Electric heating – Metal heating – By arc
Reexamination Certificate
1999-04-19
2001-10-16
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121660, C219S121730
Reexamination Certificate
active
06303897
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field Of the Invention
The invention relates to a process for the laser treatment of the inside surfaces of hollow pluralities with rotation-symmetric axes, which consist of a matrix alloy.
More specifically, this invention relates to the laser treatment of light metal engine cylinder blocks to provide a wear-resistant inner piston working surfaces. Furthermore, the invention relates to a device for carrying out such a process.
2. The Prior Art
It is known, for example from German Patent DE 3910098 A1 to weld pipes by means of a laser, whereby a rod-shaped probe with a lens system is disposed in a workpiece. In laser surface treatments, it is advantageous to also treat marginal surface coatings, especially on the inside of the workpiece, by the application of laser energy over wide surface areas.
Thus, it is an object of the invention to obtain increased amounts of alloying components in marginal coatings, as well as intermetallic compounds in these coatings, and to obtain a finer grain coating in the marginal layer, particularly in connection with cast aluminum or wrought alloys.
It is a further object of the invention to provide an increased resistance to abrasion in metallic materials, such as in engine block components and also in tools subject to wear, as well as in tubes and guide bushings.
In the prior art, wear-resistant workpieces were produced by alloying the entire material of the workpiece with an additional substance, such as, adding up to 17% silicon to an aluminum cast alloy. Where the surface is to be provided with higher wear resistance, the silicon coating provides a wear hardened surface. However, this makes the overall workpiece more brittle, which leads to substantial problems in the casting process.
SUMMARY OF THE INVENTION
Therefore, the present invention provides a treatment on workpieces for incorporating alloying components at a later time, that make a surface more resistant to wear, as well as providing a device for carrying out such a process.
The invention provides a process for alloying in substances, wherein the marginal layer is influenced by mixing the alloying components in the marginal zone by 100%. The process is preferably carried out using pure silicon powder for the alloying process with aluminum. With depths of penetration of from 0.2 mm to 2 mm, the separated silicon particles, with particle sizes of from 3 to 25 &mgr;m, make up a proportion by volume of from 17 to 50% in the marginal layer. This provides an increased resistance to wear of the aluminum matrix alloy.
It is advantageous if the laser light is directed to the surface with a distal energy of 20 to 800 J/mm, and an intensity of 0.5 to 4.0 kW/mm
2
. The laser light has to be shaped so that a
tophat
distribution takes place.
Laser light wattages of 1.3 kW to 3 kW have been successfully employed in the past, whereby both Nd-YAG lasers and high-power diode lasers were employed. Laser light with wavelengths of 1064, 808 and 940 nm is advantageous for mixing up in the marginal zone, whereby the respective process parameters have to be coordinated with each other.
This particularly applies to the feed rate to be adjusted, which may be in the range of 300 to 4000 mm/min. The feed rate is, in each case, dependent upon the intensity, i.e., the wattage of the laser light, the way in which the focus is shaped, and on the thermal conditions at the melting site. These conditions are determined by the heat source, the volume of the melt, and the cooling rate. The process parameters have to be coordinated with each other so that the process of separation of hard-substance particles takes place with the desired concentration and particle size.
Thus, by employing a laser light beam which, at the site of impact, produces a material plasma, it is possible to incorporate powder in the locally produced plasma. By introducing an alloying powder with a grain size of preferably 45&mgr; to 150&mgr;, it is possible to obtain with a laser light beam of 2 kW, a penetration depth of 1 mm over a spot diameter of 0.05 to 2 mm. This beam produces a sufficiently thick wear-resistant marginal layer, so that the latter will not detach itself from the workpiece under mechanical stress. At the same time, the selected conveying gas, for example a rare (or inert) gas, assures a separation of the plasma from reactive, oxygen-containing atmosphere. The wattage of the laser light is controlled so as to obtain a suitable distribution of primary and secondary hardened phases.
A silicon component of between 17% to 50% by volume so incorporated in the surface, advantageously assures that the elastic properties continue to be present as before in the depth of the workpiece. These properties permit absorption of the mechanical stresses, such as, for example, the working surfaces of the cylinders of an engine.
Considerable thermal problems have to be solved when an energy beam probe is disposed in a cylindrical segment of a workpiece. Apart from the heat reflected by the workpiece, the geometrical conditions lead to considerable heat at the head of the probe, which, according to the invention, is counteracted by water cooling.
Furthermore, the probe head of the invention is rotatable, so that apart from prior art equipment, where the workpiece has to be turned, the inventive probe can now remain stationary. This leads to an easier handling of the engine blocks that have to be treated because the laser light and alloying powder can be supplied in their conveying media to the probe head via a rotating passage.
Moreover, a pore-free result is possible through a suitable feed of the powder and gas when treating interior spaces. In this connection, it is necessary to make sure that with the proper guidance of the protective gas and design measures, that the applied powder will not deposit on the optics of the probe. If need be, a separate protective gas can be fed to the probe next to the beam of energy in addition to the conveying gas.
The probe is capable of treating the workpiece provided a few conditions are met. First, there should be a laser light wattage of approximately 2 kW on a beam spot having a diameter of about 0.5 to 2 mm, a feed rate of 300 to 1500 mm per minute, and a gas conveying rate of about 10 to 20 liters per minute. In addition, a powder feed of up to 10 grams per minute has to be added. For example, if the probe is focused on a surface site defined by spiral migration, then upon completion of one turn (or rotation) of the probe, the surface site being treated may migrate over the entire surface area to be treated by simultaneously lowering the probe. Thus, from 17% to 50% by volume of silicon is produced on the surface layer through alloying, whereby excess silicon powder is removed from the interior of the workpiece by transporting the excess powder away, especially together with the conveying gas.
The probe can then be focused in the space provided on a spirally migrating surface site. This occurs with an increased laser light wattage of 4 kW focused on a beam spot with a diameter of between 2 to 4 mm, and with a feed rate of between 1500 to 4000 mm per minute, and having a conveying gas feed of about 30 liters per minute, and wherein a powder feed rate of up to 20 grams per minute is added.
The invention is primarily suitable for treating internal spaces, for example, cylinders and tubes having a shaft ratio in excess of 1=10 (measured as the diameter to depth ratio), whereby the rod-shaped probe can be employed in a particularly favorable manner starting with diameters of 50 mm and more. Both the feed means for feeding the silicon powder in a conveying and protecting gas, and means for guiding the laser beam are arranged in the probe. The means for guiding the laser beam projects a beam of laser light via a collimating lens system onto a reversing mirror formed in the head of the probe. The focus can be formed with the reversing mirror, the latter being made of copper and cooled with water due to the high process temperatures. Furthermore, the mirror is preferab
Bady Torsten
Bohling Michael
Feikus Franz-Josef
Fischer Alfons
Lensch Gunter
Collard & Roe P.C.
Elve M. Alexandra
VAW motor GmbH
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