Thin-wall bearing and the casting gravity forming technique

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Thin section iron castings is casting technology development direction. In the automotive, aviation, aerospace, electronics, and other fields realize castings and, in particular, the thin light alloy castings , has even more significance. The filling is thin section iron castings manufacturing technology key. By centrifugal force, and the electromagnetic force, stress the filling casting method are replacing the traditional gravity casting method. Among them the gravity casting method has the advantages of the most. This paper expounds the basic concepts of the thin section iron castings and characteristics, and then introduced the gravity of the filling process parameter calculation and related technical problems.
The basic concept of thin section iron castings and features
Thin section iron castings is a relative concept. It does not say what the wall thickness casting for how much is defined as thin section iron castings. But relative to the heavy casting, a speaking, thin section iron castings have at least the following features:bearing corp
(1) in the thin section iron casting s pouring process, by the liquid metal surface tension force caused by Laplace plays an important role. Set of liquid metal surface tension for sigma, wall thickness casting for δ, Laplace force, the expression of the P for P = four sigma/δ. A study suggests that the aluminum alloy plate for casting parts, when casting wall thickness about less than 4 mm, Laplace force will be the filling of a great influence on the flow state. For large thin section iron castings, viscous force will also become cannot be ignored. All kinds of alloy critical wall thickness values are shown below. The table of value is based on the liquid metal and type wall is not for the calculation of the premise that wet. In fact, the modelling of different materials, due to the different wetting Angle, critical wall thickness numerical will also change.
(2) the thin section iron castings should have the meaning of precision castings. Because for just a few mm thickness thin section iron castings, may not set big machining allowance or process allowance. So thin section iron castings should be precision castings, its size accuracy (with tolerance grade said) should be below the CT6 (HB6103-86).
(3) heat transfer in the filling process factors play an important role. When the filling process flow and heat transfer process influence each other. The temperature field and the shrinkage of castings, loose, owe cast, cold septum, oxidation casting defects such as the formation of the inclusion of fluid dynamics factors and heat transfer determined factors.
3 the gravity of the filling process parameters
The gravity of the filling characteristics are the liquid metal in the pressure difference under the effect of gravity and other surface force and filling cavity. The filling anti-gravity main parameters the filling pressure, pressure speed (the filling speed), pouring temperature and mould temperature. The four parameters can be divided into two kin ds, one kind is thermal parameters, such as mould temperature and pouring temperature; Another kind is fluid dynamics parameters, such as the filling pressure and the filling speed. The filling pressure depends mainly on the structure of the castings, appearance, wall thickness and cavity exhaust ability, the physical properties of liquid alloy, and so on, and the filling speed, and thermal parameters have close relationship. If ZhunShu Bi is far less than 0.1 (thin section iron castings generally satisfy the conditions), may think flow section temperature gradient is zero. In this case, usable lumped parameter, we educe a micro yuan casting section office of body heat balance equation;
α (T prearcing) ds. Dt =-dv.. с ρ. Dt (1)
Type of α-heat transfer coefficient;
T -the body temperature; $
-prearcing mould temperature;
S-the yuan body and casting contact area;
V-the yuan the volume of the body;
T-time;
ρ-the den sity of liquid alloy;
с-the density of liquid alloy and specific heat.
Will type (1) the integral variable and used in casting respectively in the adjacent first I, (I 1) and (I 2) part, available:
T I = Ti (T I 1-Ti) exp (Ω I) (2 a)
T I 1 = Ti 1 (T I 2-Ti 1) exp (Ω I 1) (2 b)
T I 2 = Ti 2 (T I 3-Ti 2) exp (Ω I 2) (2 c)
Make T n 1 for the liquidus temperature alloy T l, T 1 for alloy pouring temperature T p, utilizing the above the recursive relations, available:
Type, Ω I = α hi / (Ri ρ с ω I)
Hi-cavity in part I height;ball roller bearing
Ri-casting part I of the module.
ω I-liquid through the first part I of flow velocity;
If all the parts of the mould has the same temperature, or Ti = prearcing, then can launch:
Type, Δ P-the filling pressure, it is used to ba lance the gravity, Laplace force and hydraulics resistance; DP (t) / dt for compression speed.
Type (4) which is considered the heat transfer of the filling process parameters calculation formula. This type of the meaning of a shape of the complex will be casting into volume V, surface area for S simple casting compute, thus facilitate applied in practical production. Although the horn of mathematics reading from it. Equation (4) can have an array solution, but parameters have their own value range. For silicon aluminium alloy of the gravity casting is concerned, 680 ~ 730 ℃ is a better the filling temperature range. Too high, easy cause oxidation inspiratory, the burning of the elements and shrinkage cavity loose; Low alloy is lower the liquidity, easy cause casting outline is not clear and produce flow mark. Mould temperature is too high, make solidification speed is reduced, casting organization bulky, air tightness poor. Metal type of thin-walled casting mould tem perature most sensitive. Studies show that, for metallic thin section iron castings, should its preheat to near heat balance be pouring temperature, at the same time, also should maintain a certain amount of pouring frequency, make casting is always in the thermal equilibrium state. High pressure will be increased speed speed amplitude, casting a sharp change in cross section when can cause even jet, the filling speed against gravity generally should not be more than 1 m/s.
In aircraft engine a aluminum alloy castings for example. Casting size outline for 230 mm x 195 mm × 55 mm, weight 700 g. Using revestment shell mould castings method. The alloy component as: Si: 7%, Mg: 0.3%, Ti: 0.2%, the others for Al. First of all, according to part 3 d CAD model and the relevant procedures calculated casting module (R = 2.16), and determine the size of each unit gating system, and then, according to the given pouring temperature (700 ℃) and mould temperature (250 ℃), a ccording to the type (4) calculated the compression speed dP (t) / dt = 2000 Pa/s.
4 last word
This paper puts forward the concept of thin section iron castings and expounds the basic characteristics. Think, Laplace force is the filling of the main influence factors fluid dynamics. Bi number is far less than 1 in the case, can use lumped parameter, the filling equation is derived and the filling process parameters are calculated. Because the gravity casting with the filling speed adjustable characteristics, therefore, in determining the process parameters, should first make sure pouring temperature and mould temperature, then, accurate adjustment for the filling speed (by compression speed sure). The aluminum alloy of the production practice show that the proposed method is more successful.bearing roller bearings