F. Czerwinski

The oxidation behaviour of an AZ91D magnesium alloy at high temperatures

Abstract As-cast AZ91D magnesium alloy was exposed to air in the temperature range from 470 to 800 K for time intervals up to 10 h. Thermogravimetric measurements revealed three distinct stages of the reaction where an initial formation of protective oxide was followed by an incubation period with a subsequent transient to non-protective oxidation, at a rate either constant or sharply increasing over time. The approximate temperature and time frames for an onset of each stage were identified. A strong link was found between the oxidation kinetics and the scale morphology. The initial protective film, grown anisotropically over the microstructural features of the substrate, was transformed to oxide ridges. The non-protective oxidation was associated with a formation of oxide nodules and their further coalescence into a fine-grained scale of a loose structure. All the morphologies were comprised of a randomly oriented magnesium oxide MgO with traces of MgAl 2 O 4 spinel. The oxidation mechanism represented a complex reaction where morphological and phase transformations within the alloy substrate were accompanied by magnesium sublimation/evaporation and subsequent condensation within the scale pores or cracks, and superimposed on the reaction with oxygen. Some implications for the high temperature processing of magnesium alloys are discussed.

The early stage oxidation and evaporation of Mg-9%Al-1%Zn alloy

Thermogravimetric technique was used to determine the oxidation and evaporation behaviour of AZ91D magnesium alloys with 5 and 10 ppm of beryllium at temperatures between 200 and 500 °C. Depending on temperature and time, the alloy experienced protective or non-protective oxidation with linear or accelerated oxide growth kinetics. During reaction in an oxidizing atmosphere, the additions of beryllium delayed the transient from the protective to non-protective scale formation. In an inert atmosphere, increased beryllium contents reduced the magnesium evaporation rate.

The reactive element effect on high-temperature oxidation of magnesium

Abstract It was discovered over 75 years ago that minor additions of elements with high affinity to oxygen exert a profound effect on oxidation resistance of many metals and alloys. In this report, progress in understanding the ‘reactive element effect’ (REE) and opportunities created for controlling the high-temperature oxidation of magnesium alloys were reviewed. Magnesium, the lightest structural metal, exhibits high affinity to oxygen and, as opposed to heat-resistant materials, does not form a dense, slow-growing and protective oxide layer. However, additions of small amounts of reactive elements (REs) to magnesium alloys produce a number of positive effects in improving their oxidation resistance. What is of crucial importance for magnesium is that RE benefits are also extended to its ignition, flammability, evaporation and liquid-state reactivity. The mechanism by which these elements influence oxidation and other related behaviour remains, so far, unclear. Although no specific theory explaining the REE on magnesium has been developed, growth kinetics, surface morphology and internal microstructure of magnesia films and scales, doped with REs show characteristics typical for high-temperature materials that form protective oxides.

Dislocation slip distance during compression of Al–Si–Cu–Mg alloy with additions of Ti–Zr–V

Abstract The plastic deformation behaviour of the Al–Si–Cu–Mg alloy with micro-additions of Zr–V–Ti was measured in the temperature range of 298–673 K and the true stress–true strain compression curves were used to calculate the dislocation slip distance (DSD). A new constitutive equation for the temperature dependent DSD was developed, based on Mott’s theory of strain hardening. The DSD predicted by the model was in good agreement not only with values achieved for the Al–Si–Cu–Mg alloy tested but also for other Al based and Pb–Sb alloys with deformation data available in the literature. A comparison of deformation and microstructure suggests that the grain refinement during hot compression deformation occurring due to continuous dynamic recrystallisation is responsible for a drastic growth of the DSD.

Modern Aspects of Liquid Metal Engineering

Liquid metal engineering (LME) refers to a variety of physical and/or chemical treatments of molten metals aimed at influencing their solidification characteristics. Although the fundamentals have been known for decades, only recent progress in understanding solidification mechanisms has renewed an interest in opportunities this technique creates for an improvement of castings. This review covers conventional and novel concepts of LME with their application to modern manufacturing techniques based not only on liquid but also on semisolid routes. The role of external forces applied to the melt combined with grain nucleation control is explained along with laboratory- and commercial-scale equipment designed for implementation of various concepts exploring mechanical, electromagnetic, and ultrasound principles. An influence of melt treatments on quality of the final product is considered through distinguishing between internal integrity of net shape components and the alloy microstructure. Recent global developments indicate that exploring the synergy of melt chemistry and physical treatments achieved through LME allows creating the optimum conditions for nucleation and growth during solidification, positively affecting quality of castings.

