The Metallurgy of Homogenisation

Abstract

Homogenisation of aluminium alloys is the high temperature heat treatment (450-600 °C) performed after casting and consists of three distinct steps; heat-up, soak and cooldown. This review considers the metallurgical importance of homogenisation and how it impacts on the further processing and final properties of some aluminium alloys, with emphasis on homogenisation of extrusion billet. The introduction of continuous homogenisation has significantly improved the temperature uniformity of homogenisation allowing the soak time to be minimised. Batch homogenisation, however, provides flexibility in practices tailored for different aluminium alloys. Soft 6060 and 6063 alloys are best homogenised at a higher soak temperature than harder alloys such as 6061 and 6082. The homogenisation cooling rate can also impact on the behaviour of the billet during extrusion processing as well as affecting the final mechanical properties. An understanding of the microstructural changes occurring as a result of homogenisation allows the cast house to ensure that the billet processing meets the customer requirements.

Effect of cooling rate on precipitation during homogenization cooling in an excess silicon AlMgSi alloy

“…Only such a billet can fulfill its age hardening capacity in the final controlled aging treatment. A cooling rate of around 300-500°C h −1 is often considered adequate [2]. It is clear from the foregoing that precipitation during homogenization cooling is critical not only for the quality but also for the extrudability of the billet.…”

“…Higher throughput rates, lower breakout pressures, shape accuracy with improved dimensional tolerances, a surface free from streaking, pick-up, and die-lines and superior mechanical properties all justify the cost of a homogenization treatment [2]. A homogenization cycle consists of three consecutive steps: heating, soaking and cooling [3].…”

Homogenisation Efficiency Assessed with Microstructure Analysis and Hardness Measurements in the EN AW 2011 Aluminium Alloy

“…However, these alloys exhibit low extrusion rates, require high extrusion pressure, and are therefore assigned a low extrudability index [1,2]. As ingots are cast, non-equilibrium solidification usually results in micro-sized eutectic phases with a relatively low melting point [3][4][5] that are likely to melt and cause irreversible damage during thermomechanical treatments at high temperatures [6]. The low-temperature annealing is less effective in eliminating eutectics and micro-segregation of dissolved elements due to insufficient solid solubility and limited diffusivity of alloying elements.…”

“…The development of the microstructure and the effect on the final properties are influenced by both alloy additions and heat treatment parameters [11,12]. In addition, precipitation is also controlled in this way [4].…”

Optimization of homogenization for a low alloyed AlMgSi alloy

“…The most critical step in this regard is the homogenization treatment [1][2][3][4]. Homogenized billets enjoy not only high throughput rates and low breakout pressures, but also superior shape accuracy and surface quality as well as higher mechanical properties after artificial aging of the extrudate [5]. While the uniform features attained at the end of soaking are favorable, a billet with Mg and Si entirely in solution is difficult to extrude owing to a high flow stress that impairs extrudability [6][7][8].…”