ENHANCED MECHANICAL PROPERTIES

Field of the Invention

The invention relates to the production method for aluminum- copper-magnesium matrix hybrid composite comprising MgO (Magnesium Oxide) and CNT (Carbon nanotube) exhibiting enhanced mechanical properties and the composite material obtained by this method.

Known State of the Art

Today, there are composite materials obtained by using additives to be used in a region for a specific purpose, which exhibit characteristics specific to that region and by making bonds. There is a need for new engineering materials with better operating performance in many industries such as defense, automotive and aerospace industries. This demand has resulted in the development of aluminum matrix nano hybrid composite materials. The important reasons for these materials to become an alternative to many materials are their lighter weight, high toughness and rigidity, excellent wear resistance, superior fatigue strength, high compression, yield and tensile strength. Their specific strength, adaptability to many production methods, corrosion resistance and aesthetic appearance are their other striking properties .

Aluminum is a material that is commonly used in the industry due to its superior properties. However, alloy and composite materials are obtained by adding different metal and ceramic materials to strengthen the present properties of aluminum and to improve its weak properties. The composite materials in the known art do not provide different mechanical properties together. This limits the usage area of the product. The interface between the reinforcing particles and the matrix has a significant impact on the mechanical behavior of aluminum matrix composites. The transmission of external forces applied by the matrix to the reinforcement phase is possible by forming a strong interface between these two components. Therefore, the interface between liquid metal and reinforcement phase is critical.

The problem of heating that may occur at the interface causes pores and a decrease in the transmission of the force to the reinforcing particles, aggregation during solidification, thus forming a heterogeneous microstructure, which adversely affects the mechanical properties of the material .

Although aluminum has some superior properties, particularly corrosion resistance and lightness, it has low tensile, yield and compression strength and low hardness, which limits its usage area due to its being easily broken during applications .

Other problems in the production of metal matrix composites by casting method are interface problem and tendency to aggregate, which occur when the reinforcement phase is not completely heated by the liquid metal. A poor interface reduces the efficiency of force transfer from the matrix to the reinforcement material and consequently its strength decreases. Gas bubbles that will occur in the molten material causes the formation of pores in the material after casting and this formation adversely affects the mechanical properties of the material. Another problem in the production of composite by the casting method is oxidation.

There is a need for a production method that improves the mechanical properties of composite materials by solving the problems related to the production of composite materials mentioned above.

Non-patent document "Microstructure and mechanical properties of AZ91 alloy reinforced by carbon nanotubes coated with MgO" (Yuan, Q., Zeng, X., Liu, Y., Luo, L., Wu, J., Wang, Y., & Zhou, G. (2016)) is an example of the known state of the art. This document describes the microstructure and mechanical properties of AZ91 alloy that is reinforced by carbon nanotubes coated with MgO. The document describes that carbon nanotubes (CNT) are used as a novel reinforcement in Mg based composites due to their mechanical properties. However, in general, it was stated that due to the poor interfacial bonding between the CNT and the matrix, and the aggregation of CNTs, the nanoscale CNTs did not fully impart their exceptional properties to the Mg matrix, limiting their applications in the Mg-based composite. In this study, a new method has been developed to increase the interfacial bonding strength by coating the surface of CNTs with magnesium oxide (MgO) nanoparticles. The detailed crystallographic analysis conducted by transmission electron microscopy (TEM) confirmed that two types of bonds were formed at the CNT/MgO interface. These bonds were nanoscale contact bonds and diffused interfacial bonds. The yield strength and elongation of AZ91-3.0 MgO+CNT composite were enhanced by 69.0% and 22.9% respectively when compared to the AZ91 alloy. In D1 document, the mechanism of strengthening of AZ91 alloy composites reinforced by MgO and

CNTs was discussed in detail. However, the document does not describe a production method based on coating a reinforcement with a different material.

