Article
Articles, Issue 50 - Autumn 2013

Structural Aluminium Anodising

Whilst it is well known that Lotus manufactures its own chassis at Lotus Lightweight Structures in Worcester, it is little known that Lotus also has its own anodising facility in Wellingborough, which is the world’s largest producer of structural chassis aluminium anodising. The plant can currently produce in excess of 0.5 million square metres per annum and has processed over 10 million automotive components. 

Andy Wilson, Head of Lotus’ anodising facility explains the process, and history of this world class facility.

Previous to the Lotus Lightweight Structures acquisition in September 2009, the Wellingborough facility specialised in luggage carrier systems, as a Tier 1 supplier for Vauxhall/Opel providing aluminium powder coated roof rail assemblies.

In 2003 we developed a chromate pre treatment process and fully extruded aluminium component service for a bonded aluminium space frame which served the Gibbs Aquada amphibious vehicle capable of a land speed greater than 100 mph and a water speed upwards of 30 mph.

Over the next 12 months we became renowned in the UK automotive market as an extruded aluminium finishing SME with an expanding portfolio supplying welded aluminium sumpguards for the Freelander and also as a Tier 1 supplier for Lotus Cars.

By the Summer of 2004, there were plans to construct an anodising plant after a nomination from Aston Martin for their next generation lightweight aluminium chassis. The Wellingborough facility with the ability of converting extruded aluminium into an anodised component under ‘one roof’, and less than one hour’s travel from the Aston Martin production facility at Gaydon excited both parties.

Later that year we were awarded the contract to provide both aluminium components and provide a Tier 1 service for all chassis component anodising services. The plant entered service for pre-production build by February 2005.

The anodising plant itself is unique in that it is an amalgamation of a electropaint system and advanced state of the art hardware. The electropaint conveyor system, a series of dip tanks, lip extraction fans, the transporter system and load/unload shuttles were upgraded and utilised. In parallel, plant equipment was sourced Europe wide. State of the art pulsed current control rectifiers and a ion exchange sulphuric acid recovery system were sourced from Italtecno in Italy.

The tank burner control systems were purchased from Lanemark International, chillers capable of providing hard anodising as well as soft from ICS.  The plant control architecture was designed and installed by MPS.

Every aspect of the development was managed in-house including the installation of new mains gas and towns water supplies to the design and manufacture of flight bars, carriers and racking systems.

The anodising process

Anodising is an electrochemical process for producing stable oxide films on the surface of metals and has rapidly developed since it’s early industrial use in the 1920s. However, it is still the most recently developed of the commonly used ‘wet’ metal finishing processes (one carried out in an aqueous solution).

The process derives its name from the fact that the metal part to be treated becomes the anode in an electrolytic cell. It is ideal for producing stable oxide ‘barrier’ films on the surface of metals to enhance corrosion/chemical resistance or creating physical surface properties to increase abrasion resistance and hardness as well as preparing surfaces with increased bonding strength. Anodising aluminium can be achieved in a wide variety of electrolytes, employing varying operating conditions including concentration and composition of the electrolyte, presence of any additives, temperature and electrical parameters. There are three main phases to the process.

Surface preparation 

In which a chemically clean surface (free from lubricants/oils/corrosion products and the naturally occurring aluminium oxide) must be achieved. This involves an alkaline soak cleaner maintained at a temperature of between 65-75 ˚C in which the free alkaline saponifies these compounds and thus makes them water soluble. Additions of surfactants reduces surface tension and thus enhances the degreasing effect by promoting ‘wetting’. After this cleaning stage an alkali etch is performed in a caustic solution at temperatures between 60-67.5 ˚C to further saponify the lubricants whilst dissolving a small amount of the naturally occurring aluminium oxide from the substrate surface. The final preparatory stage is a hydrofluoric/nitric acid dip to remove the smut formed by the alloying elements of the aluminium. All alkali and acid process stages are interspersed with a series of cascading/recycling towns water rinses.

Anodising 

The aluminium substrates are now ready for the anodising process. Each part is already mechanically fixed and electrically connected to the carrying flight bar and is immersed in a solution of Sulphuric acid at a temperature between 17-19 ˚C. The flight bar is then automatically connected to the positive terminal of a 5,000 A DC rectifier which then surveys the electrical conductivity between the anode (parts) and the static cathodes.

By presetting the anodising current density and requested number of microns (thickness of the oxide film), the rectifier delivers a constant current over a constant time. As a current is passed through the electrolyte, the negatively charged ions migrate to the anode where one or more electrons are discharged creating a build up of aluminium oxide at the surface. Hydrogen is subsequently released at the cathode which is then removed from the liquid surface by a lip extraction system. The anodic film is duplex as it consists of two layers and during anodising the non-porous barrier layer (insulator) is formed first adjacent to the aluminium substrate.

