Aluminium is one of the most important materials available. It is widely used in various industries for a huge range of applications. Aluminium comes with many advantages, some of which are low density and corrosion resistance ability. These advantages have made aluminium the backbone of multiple industries such as Aerospace Industries, Transportation Industries as well as Building Industries.
In this article, we would be focusing specifically on this important metal. We will go over aluminium machining and then we will head over to the surface finishing options that are used in case of aluminium.
Aluminium being a highly lightweight, workable and formable material makes it fit and suitable for machining processes. Aluminum Machining is very common throughout various industries. In this section, we will be covering the basic parameters that must be taken care of during Aluminum Machining.
- Important Properties: The density of Aluminum alloys is such that it allows for high rotation speeds. The thermal conductivity helps in heat dissipation. The modulus of elasticity of Aluminum needs proper and definite chucking and clamping arrangements in order to avoid deformation and distortion.
- Cutting Force: The cutting force required for Aluminum is comparatively lesser than what is required for steel. This makes chip removal three times higher in Aluminum in comparison with the same level low-carbon steel.
- Lubrication: Lubrication is done to ensure that heat dissipation is done properly. Apart from that, lubrication also helps in preventing swarf (the fine chips produced during machining) from bonding to the tools as well as in removing swarf from point of machining.
After the process of aluminium machining is complete, we have to turn our focus to the surface finish of aluminium. Surface Finish of aluminium can be improved by various post-processing methods. Now, we will be covering the major methods and how they work with aluminium one by one.
Source: Badger Anodizing
In Bead Blasting, the surface is made smoother by putting the aluminium surface under the application of fine glass beads at high pressure with the help of compressed air. It is done in such a way that no damage is done to the surface. This leads to the removal of certain surface deposits which in turn results in a smoother finish.
Bead Blasting is used on aluminium whenever a satin or dull finish is required. Apart from that, bead blasting is also used to achieve a surface finish that is rough as well as consistent. In case a uniform rough finish is required, coarser glass beads are used. It improves the looks and the presentation of the surface to a great extent.
Glass beads can be replaced by other materials like aluminium oxide, silica carbide, etc. but these materials leave the surface with a much greyer finish. Glass bead blasting allows the surface to retain its base colour and hence is recommended.
Anodizing is another post-processing method used to improve the surface finish of aluminium. It is a simple electrochemical process in which a protective coating of aluminium oxide is formed on the surface of aluminium. Aluminium oxide is a substance which is hard, durable and weather resistant in nature used for protecting the surface of aluminium.
The lifetime of the finish, i.e. how long it will last, depends on the thickness of the protective coating. The coating can also be coloured by dyeing to add various colours to the aluminium product. The coating may exhibit some tones of bronze as a result of diffraction phenomena produced by the coating. If we talk about the structure of this coating, it consists of many small hexagonal pores which are filled with a substance called a seal. This substance hydrolyzes these pores and fills them with inert aluminium oxide.
There are two types of Anodizing used for the post-processing of aluminium materials.
Anodizing Type I
This Anodizing process uses Chromic acid. It results in the thinnest of the anodic coatings in comparison to other types. Even though it’s thin, if the material is sealed properly, it provides equal corrosion protection to aluminium as its alternative types.
The anodic coating appears much greyer in colour. Also, because of its thinness, it absorbs fewer colours when dyed. This limits its ability to provide a better looking and aesthetically pleasing substance. However, it can still be dyed black and be used as a non-reflective and protective coating for optical components. Also, in order to get the type I anodizing process to accept the black dye, temperature of chromic acid needs to be raised which cannot be done every day. Hence, this process is scheduled accordingly.
This type of anodizing forms an anodic coat which is usually of the order of 0.508 µm to 2.54 µm for each aluminium surface.
Anodizing Type II
This Anodizing process makes use of Sulfuric Acid. It is also referred to as Standard or Decorative Anodizing. It is the most commonly used method for anodizing aluminium. The coating formed in this process is actually formed of 67% penetration inside the substrate and a 33% increase over the original part dimensions.
It is used when hardness and resistance to abrasion are required in our final product. Also, since parts might come under the application of considerable stress, the presence of possible corrosive acid residue should be avoided.
Sulfuric Acid Anodizing is known for producing parts which have a smooth and aesthetically pleasing surface coupled with provision of good corrosion resistance and limited wear resistance.
It can produce coatings with thickness up to 25µm. Further, the coating thickness depends on the colour. It can vary between 8-12 µm for black dyed parts and 4-8 µm for clear and un-dyed parts.
Powder Coating refers to the spraying of electrostatically charged paint which adds a thin layer of protective polymer on the surface of aluminium. It is important to note that powder coating does not require any solvent. It is important that the surface of aluminium is pre-treated properly.
The parts are pre-treated using phosphating or a chromate coat. This increases its corrosion resistance. Then, the aluminium is coating with dry powder with the help of an electrostatic spray gun and then cured in at high temperature.
Powder Coating brings with it support for a wide variety of colours. Also, multiple layers can be used to create a thick coating. The thickness of the coat varies from 18 µm to 72 µm.
We can understand the method in a step by step process:-
- Pre-treatment of Aluminum Surface: In this step, the aluminium surface is checked and cleaned so as to remove dust or grease or any other foreign particle that shouldn’t be on it. This way, good adhesion is ensured.
- Application of Powder: An electrostatically charged spray gun is used to spray the powder at the aluminium surface. The positive charge of the powder helps it bond with the electrically grounded surface.
- Curing Process: In this step, the work-piece is put into the curing oven. Heating occurs and the work-piece is baked until there is a uniformly melted coating.
- Cooling Process: Once the work-piece is baked, they are removed and left to cool. Once they are cooled down properly, a smooth and hardened coating will be achieved.
With the necessity of Aluminum in the humongous number of industries, it is important to learn the basics of aluminium machining as well its surface finishing methodologies. In this article, we have gone over both of those topics. There are various applications where these methodologies may be put to use. Thorough research should be done before deciding on which one methodology should be fixated and chosen.
I believe you must have taken up some new knowledge about Aluminum and its multiple surface finishing methods.