This invention relates to the surface finishing of components, in particular to the surface finishing of components having awkward shapes which make mechanical polishing difficult and more particularly to such components made by an additive manufacturing (AM) process.

Additive manufacturing is being promoted as bringing about the next manufacturing revolution. This technology enables the fabrication of components of very complex design, including designs that can have enclosed cavities and interlocking parts. The technology is particularly suited to high value parts, low batch manufacturing, and mass customisation. Thus this technology is suitable for aerospace components, components for ships, for radar structures and dishes, etc. Currently, limitations to the application of AM technology are the result of typical surface roughness of AM components. This roughness can affect the dynamic mechanical properties of the component. Components can currently be sand blasted or computer numerically controlled (CNC) machined to overcome these limitations. However, CNC machining is expensive and sand blasting does not always provide the tolerances and control required. According to a first aspect of the present invention, there is provided a method of surface finishing a component, the method including the steps of surrounding the component with a liquid slurry containing particles of hardness at least as great as a surface of the component to be surface finished and agitating the slurry with acoustic waves to cause the particles to vibrate back and forth with sufficient energy to abrade the surface of the component for the required surface finish.

This invention thus provides to an easy-to-use process for improving the surface finish of typical AM components and which is equally suitable for polishing open surfaces and enclosed surfaces and cavities. The slurry and component may be contained in a container and at least one acoustic transducer may be placed in contact with a surface of the container and vibrated against that surface. The surface is likely to be an outer surface of the container.

The vibration frequency is preferably high and more preferably in the ultrasonic range. This has the advantage that surface finishing will be faster than with a lower frequency. Also, the resulting surface finish may be smoother, is so desired.

In order to avoid regions on the component of over-polishing or under- polishing, the acoustic waves may be formed at varying frequency. Also, for similar reasons, the position of the at least one acoustic transducer may be varied, during surface finishing.

Further, to ensure even and consistent polishing of the component, the position and/or orientation of the component may be varied during surface finishing. In order to achieve this, the component may be rotated within the container, during surface finishing, for example on a carousel. Time taken for the surface finishing will vary according to the degree of surface finishing, e.g., polishing, required, also the efficiency of the process. Thus, the particular selected combination of material to be surface finished and slurry particles, the vibration power and frequency, the efficiency of coupling of the vibrations into the slurry and the shape of the component will all affect the time taken. The time taken may be a number of hours.

It will be appreciated that the method of the invention may be carried out at room temperature but heating due to the ultrasonic energy created may raise the operating temperature. Suitable cooling may be applied if necessary, for example if a particularly temperature-sensitive plastics material is being surface finished.

The method of the invention may be applied to any size of component. However, at present, ALM components may be generally limited to a size of approximately 250mm. This will increase with time and development of the ALM process. Any shape of component may be treated by the method of the invention, so long as the slurry can gain access to the surface required to be treated. Thus, in particular, components including hollows and cavities such as pipes, components with complex joints such as fluid pipes through aircraft and submarines, ventilation and air conditioning systems piped through aircraft, components with internally movable parts such as ball joints, one piece hinges, may be surface finished by the method or apparatus of the invention.

According to a second aspect of the invention, there is provided apparatus for surface finishing a component, the apparatus including a container for the component, slurry in the container, the slurry containing particles of hardness at least as great as that of a surface of the component to be surface finished, at least one acoustic transducer coupled to the slurry, the acoustic transducer being arranged to agitate the particles in the slurry whereby to surface finish the component. The invention now will be described by way of example with reference to the accompanying drawing which shows, schematically, apparatus according to the invention.

Referring to the drawing, the figure shows a metal container in the form of a circular tank 1 . The tank is filled with slurry 2 and has a pair of movable acoustic transducers 3, 4 rotatably attached to an outside surface 5 thereof. Inside the tank 1 is shown an example component, in this case an adjustable spanner 6 mounted on a rotating carousel 12. It will be noted that the spanner 6 is complex in construction and has open surfaces 7, cavities 8 and interlocking parts 9, 10. The tank 1 is shown as circular but may be other shapes, where more suitable. A circular tank has the benefit of enabling easy movement of transducers 3, 4 around the tank, in use.

The slurry 2 is made from a chemically inert liquid, here water, and polishing particles (not separately shown). Polishing particles must be selected for their size and hardness, for the component being treated. Their hardness needs to be as great, or greater than, the material of the component 6 and to be sufficiently small as to form a suspension in the liquid. Larger particles may be used for rough polishing and smaller particles may be used for a finer polish.

Examples of hard polishing particles for the slurry are garnet, tungsten carbide and aluminium oxide. Other materials of hardness which is selected for the hardness and type of the surface to be polished may also be used. For aerospace components, typical ALM-produced materials to be surface finished are a titanium alloy, Ti6A14V and aluminium alloys. Other suitable materials typically made by ALM, are steels and nickel alloys. However, it will be appreciated that the method of the invention is not limited by the material of the component. Plastics, for example laser sintered plastics such as nylon and other thermoset and thermoplastics, are equally suitable for surface finishing by this method.

In use, acoustic waves are transmitted from the acoustic transducers 3, 4 which cause the polishing particles to move back and forth in the liquid to locations determined by positions of acoustic nodes and antinodes. Abrasive action between the polishing particles and the surface of the component 6 leads to surface polishing taking place.

Acoustic frequency is determined by the requirement for sufficient particle movement; by the size of the polishing particles relative to the wavelength of the ultrasound, and by the acoustic impedance difference between the chemically inert medium (water here) and the particles. Particles such as garnet, tungsten carbide and aluminium oxide, with a size comparable to the acoustic wavelength, will result in a large scattering cross section. Attenuation of the signal within the water depends upon the particle volume fraction; the acoustic frequency, and the particle size and type. In order to provide sufficient particle excitation within the tank and to overcome attenuation, a judicious choice of frequency and power levels is required in order for the required degree of particle excitation to reach all required areas of the component. Acoustic powers of several tens or hundreds of watts are envisaged, with frequencies of typically 10MHz - 100MHz for wavelengths of approximately 150 to 15microns in water. In order to prevent standing waves and hence fixed locations of nodes and antinodes, which may cause abrasion/polishing only in selected areas, the transducers 6 may be continuously scanned across frequencies. However, if selected areas, only, of the component 6 are to be polished then standing waves can be deliberately generated by using fixed frequencies.

Moving the transducers 3, 4 around the tank 1 , as shown by the arrows, will also prevent the generation of standing waves and this technique may be used in combination with frequency variation.

Transducers 3, 4 could also be coupled from the bottom 1 1 and a top (not shown) of the tank. This will further increase acoustic field coverage and may enable an increase in the power coupled into the slurry. Clearly, the tank 1 needs to be full of the slurry, in order for the acoustic waves from a transducer coupled to the top to be effectively transmitted into the slurry.