This conducts current only due to it’s low thickness. The second porous layer is then formed due to the dialectic breakdown which occurs resulting in the formation of small pits. The electrolyte then penetrates into these pits and further oxide growth occurs with the oxide expanding to form a porous structure.

Anodising Plant article

The pores are typically 10-20 nm in diameter and the control limit thicknesses for structural anodising is 2-10 µm (typical width of a human hair is 50 µm).

Plant quality systems

Every load is quality assured at several stages. This includes full traceability of the parts from material receipt to ship.

The parts to be loaded onto the system are scanned and given a unique works order. The part information is subsequently scanned onto the LLS manufacturing resource planning (MRP) system and also into the plant process operating system which hard codes the relevant anodise program and designates a unique flight bar number.

All stage timings, temperatures, anodising profiles and water quality are data logged and recorded.

Each flightbar also contains a number of extruded test samples for adhesive bonding certification. The anodised coupons are adhesively bonded and tensile tested using an in-house Instron tensile test machine. Each bond durability test consists of a ultimate failure load and the energy released at peak load. Off line chemistry certifications are performed daily in our plant laboratory and we also have a highly accurate Hach Lange DR2800 spectrophotometer to measure the concentration of the additive, the total suspended solids and turbidity of the seal.

All chemistry values are electronically recorded and SPC charts are produced daily to track progress and identify long term drift.

Although sulphuric acid anodising technology is almost 100 years old and many anodising companies use this technology to Qualanod standards for architectural applications, there are only a handful of companies in the world capable of producing to the high specification that structural bonding applications require.

Most anodising specifications suggest the required quality systems only, however, the LLS anodising specification details all chemistry, time and temperature controls. If we take one process stage alone (e.g. hot seal), we have seven controlled/measured variables; minimum solution temperature and turbidity, maximum conductivity and total suspended solids minimum/maximum immersion times, seal additive concentrations and pH.

We currently anodise over 500 unique part geometries manufactured from four alloy types, Al6060/3 extrusions, Al5074 sheet, Al5083 superform and Al356 castings for which we have approximately 30 flight bar jigging arrangements some of which are bespoke to unique geometries.

The jigging is not to be an overlooked part of the overall process. They must mechanically attach and at the same time provide individual electrical paths to each part.

All LLS jigging solutions are designed in-house and predominantly manufactured from Grade 2 wrought titanium. Again many traditional anodisers use aluminium jigging although it is less expensive and has greater electrical conductivity than titanium, it is chemically attacked by the anodising process and needs constant chemical or mechanical stripping to remove the electrical insulating aluminium oxide. Like the aluminium part, the jig also becomes anodised from the process. However titanium does not have the same passivation rate as aluminium to form an oxide so stripping is not required between cycles.

Data management system

A key element in making all the aforementioned happen is our bespoke data management system (DMS). It is a modular package for process control, data monitoring, process alarming and plant fault diagnostics, which has also been extended to provide advanced features such as work history logs, production reports and maintenance schedules.

Networking also allows the DMS to provide multiple terminals for access to live and historical information, both locally and remotely. The plant graphics show status, mode and movements of transporters and loads, together with tank process information which allows the operator to obtain a quick and clear view of the current status of the plant.

Anodising Plant article2

When work enters the plant, the processing information must be selected, and by using an integrated part library system the operator is only required to enter the component part number, batch number and quantity which is all achieved by barcode scanning, thereby eliminating any keying errors.

Multiple part types may be jigged together with the system automatically checking for compatibility between them. The unique anodising profiles are then automatically obtained when the part number is entered. An overview of the plant and processing parameters are provided in real time and the work that is in each stage is dynamically displayed, together with the transporter current motion and positions, including processing parameters, associated alarm limits and set points. The plant transducers monitor, control and display all process parameters and this data is collected by the front end control system via digital and analogue inputs and then passed on to the DMS.

The DMS continuously records information about the activity of the plant, which can then be displayed in various reports allowing selections of time periods to view based on shifts, weeks or for a user defined period.

Summary

The Lotus Lightweight anodising facility is one of the most advanced of its type globally and provides a level of quality, control and data recording that is beyond that demanded by most OEMs. The staff operating the plant are highly dedicated and have a full understanding of the demands of the automotive sector. LLS is already expanding its offerings to other sectors, as well as taking on new projects from automotive OEMs.

Writer: Andy Wilson PhD⎢Head of LLS Anodising
Lotus Lightweight Structures Wellingborough

Information on Lotus Lightweight Structures Wellingborough

About lotusproactive

Lotus proActive is an e-magazine published quarterly by Lotus Engineering, covering engineering articles, industry news and articles from within Group Lotus (Cars, Engineering, Originals and Racing).

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