Influence of annealing and deformation on optical properties of ultra precision diamond turned and anodized 6060 aluminium alloy N. Tabrizian a,c, ⁎, H.N. Hansen a , P.E. Hansen b , R. Ambat a , P. Møller a a Technical University of Denmark, Department of Mechanical Engineering, Technical, Building 204 and 427, 2800, Lyngby, Denmark b Danish Fundamental Metrology, Matematiktorvet 307, 2800 Lyngby, Denmark c Bang & Olufsen Operation A/S, Technology Department, Peter Bangs Vej 15, 7600 Struer, Denmark abstract article info Article history: Received 20 August 2009 Accepted in revised form 2 February 2010 Available online 13 February 2010 Keywords: Aluminium oxide Appearance Microstructure Roughness BRDF Influence of cold forging, and subsequent heat treatment and diamond turning on optical quality of anodized film on 6060 (AlMgSi) alloy was investigated and compared with microstructural changes. Heat treatment of the samples was carried out either prior to forging, post-forging, or both. The surface of the forged material was then diamond turned to a mirror like finish. The diamond turned samples were subsequently anodized in a sulphuric acid bath. The microstructure of the samples was analysed using optical microscopy (LOM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Colour/brightness measurements were carried out using CIE Lab system. An optical method was used to measure the thickness of the oxide film and roughness of the surface was measured before and after anodizing using stylus, a mechanical instrument, and bidirectional reflection distribution function (BRDF), an optical instrument. Results indicated that the post-forging heat treatment had a great influence on the appearance of the anodized layer, which was also a function of the deformation introduced prior to heat treatment. The effect was assumed to be attributed to the change in microstructure, especially the distribution and the amount of the intermetallic particles such as elemental Si and Mg 2 Si. Roughness of the oxide film was also found to be a function of the heat treatment and deformation condition. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Unlike many engineering applications, the use of aluminium for architectural and decorative applications needs high optical quality anodized surfaces. Aluminium 6060 alloy is one of the common commercial alloys used for design applications and architecture — mainly because it has a glossy appearance after successive machining and the anodizing process [1–6]. During anodizing, aluminium is converted by an electrochemical reaction into a nano-porous oxide film consisting of Al 2 O 3 . This thin film, which is transparent and hard in nature, is several hundred times thicker than natural oxide (∼ 10 nm), and consists of a nanometre sized porous structure which has a hexagonal pattern. However, the colour of the oxide layer formed during the anodizing is not always pleasing and variation in colour is common. Light is differently scattered, refracted and reflected from this film depending on the morphology of pores, compositional variations, incorporated particles, and microscopic roughening of the substrate surface due to pre-processing conditions. Variation in colour and transparency of the anodized layer is reported [1,2,5,7] to be controlled by the anodizing conditions, and a number of microstruc- tural features related to the substrate surface namely the type of second phase particles and their stability during the anodizing process, and grain orientation effect on dissolution rates in the anodizing bath. The microstructure and surface conditions of the aluminium substrate prior to anodizing is difficult to control when the aluminium substrate undergoes many pre-processing steps such as heat treatment, forging, extrusion, rolling, etc. For most decorative products, the last step before anodizing is often one or more machining processes such as diamond turning, polishing, or brushing depending on the desired surface appearance. However, when a bright anodized surface is required, where discolouring and non- uniform oxide film is unwanted, the above mentioned factors are critical and need to be studied and controlled [1–3,15]. During forging process the material is forced to take shape by plastic deformation. This results in the development of anisotropic strain —all through the material with grain breakage, dislocations/ cavities and in some cases development of precipitations after annealing [2,8,9]. On the other hand subsequent machining intro- duces a deformed layer at the surface which is more electrochemically reactive and has a very fine grain structure [10,11,17]. The mentioned factors are affecting chemical composition and microstructure of the oxide film and its appearance. Surface & Coatings Technology 204 (2010) 2632–2638 ⁎ Corresponding author. Technical University of Denmark, Department of Mechanical Engineering, Technical, Buildings 204 and 427, 2800 Lyngby, Denmark. Current address: Kongsvang Allé 29, 8000 Århus, Denmark. Tel.: +45 7220 2459; fax: +45 7220 1019. E-mail address: naja.tabrizian@teknologisk.dk (N. Tabrizian). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.002 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat