AIMSS Guide – The Safe Use of Magnesium Castings
The racing of historic cars is rapidly increasing in popularity in Australia and around the world. Many historic cars, aged well over 30 years and still racing at speeds up to 250 km/h, have components made of magnesium and other metals for which the ageing process is little understood. The aim of this report is to give competitors a better understanding of how magnesium components in historic race cars are affected by age and stress and how they can monitor and manage any risks that may arise from the long term use of these castings.
1. What is magnesium?
2. Where magnesium castings are typically used.
3. Identifying magnesium castings
4. Resistance to fatigue, impact and corrosion.
5. Checking the integrity of magnesium castings.
6. Repair and storage methods.
8. Identifying a repaired or tested casting.
What is magnesium?
Magnesium is the lightest of the structural metals, weighing 75% less than steel being around a third lighter than aluminium. Magnesium castings exhibit remarkable damping capacity.
The commercial possibilities of the electrolytic method of production that produced commercial magnesium, were first created in 1909 by a German company; Chemische Fabrik Grissheim Elektron.
It became available in commercial quantities in the 1920’s and began being used as a structural component in aircraft and the automotive industry in the early 1930’s.
In 1936 the UK based Magnesium Elektron Company who specialised in the production of design, manufacture and supply of high performance materials, promoted magnesium alloy products under the now familiar name of “Elektron”.
There are a number of different magnesium alloys, but for the purpose of this exercise, the differences are such that the products in historic race cars that are made using a magnesium alloy can all be addressed under the generic term “Magnesium”.
Where magnesium castings are typically used.
The advantages of using magnesium castings in racing cars was soon realised by manufacturers like Mercedes, Auto Union, Maserati and Alfa Romeo in the late 1930’s who quickly replaced aluminium castings and some steel forgings with this new lighter product. In the early post WW2 years companies like Cooper, Connaught, Lotus and others began by using cast magnesium wheels as a way of reducing unsprung weight, quickly followed by various suspension components such as uprights and hubs. In sheet form magnesium was sometimes used for bodywork, replacing the heavier aluminium panels. In the search for lightness some designers wanted almost any casting or bracket to be made from magnesium.
As racing cars were usually designed for a short life; in most cases they were expected to have a useful life of one or two seasons, designers were not concerned about the longevity of highly stressed components as they were either replaced or wore out in the car’s competitive life. However, many of the restored racing in historic events today still incorporate the original castings that may have serious corrosion or stress problems. A number of which are surfacing today.
The castings that we are concerned about primarily are those that are in heavily stressed locations and may be subject to high loads and impact. The most common of these are wheels and suspension components (fig. 1). These products are subjected to high stresses, occasional impacts and corrosive conditions from rain and other liquids. Other typically less stressed products however should not be overlooked during maintenance or repairs following an incident, because of potential corrosion and or stress failure in the future.
Identifying magnesium castings
Externally products made from magnesium look very much the same as aluminium castings and while the radical difference in weight from a similar looking aluminium casting would make the magnesium product obvious, there will be few times when this sort of comparison would be available.
There are a number of ways to identify if a product has been made from a Magnesium alloy:
1. Testing swarf:
Using a small drill bit, drill into the casting at a non stressed area slowly so it produces a long strand of swarf. Place on a sheet of paper and set it alight, Magnesium will burn with a bright white light. Cast aluminium will not.
2. Spectroscopic analysis:
The most accurate way a magnesium product can be identified is by X-Ray Fluorescence Spectroscopic analysis (XRF) as used by Metallurgist. While this may seem out of reach for most people, it is the method also used by scrap metal merchants to identify the products they receive. Many will have a Spectroscope and will be able to identify the product’s material.
3. Contact a race car engineering or aircraft company that deals regularly with race car or aircraft components and has the facility to test magnesium components;
(See Appendix 1 below)
Stephen Hooker; Metallurgical Testing and Consulting Engineers
Resistance to fatigue, impact and corrosion.
Magnesium castings are subject to both stress failure and crystalline corrosion. While magnesium castings have excellent resistance to fatigue, due to their relative ductility, stress failures are becoming a problem as historic cars mount up more and more mileage than their original designers envisaged…
Repeated stress may cause eventual failure, but in the case of magnesium, it can also cause ‘stress corrosion’, which leads to brittle failure, also known as corrosion cracking. Salty atmospheres and even distilled water can be responsible for initiating corrosion in magnesium castings.
