Bruce Blakeley has worked for TWI since April 2003. As a senior project leader in the NDT department, he is responsible for two collaborative projects, 'Hull Inspector' and 'Saferay'. This paper is based on the results of a previous collaborative project - Magcast.
Magnesium is often used in castings for the automotive and aerospace industries. These castings weigh just 60% of aluminium, leading to significant cost and weight savings. Unfortunately, they suffer from porosity, which severely reduces the structural integrity of the component. Bruce Blakeley explains how the Magcast real-time radiographic inspection system can be taught to recognise areas of porosity within complex magnesium castings; it can then automatically inspect a batch of castings, using automated defect recognition software.
Magnesium is the lightest structural metal, weighing a fraction of aluminium, yet offering comparable structural strength and rigidity. It is also one of the most abundant metals on earth. Aerospace and automotive companies are keen to take advantage of this, as magnesium castings can be used to reduce the overall weight of cars and aeroplanes, with obvious advantages such as reduced fuel costs leading to cleaner, greener vehicles.
Components made from magnesium castings often suffer from porosity - small gas bubbles which form in the molten magnesium as it sets in the mould. These gas bubbles typically form where the casting changes from a thin to thick wall section. This is serious because the overall strength of the component may be dramatically reduced, jeopardising the structural performance of the component. Failure of such a component could lead to the vehicle crashing, potentially causing loss of life.
Inspection methods
There are several possible solutions to this problem:
- A representative sample of components may be sectioned through areas where porosity is suspected.
- Radiographs can be taken of a small sample, using film and an X-ray source.
- Digital radiography can be used to inspect the components automatically.
The magnesium casting process can often be difficult to control precisely. Any deviation from the ideal parameters may cause porosity. Samples gathered on one day may reveal little or no porosity, while the next batch of samples, collected on a different day, may tell another story. Ideally all cast components would be inspected, but obviously this is impossible using sectioning.
The second solution is to take radiographs of a sample of components. In this technique, the sample is placed on a radiographic film with an X-ray source, similar to medical practices. The radiation from the X-ray source casts a 'shadow' of the various thicknesses of the component, revealing small dark spots which indicate porosity, see Fig.1. These films are then interpreted by experts familiar with the component, the casting process and the interpretation of radiographs. This method offers several advantages to sectioning, but is still only used on a small sample of the components. It requires a great deal of skill and is a relatively slow process. By the time porosity is diagnosed, an entire batch of faulty components may have been produced at great expense.
The ideal inspection technique would be capable of inspecting every component as it comes off the production line. This would ensure that no defective safety critical component would ever find its way into an aircraft or car. The system would include defect recognition software to improve the accuracy and speed of the inspection. Each component would then be inspected for porosity, and sentenced before any more value is added to the component.
The Magcast system
To achieve these objectives a consortium of European companies, research establishments and universities was formed into the Magcast Consortium. This consortium includes representatives from throughout the supply chain, including foundries, service providers and suppliers of radiographic equipment. This two year project was partly funded by the European Commission under the CRAFT program of co-operative research.
The Magcast system consists of a radiation proof cabinet, see Fig.2, a manipulator for positioning the sample, see Fig.3, a stabilised X-ray source see Fig.4, and a digital X-ray detector similar to the detector chip in a digital camera Fig.5. A fault-free sample is placed in the cabinet, and a series of radiographs taken of the object from various directions. This 'teaches' the system how to manoeuvre the sample around within the cabinet, and take the relevant radiographs of suspect areas. The castings to be inspected are then loaded into the system individually; radiographs are taken and then compared to the 'golden images' of the perfect casting. Automated defect recognition software compares the two images, and recognises areas of porosity, see Fig.6.
Benefits
The benefits of the system include:
- Manual mode for inspecting individual castings in real-time
- Teaching mode, where the system is taught how to take a series of radiographs of the component from various angles and positions. 'Golden images' of a perfect casting are stored in memory.
- Inspection mode - a series of castings is inspected following the 'teaching mode'. The resulting radiographs are then compared to the golden images.
- Automated defect recognition software compares the two images, and recognises areas of porosity.
This allows magnesium casting companies to inspect 100% of their components for porosity, before any more value is added to the casting. It also enables the user to correct their casting parameters if too many castings fail. This prevents expensive production runs of faulty castings.
Acknowledgements
Magcast is co-ordinated and managed by CIT Ltd, and is partly funded by the EC under the CRAFT programme ref: G5ST-CT-2002-50163. It is a collaboration of the following organisations: CIT Ltd, Balteau NDT SA, UKRC Ltd, InnospeXion ApS, Spree Engineering & Testing Co. Ltd, TWI Ltd, CEA Leti, Technical University of Sofia
