US 8,323,729 to Inoue et al.
disclosed a process for producing a metal member comprising a shot peening treatment, including projecting particles onto the surface of a metal material comprising an aluminum alloy using compressed gas to provide fatigue strength properties of the metal member; and a chemical conversion treatment including forming a film on the surface of the metal material by performing a chemical conversion treatment following the shot peening treatment, to provide corrosion resistance of the metal member.
In order to enable both fatigue resistance properties and corrosion resistance of the metal member, it requires two steps, namely, a first shot peening on the metal surface and then a further chemical conversion treatment for forming a protective film on the shot-peened surface.
So, it is a complex surface treatment with increased production cost of the metal member.
The present inventor has found the drawbacks of the conventional method, and invented a new dynamic impacting method for simultaneously peening and film-forming on a substrate of a work piece or structural object.
The object of the present invention is to provide a dynamic impacting method for a substrate surface. Aspects include simultaneously peening a substrate surface and forming a thin film of metallic glass or amorphous metal, generally an alloy as is known, on the substrate surface. We find that the method can increase the surface hardness, fatigue resistance, fracture strength and corrosion resistance of the substrate simultaneously.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic illustration showing a dynamic impacting method as performed in the present invention.
Fig. 2 is a sectional illustration showing the surface of a treated substrate obtained in accordance with the method and embodying the present invention.
In accordance with the present invention, particles of metallic glass or liquid metal alloy are provided for shot peening and film-forming on a substrate, preferably a metal substrate or an alloy substrate of a work piece or an engineering structural object; the substrate is not limited in the present invention.
The process steps of the present invention comprise:
1. Preparation of Metallic Glass particles
A raw material of metallic glass or amorphous metal alloy is prepared by adjusting a proper atomic percentage of the elements forming the metallic glass.
The raw material of metallic glass is then put into a vacuum furnace for melting the metallic glass and then quickly cooled and atomized such as by high-speed fluid or gas to produce metallic glass particles.
The metallic glass particles are then collected and classified into several grades, for instance, a particle size of 5∼10 µm, 10∼20 µm, 20∼50 µm, 50∼100 µm or 100∼300 µm. The smaller the particle size, in general the finer and denser the peened surface on the substrate will be.
2. Bombardment of the Metallic Glass particles on the substrate
The metallic glass particles 1 are bombarded against a surface of the substrate 2 as shown in Fig. 1. The metallic glass particles are ejected through a nozzle or gun 11 driven by a compressed gas, desirably being or including argon, to dynamically bombard the substrate surface and thereby harden and smoothen the corrugated or rough substrate surface.
Substantially, the substrate 2 has its upper surface portion hardened to be a hardening zone 21 as shown in Fig. 2. Since the metallic glass particles 1 are continuously bombarded on the substrate surface, the above-mentioned corrugated or rough surface will then be smoothened by the further bombardment of metallic glass particles, forming a metallic glass thin film 10 over the hardening zone 21.
By means of this processing, the hardened zone 21 can increase the hardness, fatigue resistance and fracture toughness of the substrate according to known principles, while the metallic glass thin film 10 may further increase the corrosion resistance of the substrate. Comparatively, the described process of this invention can increase the hardness and the corrosion resistance simultaneously, rather than by the two-steps as disclosed in the prior art of US 8,323,729
as early depicted in the "Background of the Invention" of the Specification.
Desirably the bombardment of metallic glass particles on the substrate may be further divided into two sub-steps. The two sub-steps can be an initial bombardment at a higher pressure for hardening the surface, in accordance with known principles, and whereby the resulting treated surface is generally roughened, and a subsequent bombardment at a lower pressure whereby the metallic glass particles combine or coalesce to form a smoother film over the roughened substrate surface. The two sub-steps can be as follows.
A. High-Pressure Bombardment
The metallic glass particles are bombarded against the substrate surface at a speed of at least 10 meters/second, e.g. as driven by compressed gas (such as argon gas) under a high pressure of e.g. 5∼15 bars to obtain a hardened but rough substrate surface.