Aging characteristics of the Al-Si-Cu-Mg cast alloy modified with transition metals Zr, V and Ti

The hypoeutectic Al-7Si-1Cu-0.5Mg base alloy was modified with different contents of Zr, V and Ti. The wedge-shape samples with varying solidification rates during casting were subjected to isochronal aging at temperatures up to 500 °C. Moreover, as-cast and solution treated alloys were subjected to long-term isothermal aging at 150°C. As a reference, the A380 alloy, seen as commercial standard for the automotive application target, was used. The modified alloys exerted different aging characteristics than the A380 grade with higher peak hardness and lower temperature of alloy softening. Besides, the influence of the applied solidification rates on hardness after aging was less pronounced in modified alloys than in the A380 grade. For three combinations of Zr, V and Ti tested with contents of individual elements ranging from 0.14 to 0.47%, no essential differences in aging characteristics were recorded. The results are discussed in terms of the role of chemistry and heat treatment in generating precipitates contributing to the thermal stability of Al based alloys.

Effect of Hot Rolling and Equal-Channel Angular Pressing on Generation of Globular Microstructure in Semi-Solid Mg-3%Zn Alloy

A combination of hot rolling and equal channel angular pressing (ECAP) was explored to generate globular microstructures in the Mg-3%Zn alloy after re-heating to the semisolid state. It was found that the single-step deformation of as-cast alloy via hot rolling at 350°C with a thickness reduction of 50% refined the alloy microstructure by creating deformation bands of the Mg (α) phase with a size of the order of tenths of micrometers. After re-heating to 630 °C, the microstructure transformed into spheroidal morphologies with an average globule size of 82 μm. An additional deformation of the hot-rolled alloy by the ECAP method at 250 °C further refined the alloy microstructure to sub-micrometer grains of lath and equiaxed shapes. After re-heating of this microstructure to 630 °C the average globule size reached 62 μm, which is roughly 25% smaller than that achieved for the hot-rolled precursor. The role of strain-induced melt activation (SIMA) techniques in generation of globular morphologies in Mg-based alloys after partial re-melting is discussed.

Gas-Enhanced Ultra-High Shear Mixing: A Concept and Applications

The processes of mixing, homogenizing, and deagglomeration are of paramount importance in many industries for modifying properties of liquids or liquid-based dispersions at room temperature and treatment of molten or semi-molten alloys at high temperatures, prior to their solidification. To implement treatments, a variety of technologies based on mechanical, electromagnetic, and ultrasonic principles are used commercially or tested at the laboratory scale. In a large number of techniques, especially those tailored toward metallurgical applications, the vital role is played by cavitation, generation of gas bubbles, and their interaction with the melt. This paper describes a novel concept exploring an integration of gas injection into the shear zone with ultra-high shear mixing. As revealed via experiments with a prototype of the cylindrical rotor–stator apparatus and transparent media, gases injected radially through the high-speed rotor generate highly refined bubbles of high concentration directly in the shear zone of the mixer. It is believed that an interaction of large volume of fine gas bubbles with the liquid, superimposed on ultra-high shear, will enhance mixing capabilities and cause superior refining and homogenizing of the liquids or solid–liquid slurries, thus allowing their effective property modification.

High Temperature Creep Evolution in Al-Si Alloys Developed for Automotive Powertrain Applications: A Neutron In-Situ Study on hkl-Plane Creep Response

Recent trend in the automotive industry towards lightweighting and downsizing the powertrain components, without compromising the power output, have led to increased engine power density. Increased power density frequently requires these lighter components to operate in conditions of increased temperature and pressure, which is challenging for many aluminum alloys in use today in the powertrain manufacturing. Meeting the challenge requires not only improving high-temperature performance of known alloys or developing new ones, but also developing new advanced techniques to understand the long-term behaviour of the alloys.

Effect of Strain Level on the Behavior of Intermetallics and Texture of Al-Si-Cu-Mg Alloy Modified with Transition Metals

T uniaxial compression test was used to assess an influence of strain amount on the behavior of precipitates and texture of the Al-7%Si-1%Cu-0.5%Mg alloy, modified with micro-additions of V, Zr and Ti in as-cast and T6 heat treated conditions. As revealed through metallographic examinations, fracturing and re-orientation of the second phase particles increased with increasing compression strain. For both conditions of the alloy, the intermetallic particles experienced substantially more frequent cracking than the eutectic silicon. At the same time, the precipitates in the T6 heat treated alloy were also more resistant to rotate within the alloy matrix as a result of nano-size Al3X (X=Zr, Ti, & V) secondary precipitates. The crystallographic texture was measured and correlated with deformation behavior of the alloy. The weak texture of {011} and {111} , {112} and {111} components. The intensity of the components differed depending on the strain amount and the state of precipitation where the T6 heat treated alloy always exhibited lower intensity all over the strain. It is concluded that the texture formation in studied alloy is controlled by precipitates formed during T6 heat treatment.