Electric heating – Metal heating – By arc
Reexamination Certificate
2002-05-17
2003-12-30
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121710
Reexamination Certificate
active
06670571
ABSTRACT:
This invention relates to a method of operating on a workpiece, for example to modify or prepare a surface topography of the workpiece or parent material in order to increase the degree of mechanical friction or the degree of mechanical interlock with respect to another material or body, or to create holes in the workpiece.
Various methods have been used to increase the surface roughness of materials which have been made relatively smooth by machining, polishing, forming against a flat mould and so forth. These roughening methods include surface deformation by mechanical means such as grooving or knurling, or chemical means such as etching.
Alternatively, material has been added to the parent material surface by means, for example, of arc welding with a consumable filler wire. In one example small drops of metal have been deposited onto a metal surface, utilising the consumable electrode gas metal arc (GMA) process in which the current is modulated such that molten drops are detached from the melting wire electrode in a discrete and regular manner. These drops serve as raised dimples on the otherwise relatively smooth parent material surface and have been used to reduce the tendency to slip between the parent and another body.
EP-A-0626228 describes a method of modifying the surface of a workpiece which is to be joined to another member comprises exposing a series of locations on the workpiece surface to a power beam whereby at each location workpiece material is melted and spaced laterally under the influence of the power beam and then allowed to solidify so as to form a cavity surrounded by an area of solidified material which is raised with respect to the adjacent workpiece surface and has a re-entrant profile.
In accordance with a first aspect of the invention, we provide a method of operating on a workpiece, the method comprising causing relative movement between the workpiece and a power beam in a traverse direction so as to expose a series of locations on the workpiece to the power beam; and at each location causing the power beam to move relative to the workpiece in a predetermined manner having a component transverse to the traverse direction, whereby at each location, workpiece material is melted and displaced by the power beam so as to form a cavity or hole.
The advanced texturing technique claimed here differs fundamentally from the previously known and applied method. In the advanced form, the power beam is specifically caused to move (manipulated) in a predetermined manner at each location, instead of being static. The power beam may be pulsed so as to expose each location, although preferably it is continuous. The typically relatively small, high frequency “secondary deflection” motions of the beam at each location have the effect of markedly altering the redistribution of the molten material generated at each location. In general the movement of the beam is predetermined by an initial step of selecting the desired movement.
The invention can be used to form cavities in the surface of the workpiece or to create holes, typically extending through the workpiece. The formation of a hole can be carried out with or without a backing substrate. Furthermore, when a cavity or hole is formed, preferably the step of causing the beam to move in the predetermined manner is performed after the cavity or hole has been formed. This enables the shape of the cavity or hole to be accurately determined. In many cases a manipulation step may also be used during the cavity/hole formation where the diameter of the cavity/hole exceeds that of the beam.
The secondary deflection is preferably at such a frequency that at least one complete pattern repeat (or at least direction reversal) is performed at each location, and preferably an integer number of repeats is carried out at each position. For 1000 holes per second in steel, a secondary frequency of approximately 1000 Hz, 2000 Hz or 3000 Hz is appropriate. In this case the cavities would be ~0.5 mm in depth and width, on a 0.7 mm pitch. Higher secondary frequencies may be required with smaller diameter cavities, and the material properties also influence the optimum frequency.
The “secondary deflection” used may take various forms, including circular, linear, ellipsoidal and/or simple geometric figure shaped motions, each having at least a component in the direction transverse to the traverse direction. The secondary deflection may also be combined with other suitable variations of temporal and spatial beam power density distribution (e.g. pulsing of beam current and/or varying the beam focus position) to achieve the desired effect and allow finer control over the cavity or hole formation process.
In general, the duration of the beam's interaction is carefully controlled so that the secondary beam deflection is consistently applied at each location, i.e. the secondary and primary beam motions are correctly phased in relation to one another so that the texturing effect is reproducible and does not vary in a non-systematic fashion.
In the case of cavity formation, suitable raised portions or dimples with re-entrant profiles may be generated on the otherwise comparatively smooth material surface from the parent material by rapidly melting small localised regions by means of a (typically focussed) power beam such as an electron beam impinging on the parent material. The high power density beam is rapidly moved from point to point to produce a series of such raised dimples. By closely spacing the dimples a semi-continuous line or ridge can be produced. It is noted that the raised material comes from melting and displacing the molten material, thus leaving a small cavity in the parent. Some material may be lost by vaporisation under the high power density of the applied beam.
In use, when a workpiece provided with cavities is adhered to another member, both the cavity and the raised dimple can contribute to mechanical interlock.
The adhesion to textured surfaces of materials from the vapour phase is also modified and enhanced.
The re-entrant profile further increases the mechanical forces required to separate the parent from, or slide the parent over, another member.
The re-entrant profile is beneficial in retaining, for example, an adhesive, especially an adhesive which does not itself bond to the parent, or only bonds with a low strength. Alternatively the bond strength between a non-adherent material, e.g. a low friction polymer, and the parent material surface may be increased substantially by the interlocking nature of the re-entrant profile.
In addition, the processed surface is substantially cleaned and freed from any minor contamination by the action of the beam, this increasing the bond strength obtained with any subsequent adhesive bonding operation.
Where a series of closely spaced dimples form a semi-continuous ridge, this ridge preferably also has a re-entrant profile.
For the purposes of increasing the overall bond strength of an adhesive, it is also noted that the adhesive preferably also enters the cavity as well as the re-entrant surface on the dimple to increase its bonding shear strength.
Preferably the surface topography utilises the existing parent material so no additional material is deposited onto the parent. The technique of changing the surface topography relies on utilising the melting/vaporisation capability of a power beam or high power density heat source typically brought to a focus in the region of the work surface.
Preferably a portion of the movement in the predetermined manner causes an area adjacent the location to be heated. Such an area may surround the particular location or may constitute another location. The adjacent location may be heated either before or after the formation of the cavity or hole, so as to effect a pre-formation or post-formation heat treatment respectively. The workpiece surface in the adjacent region can be either heated to below or above its melting point.
The form of the predetermined movement at the location may be similar to that of the portion of the predetermined move
Heinrich Samuel M.
Novack Martin
The Welding Institute
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