Method of laser casting copper-based composites

Details Method of laser casting copper-based composites Method of laser casting copper-based composites

2002-01-31

2004-04-20

Jenkins, Daniel J. (Department: 1742)

Powder metallurgy processes

Powder metallurgy processes with heating or sintering

Making composite or hollow article

C419S007000, C419S009000, C419S046000, C419S047000

Reexamination Certificate

active

06723278

ABSTRACT:

FIELD OF INVENTION
This invention relates to a method of laser casting, for example, a copper-based material. In particular, the invention presents a method to directly melt pure copper (Cu) powder with the help of other elements (X) such as, for example, (nickel (Ni), iron (Fe) or tungsten (W)) using CO
2
laser. Using this technique, Cu alloys (Cu+X) and composites Cu−Y and Cu−X−Y (Y=tungsten carbide (WC), titanium carbide (TiC), titanium (Ti) and graphite (C)) can be synthesised from elemental powder mixtures which are prepared by mechanical mixing or milling processes. The developed laser casting process may advantageously be used to fabricate complex three-dimensional objects, by multi-layer overlapping, which may be used in electrical discharging machining (EDM) electrodes, rapid die and mould tooling, or other system components.
BACKGROUND ART OF THE INVENTION
The method and apparatus of selective laser sintering (SLS) are described in U.S. patents such as U.S. Pat. No. 4,863,538 (1989), U.S. Pat. Nos. 4,938,816 and 4,944,817 (1990), U.S. Pat. No. 5,076,869 (1991) and U.S. Pat. No. 5,182,170 (1993). In SLS, parts are built by selective sintering or local melting of a binder in a thin layer of powder particles using a CO
2
laser beam. The interaction of the laser beam with the powder raises the temperature to the melting point of the powder binder, resulting in particle bonding, fusing the particles to one another and to the previous layer. After an additional layer of powder is deposited via a roller mechanism on top of the sintered layer, the succeeding layer is similarly sintered and built directly on top of it. In this way, the entire solid can be built layer by layer. Each layer of the building process consists of the required cross-section of the part at a given height. The unsintered powder in each layer remains in the powder bed during processing to support overhangs and other structures in subsequent layers. The completed part is revealed by brushing off the loose powder surrounding it and the unsintered powder can then be reused. Despite of the capability of the SLS to build parts of various materials, post-processing, such as debinder and Cu infiltration, is often needed to achieve working strength. Shrinkage of the built part after the debinder and infiltration process results in distortion.
A selective metal powder sintering process was described by Van der Schueren and Druth in “Powder deposition in selective metal powder sintering” in Rapid Prototyping Journal, Vol. 1, Number 3, 1995, pp23-31. In this process particles in a Fe—Cu powder mixture were selectively bound by means of liquid phase sintering initiated by a Nd-YAG laser beam. The powder deposition mainly depended on the powder properties—in this case on the individual Cu or Fe powder properties—and resulted in compromises on the powder mixtures as well as in modifications of the deposition mechanism.
EOSINT M system, as described in U.S. Pat. Nos. 5,753,274, 5,730,925, 5,658,412, was the first commercial system for direct laser sintering of metallic powder. The word “direct” implies that the material constituents are directly laser sintered to produce a high density part requiring little or no post-processing. A related patent on parts formed by direct sintering is U.S. Pat. No. 5,732,323 which describes processing of powders based on an iron-group metal. Currently, the only metallic material that is available commercially for direct metal sintering is a bronze-nickel alloy by Electrolux and a newly developed metal powder M Cu 3201 by EOS. Direct selective laser sintering involves directly melting and consolidating selected regions of a powder bed to form a desired shape having high or full density. Direct metal laser sintering involves melting the component matrix and obtaining the appropriate amount of flow from the molten material. The appropriate amount of flow is critical and can be described as the flow that eliminates porosity, produces a highly dense part and maintains tight dimensional tolerances. The appropriate amount of flow is controlled by factors such as atmosphere, powder bed temperature and laser' energy density. Three important parameters governing the energy density are laser power, scan spacing and scan speed
2
:
A
n
=P/&ngr;&dgr;(J/cm
2
)  (1)
where A
n
is the energy density; P is the incident laser power (Watts); &ngr; is the laser scan speed (cm/s); and &dgr; is the scan spacing (cm).
If the energy density is too high, the surface begins to vaporize before a significant depth of molten material is produced. The sintered layer thickness decreases with increasing scan speed due to the shorter interaction (sintering) time. This thickness also decreases with decreasing scan line spacing if the laser beam spot is larger than the spacing. More scan overlapping will occur with smaller scan line spacing. The thermal conductivity and reflectivity of the sintered solid are higher than those of the powder. When more scan overlapping occurs, more laser energy will be transferred away by heat conduction through the sintered solid and reflected away by the sintered solid surface resulting in a decrease in the layer thickness.
The amount of light energy of the laser beam absorbed by a metallic surface is proportional to 1-R, where R is its reflectivity. The reflectivity of a material is defined as the ratio of the radiant power reflected to the radiant power incident on the surface. It indicates the fraction of the incident light that is absorbed and contributes to heating effects, and is most dependent on the electrical conductivity. A metal with high electrical conductivity has high reflectivity, for example, copper and nickel. High-density energy is required to sinter a material with high reflectivity, such as Cu. Another important characteristic is thermal diffusibility. A material with high thermal diffusibility will normally allow a greater depth of fusion penetration with no thermal shock or cracking.
At the CO
2
laser wavelength of 10.6 &mgr;m, where R is close to unity, 1-R becomes very small. High-density energy is thus required to sinter a material like copper. The difference in the value of R becomes important at long wavelengths. For copper at 10.6 &mgr;m, 1-R is about 0.02, whereas for steel it is about 0.05. As steel absorbs 2.5 times as much of the incident light as copper, it is easier to melt steel with a CO
2
laser than metals such as aluminum or copper. Attempts to coat the powder surface to improve heat absorption or reduce reflection are not always effective because of poor thermal coupling between the coating and the powder. The reflectivity problem has been a barrier to the application of CO
2
lasers to the melting of metals such as copper or gold.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a method of laser casting a metal-based alloy or composite comprising:
milling elemental metal powder having a relatively high reflectivity at the wavelength of the laser with at least one other material which absorbs laser energy more readily than said elemental metal powder to form said metal-based mixture; and
laser casting said metal-based mixture; wherein said milling is conducted for a period sufficient to form at least a partial coating of said at least one material on particles of said elemental metal powder.
More particularly the invention provides a method of laser casting a copper-based alloy or composite comprising:
milling elemental copper powder with at least one other material which absorbs laser energy more readily than elemental copper powder to form said copper-based mixture; and
laser casting said copper-based alloy or composite by application of a laser to said copper-based mixture; wherein said milling is conducted for a period sufficient to form at least a partial coating of said at least one material on particles of said elemental copper powder.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
a
illustrates a process of laser casting in building strips;
FIG. 1
b
ill

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