Alloy concept

【China Aluminum Network】 Alloy is a kind of metal with one or several other elements (metal or non-metal can be) after the fusion of the composition of the material with the speed characteristics. The more basic, independently presentable substances that make up the alloy are called components, or simply yuan. In most cases, the components are the elements that make up the alloy. However, there are also compounds as constituents, provided that the compound does not decompose nor undergo any chemical reaction within the scope of the study. According to the number of components, they can be divided into binary alloys, ternary alloys, or multi-element alloys. For example, simple brass is a binary alloy composed of two elements: copper and zinc. Duralumin is composed of aluminum, copper, and magnesium. The composition of the ternary alloy.

â—† Copper Alloy Classification

Copper alloys are divided into brass, bronze and cupronickel. Cupronickel is a copper-nickel alloy and is mainly used to make corrosion resistant parts, resistors, and thermocouples in precision machinery and precision instruments.

In mechanical production, brass and bronze are mainly used.

● Cast brass

Alloys known for copper and zinc are collectively referred to as brass. The copper-zinc binary alloy is called ordinary brass. In addition to zinc and then adding other elements composed of multi-brass called special brass.

Cast brass has higher mechanical properties, better casting properties and lower price than bronze. Commonly used for general-purpose bearings, bushings, gears and other wear parts and valves and other corrosion parts.

● Cast bronze

Can be divided into ordinary bronze (tin bronze) and special bronze (aluminum bronze, lead bronze, silicon bronze, bronze, etc.) two categories.

â—† Copper alloy casting process

Copper alloys of various compositions have different crystallization characteristics, different casting properties, and different casting process characteristics.

1. Tin bronze: Crystallization features a large crystallization temperature range and a wide solidification zone. Poor fluidity in casting performance, easy to produce shrinkage, not easy to oxidation. The process feature is that the wall-thick parts adopt directional solidification (sequential solidification), and the complex thin-wall parts and general wall-thick parts are simultaneously solidified.

2, aluminum bronze and aluminum brass: crystallization characteristics is a small range of crystallization temperature, layer-by-layer solidification characteristics. Good fluidity in casting performance, easy to form a concentrated shrinkage hole, easily oxidized. The process features aluminum bronze casting system as bottom injection type and aluminum brass casting system as open type.

3, silicon brass: crystal characteristics are between bronze and aluminum bronze. Good casting performance (in special brass). The process features a sequential solidification process, a mid-injection gating system, and a small riser size.

â—† Aluminum alloy castings classification

Cast aluminum alloys can be divided into aluminum-silicon alloys, aluminum-copper alloys, aluminum-magnesium alloys, and aluminum-zinc alloys according to chemical composition.

● Aluminum alloy casting process

The casting properties and chemical composition of aluminum alloys are closely related, among which the Al-Si alloy is near the eutectic composition and has good casting properties, similar to those of gray cast iron. The Al-Cu alloy is far from the eutectic composition, has a large solidification temperature range, and has poor casting properties. In practical production, aluminum castings are provided with feeders for feeding, Al-Si alloys have a small solidification temperature range, high feeding efficiency for feeders, and easy to obtain dense castings. Other types of cast aluminum alloys have a wide range of solidification temperatures, low feeder feeding efficiency, and poor casting compactness.

Aluminum alloys are very easy to inhale and oxidize, so the pouring system must ensure that the molten aluminum flows in quickly and smoothly, avoiding stirring.

Various casting methods are suitable for aluminum alloy castings. When the production volume is small, sand casting can be used, and fine sand should be used for molding. For mass production of important castings, special casting is used. Metal casting has high efficiency and good casting quality. Low-pressure casting is suitable for pressure-resistant castings requiring high compactness. Pressure casting can be used for thin-walled and complex parts.

● Melting characteristics of cast aluminum alloy

Aluminum alloy is easily oxidized in liquid state. Its product is Al2O3. Its melting point is as high as 2050°C. Its density is slightly larger than that of aluminum. It is a solid inclusion suspended in the liquid of aluminum. It is difficult to remove. It not only deteriorates the casting performance, but also reduces the mechanical properties. Density decreases. Aluminum liquid also easily absorbs hydrogen, precipitates when solidified, and forms defects such as pores or pinholes.

