Chap. 3.

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.

Gas-Liquid In Situ Production of Ceramic Reinforced Aluminum Matrix Nanocomposites

The present study aims at evaluating the gas bubbling method, based on the use of dry air as a gaseous phase, for the production of Al based metal matrix nanocomposites through a proper gas-liquid reaction. In particular, Al2O3 reinforcement particles were in-situ synthesized in molten commercially pure Al through a gas bubbling oxidation technique. Dry air was injected in the melt in order to induce a controlled oxidation of the molten matrix. SEM-EDS analysis on the produced samples revealed the presence of alumina particles, ranging from the nanoto the micrometric size, demonstrating the feasibility of the process. A hardness increase on the produced samples confirmed the strengthening effect of the in-situ produced ceramic particles.

High temperature tensile behaviour of the A354 aluminum alloy

The high temperature tensile behaviour of the A354 casting aluminum alloy was investigated also evaluating the influence of secondary dendrite arm spacing (SDAS). Cast specimens were produced through a gradient solidification equipment, obtaining two different classes of SDAS, namely 20-25 µm (fine microstructure) and 40-50 µm (coarse microstructure). After hot isostatic pressing and T6 heat treatment, the samples underwent mechanical characterization both at room and high temperature (200 °C). Results of tensile tests and hardness measurements were related to the microstructural features and fractographic characterization, in order to investigate the effect of microstructure and high temperature exposure on the mechanical behaviour of the alloy.

Mechanical Behavior of Al and Mg Based Nanocomposites

This chapter provides an insight into the mechanical properties of Al and Mg based nanocomposites. Tensile and compression properties, ductility and the influence of heat treatment on mechanical behavior of both aluminum and magnesium based nanocomposites are discussed. Experimental data (hardness, tensile/compression strength, ductility) reported in recent literature works is presented and compared, to highlight the effect of different processing techniques on the mechanical response of nanocomposites.

Metal Matrix Nanocomposites: An Overview

In this chapter, an overview on both Al and Mg based nanocomposites is given, emphasizing particularly on the importance of using reinforcements at nano length scale. The strengthening mechanisms at the basis of the reinforcing action exerted by nanoparticles (Orowan mechanism, enhanced dislocation density, grain refinement and load bearing effect) is described and their contribution discussed; modelling of the above mentioned mechanisms is also presented.

Casting Routes for the Production of Al and Mg Based Nanocomposites

As previously described in Chap. 2, the different production techniques for metal matrix nanocomposites (MMNCs) may be classified depending on the matrix state: liquid, solid or semi-solid. In comparison to other methods, liquid and semi-solid state MMNCs processing techniques are particularly attractive since they are potentially scalable to industrial level for the high volume production of near-net shape components. Nevertheless, such methods pose critical issues related to the low wettability of nanosized particles, generally leading to clusterization and high casting defects content. In this chapter, the main liquid and semi-solid casting routes (stir casting, compocasting, ultrasonic assisted casting and disintegrated melt deposition, DMD) will be described; the results of recent and relevant case studies on Al and Mg based nanocomposites will be summarized and discussed, by highlighting the main drawbacks of such processes.

Tribological Characteristics of Al and Mg Nanocomposites

Al and Mg nanocomposites have good mechanical properties and are potential tribological candidates for applications where weight-reduction is a critical factor, such as in automotive, aerospace and sports. This chapter presents the tribological characteristics of various Al and Mg nanocomposites. The tribological properties of wear and friction of the light-metals nanocomposites are influenced by several factors such as: process and process parameters, composition and microstructure, mechanical properties, heat treatment to enhance mechanical properties, applied external parameters of velocity and load, and formation of in situ protective films/layers. Examples of tribological investigations of nanocomposites that highlight these influencing factors are presented in the view of providing a comprehensive outlook on the tribology of light-metals nanocomposites. Some suggestions for future work are also mentioned.

Cast Al-Based Nanocomposites Reinforced with Thermal Plasma-Synthesized Ceramic Nanoparticles

The present study aims at producing Al-based nanocomposites reinforced with low fractions of ceramic nanoreinforcement produced by thermal plasma, evaluating the strengthening effects induced by their addition to the widely used A356 (Al-Si-Mg) cast aluminum alloy. Nanoparticles were produced using a lab-scale RF inductively coupled thermal plasma system designed by simulation as to optimize the plasma operating conditions and reactor geometry. During the casting route, ultrasonic treatment of the melt was performed to better disperse the reinforcing particles into the matrix. Ceramic spherodized microparticles were also synthesized and micro-reinforced Al-matrix composites were produced with the same route for comparison. Microstructural characterization of the cast samples was carried out by optical and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis. BET analysis was also used for powder characterization. Hardness tests were performed to assess the enhancement in mechanical properties obtained by addition of nanoparticles with respect to both the microparticle reinforced and unreinforced Al-Si-Mg matrix.