Patent document US2012164429A is an example of the known state of the art. This document relates to a metal matrix composite containing carbon nanotube (CNT) infused fiber materials and methods for production thereof. Illustrative metal matrices include aluminum, magnesium, copper, cobalt, nickel, zirconium, silver, gold, titanium and various mixtures thereof. The fiber materials can be continuous or chopped fibers and for example, include glass fibers, carbon fibers, metal fibers, ceramic fibers, organic fibers, silicon carbide fibers, boron carbide fibers, silicon nitride fibers and aluminum oxide fibers. The composite materials can further include a passivation layer overcoating at least the carbon nanotube-infused fiber material and optionally the plurality of carbon nanotubes. The metal matrix can include at least one additive that increases compatibility of the metal matrix with the carbon nanotube-infused fiber material. The fiber material can be distributed uniformly, non-uniformly or in a gradient manner in the metal matrix. It is stated that non-uniform distributions is used to impart different mechanical, electrical or thermal properties to different regions of the metal matrix. However, in the document, a reinforcing material is coated with another reinforcing material, a carbon fiber is used, and a continuous infusion process is employed.

Production processes of the known state of the art do not provide a solution to overcome the drawbacks described above. Accordingly, there is still a need for a production method developed to overcome these drawbacks. Detailed Description of the Invention

The invention relates to the production method for aluminum matrix hybrid composite comprising MgO (Magnesium Oxide) and CNT (Carbon nanotube) as reinforcing materials, exhibiting enhanced mechanical properties, and the composite material obtained by this method.

The object of the invention is to improve the tensile strength, compression strength and toughness properties of the aluminum matrix composite materials and obtain a strong interface. For this purpose, using more than one reinforcing material with different physical, chemical and mechanical properties at the same time, it is provided that the material has different mechanical properties expected from it.

The production method for aluminum-copper-magnesium matrix hybrid composite of the invention, in general, comprises the steps of

-coating MgO (magnesium oxide) and CNT material as reinforcement with nickel in an electroless manner, and wrapping them with aluminum foil,

-adding MgO (magnesium oxide) and CNT material into the liquid metal that is aluminum-copper-magnesium matrix material,

-adding shielding gas during casting,

-stirring the mixture by means of mechanical mixers at a specific speed for a predetermined period of time,

-adding the gas tablet into the molten material,

-casting the obtained mixture into the preheated steel container,

-obtaining the aluminum matrix hybrid composite material, -homogenizing the obtained composite material,

-subjecting the composite material to thermal treatment. In one embodiment of the invention, the mixture comprising MgO and CNT contains 50% MgO and 50% CNT by weight. It is taken from the mixture at 0.2% by weight and added into the liquid metal that is the matrix material.

In one embodiment of the invention, the mixture of 50% MgO and 50% CNT by weight is used at 0.2%.

In one embodiment of the invention, the diameter of MgO particles that are reinforcing materials (reinforcement) is less than around 45 nm and CNT particles have a diameter of 9.5 nm and a length of 1.5 pm.

In one embodiment of the invention, the casting process is carried out at 750°C.

In one embodiment of the invention, argon with 99.999% purity is used as the shielding gas. The reason for using shielding gas is to prevent the molten matrix from contacting with air and thus avoid oxidation.

In one embodiment of the invention, the mixture is stirred mechanically at an average speed of 500 rpm for 10 minutes. The purpose of this stirring process is to prevent aggregation and to provide homogeneous distribution of the reinforcements .

In one embodiment of the invention, the steel container into which the mixture is casted, is preheated to 250°C.

In one embodiment of the invention, the obtained composite is homogenized at 500°C for 5 hours. In one embodiment of the invention, the thermal treatment is carried out at 170°C and 200°C for 2, 4 and 6 hours. Thus, the mechanical properties are further enhanced.

The purpose of putting a gas tablet into the molten material is to prevent the formation of pores.

Before adding MgO and CNT that are the reinforcement materials into the matrix material, they are coated with nickel in an electroless manner to increase the wettability properties, i.e. to form a better interfacial bonding with the matrix material. It is further coated with aluminum foil to prevent oxidation.

The method of the invention is a stir casting method.

In summary, the invention relates to a production method for aluminum-copper-magnesium matrix hybrid composite.

A composite material obtained by the method of the invention is also within the scope of the invention.

Thanks to the invention, higher hardness, tensile strength, compression strength, and lighter material and wider usage area are provided. In addition, the wettability of magnesium oxide and carbon nanotube that are the reinforcing materials, is enhanced by the electroless coating method and thus a significant improvement is achieved on the mechanical properties of the hybrid composite material produced. The mechanical properties are further improved with the thermal treatment applied.