1. Refining method In order to slow the oxidation and absorption of aluminum liquid, the aluminum alloy should be melted under the flux layer. KCl, NaCl, or the like can be added to the crucible as a flux to isolate the aluminum liquid from the furnace gas. In order to drive off the hydrogen that has been sucked into the aluminum liquid and prevent the generation of pinholes, hydrogen flooding and refining should be performed before the aluminum liquid is discharged. There are various methods, and it is relatively simple to use a bell jar to press a chloride or chloride such as zinc chloride (ZnCl2) or hexachloroethane (C2Cl6) into the aluminum liquid, and the following reaction occurs:

3ZnCl2 + 2Al = 3Zn + 2AlCl3

3C2Cl6 + 2Al = 3C2Cl4 + 2AlCl3

The boiling point of the AlCl 3 formed by the reaction is 183° C., and the boiling point of C 2 Cl 4 is 121° C. Therefore, bubbles are formed. During the floatation process, gases H 2 and Al 2 O 3 in the aluminum liquid are carried together with the liquid surface.

2, smelting equipment There are many types of aluminum alloy melting furnaces, which are generally used for coke boilers. Resistor crucible boilers can also be used. In addition, induction furnaces (power frequency, intermediate frequency) are also used.

The structure of the alloy is much more complicated than pure metal. Because the alloy consists of two or more elements, the various elements interact to form different phases. We refer to homogeneous components in metals and alloys that have the same chemical composition, the same structure, and are separated from other parts by the interface.

According to this concept, the structure of pure metals and alloys is analyzed below. When the pure metal is liquid, it is a single phase; the solid state is composed of the same element and the same crystal lattice, so it is a single phase; in the crystallization process, there are both a liquid phase and a solid phase, that is, two phases. When the alloy is in a liquid state, it is an alloy liquid having a uniform chemical composition and is a single phase. After the alloy changes from a liquid state to a solid state, the elements dissolve into each other to form a solid solution; the elements may also react with each other to form a metal compound. Solid solutions and metal compounds are the two basic phases of solid-state alloys. Therefore, in solid state, the alloy may be single-phase or multi-phase. In the analysis of the alloy structure, it is to analyze the phase structure to see if it consists of several solid solutions or metal compounds, that is, several phases.

â—† Solid solution

By solid solution is meant an alloy phase composed of solute atoms dissolved in the lattice of a metal solvent. The two groups of elements are mutually soluble in the liquid state, the solid state also dissolves in each other, and a uniform and uniform substance is formed. When a solid solution is formed, a solvent with a large content is a solvent, and a solvent with a small content is a solute. A crystal lattice of a solvent is a lattice of a solid solution.

● Classification of solid solutions

According to the position of the solute atoms in the lattice, they can be divided into the substitutional solid solution and the interstitial solid solution.

1. Displacement of solid solution A solute atom occupies the position of a node in the solvent crystal lattice to form a solid solution called a substitutional solid solution. When the solvent and solute atom diameters are not much different, generally within 15%, the substitutional solid solution is easily formed. The copper-nickel binary alloy forms a substitutional solid solution, and nickel atoms can replace copper atoms at any position in the copper lattice.

2. Interstitial solid solution The solute atoms are distributed in the solvent lattice space and form a solid solution called interstitial solid solution. The solvent of the interstitial solid solution is a transition metal with a larger diameter, and the solute is a non-metallic element such as carbon and hydrogen with a very small diameter. The formation condition is that the ratio of solute atom to solvent atom diameter must be less than 0.59. For example, in iron-carbon alloys, solid solutions of ferrite and austenite formed by iron and carbon are interstitial solid solutions.

In addition, according to the solubility of solute elements in solid solution, it can be divided into finite solid solution and infinite solid solution. However, only the replacement solid solution may become an infinite solid solution.

● The performance of solid solution

When the solute element content is small, the solid solution properties are basically the same as the solvent metal properties. However, as the content of solute elements increases, the strength and hardness of the metal increase, and this phenomenon is called solid solution strengthening. Displacement of solid solution and interstitial solid solution will result in solid solution strengthening.

Appropriate control of the solute content can significantly increase the strength and hardness, while still ensuring a sufficiently high plasticity and toughness, so the solid solution generally has good comprehensive mechanical properties. Therefore, structural materials with comprehensive mechanical properties are required, and almost all of them take a solid solution as a basic phase. This is why solid solution strengthening has become an important enhancement method and has been widely used in industrial production.

â—† Metal Compounds

The metal compound is a new phase formed by the interaction of two groups of elements. Its crystal structure is different from the two groups of elements and has metal properties. There are many kinds of metal compounds, their common feature is high melting point, high hardness, generally as a hardening phase in the alloy. Such as carbon steel Fe3C, alloy steel TiC, VC, W2C and so on.