Testing castings for damage and or potential failure;
Because magnesium castings are subject to both stress and intergranular corrosion, regular testing is important to ensure the safe operation of competition cars using these products, particularly when you consider that most competition cars were designed for short life operation and longevity typical of historic competition cars was never a consideration as a design parameter.
There are number of checking methods that can be used:
1. Dye check using a propriety dye check set can be quite useful for regular checking, but requires strict application as per the manufacturer’s instructions and will only show failures that are on clean unpainted surfaces. Paint must first be removed from painted components, as this coating would prevent the crack identification dye from penetrating into faulty areas.
Warming the component helps die penetration and visibility of cracks or corrosion, but must not exceed 38ºC to prevent drying of the penetrate dye.
Castings should be cleaned by low velocity abrasive blasting prior to testing, as this can obscure any fine fault lines. Avoid glass or shot blasting as these will peen the surface and cause metal to flow over the defects. Walnut, aluminium oxide, lime or soda blast will usually give a satisfactory result.
2. X-Ray inspection can identify potential deep faults that might not be picked up by surface testing like dye check, but it is expensive and from a practical aspect, would probably only be used on an area where there is high stress and/or hidden areas where the dye check system cannot be used. This method only finds voluminous defects and cracks that are in line with the X Ray beam.
3. Eddy current systems are good at identifying surface cracks, but are more limited than X-Ray. Eddy current testing will find substrate cracks as well, but is highly dependant on operator technique.
Repairing Magnesium Castings:
The repair of magnesium castings should only be done by experts in that field.
Magnesium can be repaired by arc and resistance welding processes (such as TIG) as well as by the oxy/gas welding processes. Critical to the welding processes is the requirement to match the welding rod with the casting material, otherwise that casting will be contaminated and be unusable. Castings need to be preheated, prior to the repair. The arc welding methods are preferred, as the heat effected zone is minimised.
Protection of Magnesium Castings
Because magnesium castings are subject to corrosion as well as stress it is important to protect the casting by excluding any contact with oxygen, in other words; air and water, especially when associated with salt solutions.
Castings should be painted with a good thick coat of paint. While to get good adhesion, it is usual to start with a dichromate, it is important not to rely on that, because dichromate is porous. Always follow up with a coat of paint over the dichromate. The paint film must be non-porous and continuous over the whole casting.
Contact with other metals will also cause galvanic corrosion at their contact point. The solution is to provide some electrical insulation between the two metals.
When fitting tyres to magnesium wheels, do not use soapy or plain water as a lubricant to make it easier to install the tyre, as both these will corrode magnesium. Instead use WD40 which will provide an adequate lubricant, without any long term corrosive effects.
Where a wheel is to be presented fully painted, except for a polished rim, the polish will only provide very short term protection and the polished area should be coated with oil or grease if the vehicle is going to be garaged for any extended period.
Storing Magnesium Castings
When storing magnesium castings the primary requirement is to exclude moisture from attacking the casting. Castings can be coated with oil, grease or wax and ideally place the component in a polythene bag with some form of desiccant to prevent condensation.
Identifying tested and/or repaired castings:
All tests and or repairs to be noted in the vehicle’s log book.
Photo Examples Below:
Fig. I Failure of an Elfin 600 rear upright. Failure started from the lighting/casting hole.
Fig. 2 Rear upright the dark patch showing the start of the stress failure.
Fig. 3 Rear upright lower mounting bolt. Stress crack apparent radiating out from the through bolt hole.
Magnesium Fabricators and Repairers:
Stephen Hooker; Metallurgical Testing and Consulting Engineers
Corrosion-Mechanical Strength of Metals by L.A.Glickman (Butterworths)
Analysing failures of Metal Components; www.keyometals.com/page.aspx
Fatigue of Metals: www.keyometals.com/page.aspx
Corrosion of magnesium and Magnesium Alloys: www.keyometals.com/page.aspx
Welding Magnesium Based Alloys: www.keyometals.com/page.aspx
Material Identification Prior to Welding: www.welding-advisers.com/Material-identification
Light Metal Research: www.arclightmetals.org.au
Magnesium information: www.magnesium-electron.com/about-magnesium
E. Caldersmith – Australian Institute for Motor Sport Safety – July 2010
Fig. 1 Elfin rear upright failure
Fig. 2 Rear upright failure
Fig. 3 Stress crack