Manufacture of lightweight metal matrix composites with controlled structure

Abstract

Lightweight metal matrix composites containing a skeleton structure of titanium, titanium aluminide, or Ti-based alloy are manufactured by low temperature infiltration with molten Mg-based alloy or Mg-Al alloy at 450-750° C., with molten In, Pb, or Sn at 300-450° C., or with molten Ag and Cu at 900-1100° C. The skeleton structure with a density of 25-35% is produced by loose sintering of Ti or Ti-based alloy powders. A primary deformation of the Ti skeleton structure before the infiltration is carried out by cold or hot rolling or forging to obtain a porous flat or shaped preform with a porosity <50% and pores drawn out in one direction such as the direction of future rolling of the composite plate. A secondary deformation of the infiltrated preform is carried out by multistage cold, or especially hot rolling, to refine the microstructure of the infiltrated skeleton structure and transform it into the textured microstructure strengthened by intermetallic phases such as TiAl, Ti3Al, and TiAl3. Subsequent re-sintering or diffusion annealing form a fully dense final structure of the resulting material having improved mechanical properties. The molten Mg-based infiltrate is alloyed with Al, Si, Zr, Nb, and / or V with the addition of TiB2, SiC, and Si3N4 sub-micron particles as infiltration promoters. The molten Ag- or Cu-based infiltrate can be alloyed with elements depressing its melting point. The method allows for control of the microstructure of composite materials by changing parameters of deformation, infiltration, and heat treatment. The method is suitable for the manufacture of flat or shaped metal matrix composites having improved ductility, such as lightweight bulletproof plates and sheets for aircraft and automotive applications, composite electrodes, heat-sinking lightweight electronic substrates, sporting goods such as helmets, golf clubs, sole plates, crown plates, etc.

Description

[0001] The present invention relates to metal matrix composites (MMC) manufactured by methods of powder metallurgy especially by infiltrating a loose sintered solid metal powder with low-melting liquid metal or alloy. More particularly, the invention is directed to MMC containing at least one component (solid powder or infiltrating melt) based on lightweight metals such as titanium and magnesium.[0002] Metal matrix composites manufactured by infiltrating with molten metal are attractive materials for structural applications not only due to their excellent properties such as stiffness, light weight, high abrasion and oxidation resistance but mainly due to the opportunity to compose materials containing combinations of metals that can be difficult or cost prohibitive when produced by methods of conventional metallurgy and machining.[0003] However, infiltrated MMC (IMMC) are usually brittle, weak at high temperatures, and exhibit insufficient flexure, fatigue, and impact strengths. Low...

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Benefits of technology

[0016] It is therefore an object of the invention to form an homogeneous, essentially uniform structure of the metal matrix composites providing significant increases of such mechanical characteristics as elongation, toughness, flexure and impact strength or fatigue resistance.

[0019] It is still another object of the invention to generate intermetallic compounds on the interface. of the titanium matrix and the infiltrated alloy and to achieve the effect of strengthening of the final microstructure by the intermetallics.

[0026] In essence, the core of the invention is to control the composite microstructure using (a) loose sintering, (b) customized deformation before and after the infiltration, (c) alloying or modifying the infiltrated metal, and (d) heat treatment realizing dispersion-strengthening. The controlled microstructure results in significant improvement of mechanical properties of the composite material.

[0027] The method allows the control of the microstructure of the composite materials by changing parameters of deformation, infiltration, and heat treatment. The method is suitable for the manufacture of flat or shaped metal matrix composites having improved ductility such as lightweight bulletproof plates and sheets for aircraft and automotive applications, composite electrodes, heat-sinking lightweight electronic substrates, as well as for sporting goods such as helmets, golf clubs, sole plates, crown plates, etc.

Problems solved by technology

However, infiltrated MMC (IMMC) are usually brittle, weak at high temperatures, and exhibit insufficient flexure, fatigue, and impact strengths.

Low dynamic mechanical properties limit the application of IMMC especially in aircraft, automotive, and rocket industries.

All of these new processes as well as conventional powder metallurgy techniques impose certain limitations with respect to the characteristics of the produced IMMC.

Besides, low porosity results in the incomplete infiltration of small porous channels by magnesium.

Even long holding time does not help, and the final structure has a remaining porosity in the central part of the block.

This randomly distributed porosity and heterogeneous structure of dense titanium skeleton decrease mechanical properties of the obtained IMMC, especially impact strength and toughness.

But ceramic matrix composites have significantly lower elasticity and impact strength than the projected IMMC.

Besides, it is technically difficult to organize high pressure evenly distributed on the area of large size porous blanks such as plates, sheets, and the like.

However, this method does not improve any mechanical characteristics of the final product due to oxide or nitride intrusions acting as stress concentrators in the composite microstructure.

This method is not effective if the infiltrated compact is manufactured from titanium or zirconium as they are oxygen-active metals themselves.

Besides, carbon-based additives also deteriorate microstructure of IMMC.

However, the absence of pressure does not produce the beneficial effects on the composite structure as well as the high pressure in the above mentioned methods.

High temperature of the infiltration process caused by high melting point of silver and copper result in their active reaction with titanium and formation of monolith casting structure.

Both those technologies can not provide fully dense uniform structure of MMC, and therefore, can not provide a stability of mechanical properties of the composite materials.

(as described in the U.S. Pat. No. 6,287,433) results also in residual porosity and insufficient strength of the obtained composite plates.

All other known processes of making IMMC have the same drawback: the irregular structure (consisting of the sintered skeleton, casting infiltrate, and residual pores) results in low mechanical properties of the composite.

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