The presence of a single-phase solid solution or metal compound in the alloy is reduced, and is mostly present as a two-phase mechanical mixture. For example, pearlite in carbon steel is a mechanical mixture composed of a solid solution (ferrite) and a metal compound (cementite Fe3C).

â—† Determination of alloy phase diagram

As mentioned earlier, the crystallization of pure metals is performed at a constant temperature and can be described by a cooling curve. The transformation of an alloy from a liquid state to a solid state occurs within a certain temperature range and can also be represented by a cooling curve. However, the crystallization process of an alloy system requires the use of phase diagrams to show clearly.

The alloy phase diagram is a graph showing the relationship among the composition, temperature and structure of the alloy and is an important tool for studying the alloy. Alloy phase diagrams are generally obtained by test methods. The Pb-Sb binary alloy is taken as an example to illustrate the measurement process of the binary alloy phase diagram.

(1) Prepare several alloys of different compositions and make their cooling curves separately.

(2) Find the crystallization transition temperatures of the respective alloys, ie, the crystallization starting temperature and the crystallization ending temperature.

(3) The crystallization starting temperature and the crystallization ending temperature of each alloy are respectively indicated in the temperature-component coordinate system.

(4) The crystallization starting point of each alloy is connected, that is, the liquidus; and the end of the crystallization temperature is connected, that is, the solidus. This constitutes a Pb-Sb binary alloy phase diagram. Other alloy phase diagrams can also be determined by this method.

The Pb-Sb alloy constitutes a binary eutectic phase diagram. Other alloys can also constitute binary eutectic phase diagrams, binary phase diagrams that form stable compounds, and binary phase diagrams.

â—† Iron Carbon Alloy

Iron-carbon alloy is a general name of steel and iron, and is a widely used alloy in industry. Iron-carbon alloys are iron-based alloys composed mainly of carbon. In the liquid state, iron and carbon can be infinitely soluble. In the solid state, carbon dissolves in iron to form a solid solution. When the carbon content exceeds the solid solubility of carbon in iron, metal compounds appear. In addition, a mechanical mixture consisting of a solid solution and a metal compound can also be formed.

The following sections describe several basic structures of iron-carbon alloys that appear in the solid state.

● Ferrite

Ferrite is an interstitial solid solution in which carbon is dissolved in a-Fe, and is often represented by symbol F. It still maintains a body-centered cubic lattice, which has a very small carbon-solubilizing capacity. It can only dissolve at 0.0008% carbon at room temperature, and has a large carbon-dissolving power of 0.02% at 727°C.

Because the ferrite carbon content is very low, its performance is similar to that of pure iron, and its plasticity and toughness are very good. The elongation δ=45%-50%. The strength and hardness are lower, σb≈250MPa, and HBS=80.

Austenitic

Austenite is an interstitial solid solution in which carbon dissolves in γ-Fe, and is often denoted by symbol A. It still maintains a face-centered cubic lattice of γ-Fe. Its carbon-dissolving capacity is relatively large. At 727°C, the dissolved carbon is ωc=0.77%, and the soluble carbon at 2.148°C is 2.11%. Austenite is a stable structure that exists at high temperatures above 727°C. Austenitic plasticity is good, and it is the required structure for most steels under pressure processing at high temperatures. Austenite is not magnetic.

● Cementite

Cementite is a metal compound formed of iron and carbon, and its chemical formula is Fe3C. The carbon content of cementite is ωc=6.69% and the melting point is 1227°C. Its lattice is a complex orthorhombic lattice, with a high hardness of HBW=800, plasticity, toughness almost zero, and brittleness.

There are different forms of cementite in iron-carbon alloys, and their quantity, morphology, and distribution have a direct effect on the properties of iron-carbon alloys.

● Pearlite

Pearlite is an austenite-eutectoid body formed by the eutectoid transformation of austenite. Its morphology is a layered complex of alternating ferrite thin layers and thin layers of cementite, also known as pearlite. The symbol P indicates that the carbon content is ωc=0.77%. Its mechanical properties lie between the ferrite and the cementite, which is determined by the pearlite lamellar spacing, which is the average thickness of a layer of ferrite and a layer of cementite.

● Layman body

Laeite is a cocrystal composed of austenite and cementite formed by eutectic transformation of the liquid iron-carbon alloy, and its carbon content is ωc=4.3%. When the temperature is higher than 727°C, the austenite is composed of austenite and cementite and is represented by symbol Ld. At less than 727°C, the leucite is composed of pearlite and cementite and is denoted by the symbol Ld' and is called metamorphic beryllite. Due to the fact that the matrix of the lanoid body is a hard and brittle cementite, the hardness is high and the plasticity is poor.

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