Malcolm J. Couper

The precipitation sequence in Al–Mg–Si alloys

Fine-scale precipitation that occurs during age hardening of Al alloy 6061 has been studied using differential scanning calorimetry (DSC), atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). It was found that the precipitation sequence is: independent clusters of Mg and Si atoms {yields} co-clusters that contain Mg and Si atoms {yields} small precipitates of unknown structure {yields} {beta}{double_prime} needle-shaped precipitates {yields} B{prime} lath-shaped precipitates and {beta}{prime} rod-shaped precipitates. A new structure is proposed for the {beta}{double_prime} precipitate. It was found that the Mg:Si ratio in the intermediate precipitates and co-clusters was close to 1:1.

Observation and Prediction of the Hot Tear Susceptibility of Ternary Al-Si-Mg Alloys

An investigation into the hot tear susceptibility of ternary Al-Si-Mg alloys has been made using direct crack observation, measurement of load response, and predictions made by a modified Rappaz-Drezet-Gremaud (RDG) hot tearing model. A peak in both the hot tear susceptibility and the load at solidus occurred at approximately 0.2Si and 0.15Mg, and then the hot tear susceptibility decreased as the total solute content increased. In general, a good correlation was found among the observation of cracks, the load at solidus, and the predictions of the RDG hot tearing model, although it was shown that correlation with the RDG model depended critically on the fraction solid at which solid coalescence was assumed to occur. A combination of these approaches indicated that when the total Si+Mg content and the Si:Mg ratio increased toward four, a decrease occurred in the hot tear susceptibility because of an increase in the amount of final eutectic formed. At the lowest Si:Mg ratio of 0.25, the RDG model also predicted a lower relative hot tear susceptibility than that measured by the load at solidus. In these alloys, the final stages of solidification are predicted to occur over a large temperature range, and hence, both the predictions of the RDG model and the measurement of the load were dependent on which fraction solid was chosen for grain coalescence. In the alloys studied in this article, the formation of small amounts of the ternary eutectic Al+Mg2Si+Si caused the highest hot tear susceptibility.

An empirical analysis of trends in mechanical properties of T6 heat treated Al-Si-Mg casting alloys

The Al-Si-Mg family of alloys is typically used in the T6 heat treated condition for high integrity casting applications. There are several variables that can influence the tensile properties of these alloys besides the actual ageing treatment employed. These are alloy composition, particularly Mg and Fe content, and secondary dendrite arm spacing or SDAS. Tensile data from samples covering a wide range of Mg content (0.3–0.7%), Fe content (0.05–0.20%), SDAS (20–60 μn) and two ageing treatments (under- and peak-aged), have been collected and analysed empirically. Concise relationships have been determined for yield strength, ultimate tensile strength and elongation to fracture in terms of one or more of the experimental variables. The observed trends are explained in terms of basic metallurgical principles and the predictive use of the empirically-determined equations is qualified. A comparison of experimentally determined properties and those calculated using the derived relationships highlights some important and clear trends.

Dispersoid Phases in 6xxx Series Aluminium Alloys

In high strength AlMgSi alloys additions of Mn and Cr lead to the formation of dispersoid phases whose primary functions are to improve fracture toughness and control grain structure. Whether or not dispersoid phases form during heating to the homogenisation temperature and which dispersoid forms is strongly dependent on the alloy composition. By correlating dispersoid features after different homogenisation heat treatments to TEM investigations into the crystal structure, it is proposed that the crystal structure and chemical composition of the dispersoids changes as the dispersoids coarsen at increased temperatures and times.

A practical method for identifying intermetallic phase particles in aluminium alloys by electron probe microanalysis

Aluminium alloys that contain Si, Mg, Fe, Mn and/or Cu usually contain one or more types of intermetallic phases that are not readily distinguishable in the microstructure by conventional microscopy methods. It has thus been a challenge to develop a method that will unambiguously identify them. A practical approach has been developed that is based on an inherent linear relationship revealed for the overall distribution of any two elements in a precipitate/matrix geometry and the first-order approximation of electron probe microanalysis (EPMA) results. Application of this approach to a direct chill cast 6082 alloy is demonstrated, and its major limitations are discussed.

Characterisation of AlFeSi intermetallics in 6000 series aluminium alloy extrusions

Alloys designed to optimise strength and extrudability have a lower alloy Mg to Si ratio than has commonly been used in AA6060 and AA6063 alloys. Intermetallic phases have an impact on alloy design since they tie up some of the Mg and Si alloy content. The effect of Mg and Si alloy content on the type of intermetallic phases present has been investigated using TEM, SEM and Thermocalc analysis. Results for Al-Mg-Si-Fe alloys with 0.46 - 0.70 wt%Mg, 0.27 - 1.24 wt%Si and 0.09 - 0.22 wt%Fe are presented. The occurrence of a-AlFeSi (various stoichiometries), b- Al5FeSi, p-Al8FeSi6Mg3, Mg2Si and Si has been found to depend on alloy composition within the ranges examined.

Hot Tearing in Al-Mg-Si Alloys with Minor Additions of Cu or Mn

A study of the influence of minor additions of copper or manganese on hot tear susceptibility in Al-Mg-Si alloys has been conducted. Testing was carried out using a laboratory scale hot tearing rig and the results were validated using an analysis of cast house cracking scrap data for 6060 and 6063 extrusion billet alloys. Mn content was found to have a strong influence on hot tearing rates.

Precipitation Sequence in an Al-Si-Mg Foundry Alloy

It is often assumed that the precipitation sequence and phases in Al-Si-Mg foundry alloys, such as A356 with 7 wt%Si, are similar to those in wrought 6xxx Al-Mg-Si alloys, such as 6063. The foundry alloys have been less extensively studied due to added difficulties in sample preparation, resulting from the high volume fraction of coarse particles of spheroidised eutectic silicon. Recent work has been successful in studying the precipitation sequence in a foundry alloy containing 0.45 wt%Mg. The work highlights some differences and similarities between foundry and wrought alloy precipitation, which may have implications for alloy design and heat-treatment.

Classification of Streaking Defects on Anodized Aluminium Extrusions

Streaking is a common problem on anodised extrusions of 6xxx series soft alloys. This paper presents various types of streaking defects on the basis of industry practice and experimental results. The streaking defects are classified according to their root causes. This provides a basis for developing effective methods for preventing the formation of these defects for the extrusion.

Effect of process variables on the formation of streak defects on anodized aluminum extrusions: An overview

Abstract Streak defects are often present on anodized extrusions of 6xxx series aluminum alloys, increasing the fabrication cost of these products. Moreover, streaking often only becomes visible after etching and anodizing treatments, rather than in the as-extruded condition, making it difficult to identify the original causes and influencing factors of these defects. In this paper, various process variables that influence the formation of streak defects on anodized aluminium extrusions are reviewed on the basis of a literature review, industrial practice and experimental results. The influencing factors involved in various processing steps such as billet quality, extrusion process, die design and etching process are considered. Effective measures for preventing the formation of streak defects in industrial extrusion products are discussed.