After the metal is melted, the liquid state structure has the following characteristics in a temperature range not higher than the melting point. 1 The atoms still maintain strong binding energy. Therefore, the atoms still have a certain regularity within a small distance, and the average atomic distance does not increase much. Metal solids consist of many grains. Liquids are composed of many atomic groups, which maintain the arrangement of solids within the atomic group. The bonds between atomic groups are different from solids because they are destroyed. This ordered arrangement within only the atomic group is called near-range ordering 2 Because the energy of the atomic heat movement in the liquid is large, each atom has adjacent atoms in the three-dimensional direction and often collides with each other to exchange energy. In the collision, some atoms transmit some of their energy to other atoms, and their own energy is reduced. The result is that the energy of some atoms exceeds the average energy of the atoms every moment, and the energy of some atoms is much smaller than the average energy. This energy inhomogeneity is called energy fluctuation 3 The energy fluctuations that exist in the liquid cause the atoms in each atom group that have greater kinetic energy to overcome the bond between the neighboring atoms, resulting in strong thermal motions in the group and in the proliferation of vacancies. -T>In addition, they can also be separated from the original group and added to other atomic groups or form new atomic groups. As a result, all atomic groups are in an ever-changing state, sometimes growing up and becoming smaller, sometimes disappearing and disappearing from time to time, as if swimming without stopping. The instantaneous change of this structure is called structural ups and downs The above three characteristics determine the basic characteristics of the liquid metal: a fixed volume, a good fluidity, a variety of physical and chemical properties close to the solid state, and away from the gas At present, a lot of research has been done on the properties of liquid metals at home and abroad. Only the private lag, surface phenomena and basic physical properties of liquid metals are described here. First, the private lag of liquid metal The private lag of liquid metal refers to the internal friction inside the fluid when the fluid flows. The private lag affects not only the flow characteristics and filling ability of the liquid metal in the mold, but also It will affect the elimination of gas and slag in the molten metal, and it will also affect the solidification of castings. Private lag has two concepts in fluid mechanics. One is power dynamics and the other is mobility. Dynamic privacy: Newton's law of liquid flow of private lag, can be expressed as: Formula see screenshot 1 where η - viscosity coefficient, also known as dynamic viscosity; S - contact area between layers; F (x) - force acting on the liquid surface; dvX/dy - velocity gradient between layers This expression only reflects the macroscopic external laws and does not reflect the microscopic internal laws. According to Frankel's liquid structure theory, the private lag coefficient can be expressed by the following formula: Screenshot 2 Where: To—the time at which the atom is in equilibrium; k—the Boltzmann constant; T—the thermodynamic temperature; U—the activation energy of an atom's dissociation; S—the average distance of the equilibrium positions of neighboring atoms The above formula shows that the private lag is inversely proportional to δ3 and is proportional to the activation energy U. Both U and δ reflect the strength of interatomic binding forces. Therefore, the nature of viscosity is the magnitude of interatomic binding force. Kinematic viscosity: The ratio of dynamic viscosity to density, that is, η v = - p The above formula shows that when the same degree of motion is the same, the density is high and the degree of movement is small, that is, the tendency of the fluid particle to maintain its own movement direction is large. Engineering uses the degree of privacy that is directly measured by the instrument, that is, relative privacy to evaluate the private lag of liquid metal. It is measured directly against the water's privacy with a specific privacy meter under the specified conditions. According to different measurement conditions, the relative private units adopted by different countries are also different. Our country adopts Ng's degree of privacy. This method is applicable to liquid metals whose low melting point is not easy to oxidize, but most of the metals and alloys are susceptible to oxidation or crystallization due to the high temperature in the liquid state. Therefore, the measurement error is large. Other methods of measurement include overhanging method, hanging ball method, pendulum method, etc. The factors affecting the privacy of liquid metal include: 1 Influence of physical properties of metal itself The physical properties of the two metals determine the size of the liquid metal, one of which is the atomic volume as a geometric factor, and the other is the absolute value of enthalpy as an energy factor. 2 temperature. When the temperature is not too high, T rises and η decreases. When the temperature is high, T rises and η rises, as shown in Figure 2-1. 3 chemical composition. The degree of freedom generally has a minimum at eutectic points, such as in Al-Si alloys. In the Fe-C alloy, when the temperature is constant, the degree of privacy decreases as the carbon content increases, as shown in Figure 2-2. 4 impurities. The quantity, status, and distribution of impurities can affect the degree of freedom of liquid metal to varying degrees. Solid impurities often increase their privacy. However, some impurities with a low melting point are in a molten state in the liquid metal, which in turn will reduce the liquid metal's privateness. Under normal circumstances, the flow of liquid metal in the casting and cavity is turbulent. It is laminar flow only in the thin part of the cavity or in the late stage of filling. The flow resistance is more affected by the degree of privacy than the turbulence in laminar flow. Big time. The greater the degree of sportiness, the smaller the convection intensity. Recent studies have shown that the macrosegregation of castings is mainly affected by convection. The greater the degree of privacy, the smaller the floating velocity of inclusions and the easier it is to stay in the casting. Fig. 2-1 Fig. 2-2 2. Surface phenomena of liquid metal For the source or dispersion of particles on the interface between liquid and gas, the density of the liquid is greater than that of the gas, so the force of the gas relative to it is much smaller than that of the liquid. Its force causes the surface layer to be in an unbalanced force field. As a result, the particle of the surface layer of the liquid metal is subjected to a force directed to the inside of the liquid, so that the liquid surface tends to shrink automatically. This is equivalent to having a tension on the surface of the liquid metal that is parallel to the surface and equal in size. This tension is the surface tension. The phenomena caused by surface tension are collectively referred to as surface phenomena. The surface tension of the liquid forces the liquid to contract inwards, creating an extra pressure. This extra pressure is called additive pressure. The amount of additional pressure is related to the radius of curvature of the liquid surface. The factors affecting the surface phenomenon are: 1 melting point: high melting point substances, the bonding force between the atoms, and the surface tension is also large. 2 Temperature: For most metals and alloys, the temperature increases and the surface tension decreases. However, for iron-carbon alloys and copper alloys, the surface tension increases with increasing temperature. There is no uniform and reasonable explanation for this anomaly. Surface phenomena generate additional pressure P = 2σ/γ, which is the surface tension, and γ is the radius of curvature, which can increase the resistance to gas evolution in the molten metal and affect the interaction between the molten metal and the mold. Convex liquids (eg, beads) r>0, additional pressure is positive; concave liquids (eg, bubbles in liquid) r<0, additional pressure is negative; plane: r→00, additional pressure is 0. When the additional pressure is positive (wet), the mold surface is smooth, but the ability to infiltrate the mold cavity is poor; when the additional pressure is negative (not wet), the metal fluid can work well in the mold cavity , but it is easy to bond with the mold (sticky sand), hinder the shrinkage, and even produce cracks Thermostatic Bathtub Faucet,Bath Hand Shower Set,Tub and Shower Set,Bath Shower Set kaiping aida sanitary ware technology co.,ltd , https://www.aidafaucet.com
