Aerospace Metal finishing

NADCAP accredited chemical processing by

Ashton & Moore Ltd. www.ashton-moore.co.uk

Address : 12 Smith Street, Birmingham, B19 3EX, W. Midlands, England

Phone : 0845 618 8196

E-mail : sales@ashton-moore.co.uk

Anodising

Aluminium develops a hard oxide surface layer in air, hard because the oxide (alumina) is a ceramic material. This layer can be thickened by an electrolytic process, the metal finishing treatment being known as anodising. The freshly formed anodic layer is porous and has to be sealed, by immersion in boiling water or a special solution, to give maximum protection against atmospheric corrosion. The film can be coloured by pigments for decorative purposes.

Some metal finishing treatments when applied to surfaces can reduce wear

Phosphating

This is used with all ferrous metals. A film is produced on the surface of the material by either chemical or electrochemical treatments at about 40 to 100 degrees centigrade.. The porous nature of the film helps to retain lubricants and resists scuffing. The treatment is less effective than nitriding in improving wear resistance.

Non-destructive testing

Defects such as cracks, inclusions and porosity may be introduced during the manufacture of components or as a result of degradation during service. Non-destructive testing NDT is the name given to the various techniques which allow inspection of the material to detect the presence, location and size of such defects without impairing the ability of the tested component to function. Destructive tests are those such as tensile testing or impact tests in which samples of the material are cut from the component, so destroying its ability to function.

Non-destructive testing includes:

Visual inspection

Examination of a component by the naked eye can reveal large surface defects. The use of a magnifying lens or microscope enables smaller surface defects to be identified. Optical inspection probes which can be inserted into cavities can be used to enable the surfaces of cavities to be examined. Such probes consist of a viewing eyepiece lens at one end of an optical fibre and an objective lens at the other. Light to illuminate the surface being viewed is also conveyed from the viewing end to the viewed end by another optical fibre, generally concentrically arranged round the viewing optical fibre. Visual inspection can only find defects which break the surface.

Dye penetrant inspection

This method is used to make more easily visible defects that break the surface of a component such as cracks and zones of surface porosity. The procedure is first to clean the surface so that it is completely free from contaminants such as oil, water and grease. The dye penetrant is then applied by brush, spray or immersion so that a film of the penetrant is formed over the component surface. The penetrant is drawn into the defects by capillary attraction. After allowing time for this to happen, the excess penetrant is removed from the surface by the use of water or special solvents. With a coloured dye penetrant, the next stage of the process is development to clearly reveal the presence of the dye in cracks. This consists of spraying the surface with chalk dust, generally suspended in a volatile carrier fluid. The penetrant liquid in the cracks is drawn into the pores of the chalk and results in some spread of the liquid. This magnifies the apparent width of cracks and makes them more clearly visible. With a fluorescent dye penetrant, the surface is viewed by ultraviolet light to make the dye-penetrated cracks visible. This method can be used for surface defects with metals, many plastics, glass and glass ceramics.

Magnetic particle inspection services

This method can be used for the detection of defects which break the surface or are close to the surface of ferromagnetic materials. When a ferromagnetic component is magnetised, any discontinuity that is approximately at right angles to the magnetising field direction will distort the magnetic field lines and if at the surface or close to the surface will result in the formation of a `leakage field'. The magnetic field in a material is generally produced by passing a heavy current through the component, by placing it in a coil through which a current passes or making it part of a magnetic circuit. Magnetic particles, a fine powder of metals or metal oxides, is then sprayed over the surface, either dry in air or a gas or wet on some liquid suspension. The particles `stick' to the regions of leakage flux and thus render them clearly visible

Eddy-current inspection services

If an alternating current is passed through a coil an alternating magnetic field is produced. This will induce eddy currents within any conducting material in the vicinity. These eddy currents will also produce a magnetic field. This produces a back e.m.f. in the magnetising coil and so alters its impedance. If the conducting material contains a crack, the

flow pattern of the eddy currents is altered and this causes a change in the magnetic field they produce and consequently a change in the coil impedance. This method can detect surface defects and some subsurface defects with metals.

Ultrasonic testing services

Ulltrasonics is widely used for the detection of internal defects in materials. Audible sound waves have frequencies ranging from about 20 Hz to 20 kHz. The waves used in ultrasonics are way beyond the audible region, having frequencies ranging from about 0.5 MHz to 20 MHz.

The simplest method, termed the A -scan, involves a pulse of ultrasonic waves being transmitted into the material and the reflections detected from the backwall and the defect. The initial pulse and the reflected pulses are then displayed on a cathode ray tube.

Radiography or x-ray inspection services

This is based on the use of X-rays or gamma radiation and is the same technique as that used for obtaining X-rays of parts of the human body in, say, looking for the fracture of a bone. Radiography can be used with most materials for the detection of both internal and surface defects.