B. Low-Pressure Bombardment
The metallic glass particles are further bombarded against the substrate surface by a compressed gas under a lower pressure, of e.g. 0.1∼5 bars to obtain a smooth and shiny substrate surface, similar to a polishing surface.
We find that the above bombardments may rapidly superimposedly form thin films of metallic glass on the substrate surface, thereby forming a corrosion resistant surface with smooth and shiny appearance.
Therefore, the finished surface of the substrate may have hardened zone 21 and metallic glass thin film layer 10 for enhancing both hardness and corrosion resistances to be superior to the prior art.
By bombarding metallic glass particles on a 6061 aluminum alloy substrate, we have found a surface nano-hardness of 23.41 GPa (2212 Hv) which is greatly increased in comparison with the surface untreated by metallic glass bombardment - only 1.13 GPa, (107 Hv).
Meanwhile, after bombardment of the metallic glass particles on a high speed steel pitch mould surface, we found a surface nano-hardness increased from 7.06 GPa (667 Hv) to 22.03 GPa (2082 Hv). Furthermore, the treated surface was not corroded or rusty (i.e. did not form oxide layer) after exposure to the air for 3 weeks.
The present invention can provide the following advantages over the prior art and conventional shot peening.
- 1. The metallic glass particles may be formed as a true spherical shape to form a smooth polishing surface after bombardment.
- 2. The metallic glass particles have high fracture strength, not easily broken to damage the processing surface, and the particles may also be recycled for re-use.
- 3. The metallic glass particles have high hardness and density to thereby increase the bombarding effect when bombardment against the substrate to form a bombarded surface with increased hardness.
- 4. The metallic glass particles when striking the substrate will be partially melted due to frictional heat when impacting the substrate surface at high speed (such as 10 m/s or higher) to a temperature higher than its glass transition temperature (Tg) so as to form a thin film of metallic glass to be adhered on the substrate surface, which will be instantly cooled to a room temperature to still keep its amorphous property. This is very important since such a metallic glass thin film as formed on the substrate surface will render a better corrosion resistance of the substrate of the work piece or structural object. A production cost may then be greatly reduced.
Conclusively, without further treatment for corrosion resistance, the bombardment of the metallic glass particles on the substrate surface, it may render the substrate surface to be corrosion resistant in addition to the increasing of hardness, the fatigue resistance and the fracture strength.
The present invention further comprises a product as made by the method as aforementioned, generally comprising the substrate with the treated surface, having the thin film of metallic glass or liquid metal alloy overlying the hardened peened surface beneath. The thin film is generally smoother than the hardened substrate surface.
A dynamic impacting method comprising:
A. providing metallic glass particles or amorphous metal alloy particles (1), and
B. bombarding the metallic glass particles or liquid metal alloy particles (1) against the surface of a substrate (2), to harden the surface and to form thereon a thin film (10) of the metallic glass or alloy for increasing corrosion resistance of the surface.
A method according to Claim 1 wherein said bombarding of metallic glass particles (1) on the substrate (2) comprises:
a high-pressure bombardment step, bombarding the metallic glass particles (1) driven by a high-pressure gas under a pressure ranging from 5 bars to 15 bars, to harden the surface of the substrate (2); and
a low-pressure bombardment step, further bombarding the metallic glass particles (1) driven by a low-pressure gas under a pressure ranging from 0.1 bars to 5 bars, to superimpose a thin film (10) of metallic glass on said substrate (2) to form corrosion resistance and a smooth or shiny surface of said substrate (2).
3. A method according to Claim 1 or 2 wherein said substrate is of metal or metal alloy, such as of steel or aluminum.
4. A method according to any one of Claims 1 to 3 wherein said metallic glass particles (1) are made by melting a metallic glass raw material in a vacuum furnace and then quickly cooling and atomizing to form the metallic glass particles (1).
5. A method according to Claim 4 wherein said metallic glass particles (1) obtained from said vacuum furnace are collected and classified into plural particle sizes for optional or selective uses.
6. A product obtainable by a method as set forth in any one of Claims 1 to 5.