Discussion on the feeding angle of metal gravity castings
Abstract: for the volume shrinkage of metal (alloy) liquid after pouring into the mold cavity, if appropriate measures are not taken, the castings designed by ourselves will produce porosity and shrinkage. In this paper, a theory called feeding angle is derived, which can effectively solve the above problems, so that the castings designed by ourselves can obtain castings with dense internal structure
key words: loose castings; Shrinkage cavity; Feeding angle
since the origin of metal casting, people in gravity casting, both at home and abroad, to a large extent, use molten metal, and then pour it into the pre cast mold, with the help of the self weight of the metal, feeding and solidification, so as to obtain the designed castings. Whether ferrous metals or non-ferrous metals, people generally still use the above methods to produce castings. Later, due to the hard exploration of casting work, many casting theories, viewpoints and methods were covered. Now the most popular casting theories are "sequential solidification", "simultaneous solidification", as well as "directional" and "diffusion solidification". Therefore, almost all foundry workers also use these theories to guide teaching, design and produce their own castings. Unfortunately, in many cases, although we seem to apply some of the above theories as guidance, in fact, the castings produced by ourselves have porosity, shrinkage or unsatisfactory internal structure compactness. Therefore, there is a problem here, that is, how to make the castings designed by yourself obtain the ideal castings with dense internal structure after casting, that is, how to appropriately make the metal castings obtain the ideal feeding: that is, to reveal the feeding essence of the castings, so as to open up the feeding channel, and finally obtain the castings with dense internal structure, which is a kind of call introduced in this paper“ θ "Theory of feeding angle.
" θ "What's the matter with the feeding angle? Now in casting books or classes, people still retain the above-mentioned casting solidification theories, but the author believes that a theory called" simultaneous solidification "is sometimes misunderstood, that is, because this misunderstanding will lead to some people's mistakes in work. We can say that before and now, we can't create this condition that the casting will solidify as a whole after pouring into the mold cavity" at the same time ". some of us, seeing that the wall thickness of a casting is uniform, mistakenly believe that when it is put into casting production, it will solidify as a whole at the same time, thus ignoring the consideration of feeding the casting. Therefore, under the guidance of this misunderstanding, it can be said for sure that many castings produced are not dense in internal organization, so now we should fully understand that simultaneous solidification is relative, that is, the whole casting After pouring (whether pure metal or alloy), it is absolutely impossible for the whole casting to solidify at the same time in an instant. For the whole casting, from the perspective of instant, it must follow the principle of starting from the surface of the casting with large temperature difference to the interior of the casting with small temperature difference and the pouring exit, and it will not end until the end of condensation, which is the solidification essence of metal castings receiving feeding in sequence
many workers have done a lot of work on the theory of sequential solidification, but so far, how to make the castings designed by ourselves achieve the ideal sequential solidification and successfully meet the sequential feeding and solidification of castings with the help of it. In order to achieve the goal of good internal quality of castings, there is no ideal calculation and control method at present. The gravity casting provided in this paper adopts a method called "gravity casting"? quot; θ "The feeding angle method reveals and can better meet and improve its purpose of sequential feeding and solidification. This method can be said to be an effective method to maximize and appropriately arrange the shrinkage cavity and porosity of castings caused by insufficient feeding amount.
let's discuss the problems related to gravity castings." θ "Feeding angle.
we know that after the molten metal is poured into the mold, it begins to appear at the moment of condensation:
α V × △ f=k... (1) (in order to analyze the feeding angle, we convert volume contraction into surface contraction)
α V metal body shrinkage (%)
△ f casting sectional area (mm, while ordinary plastic profit level is 1% ⑶%2)
k casting sectional area shrinkage mm2 (supplementary shrinkage)
and with the extension of time, they are rapidly doing α V × △ f is not finished until the casting is completely solidified
next, we can assume that the volume shrinkage of tin bismuth alloy is α If V is close to zero, then: multiply zero by △ F, and then the reduction value is close to zero, so Sn Bi alloy does not need to consider the shrinkage
about the shrinkage of metal castings, i.e α Determination of V
localization of decoration materials in North Korea: perforated plastic tiles know that after pouring molten metal, liquid shrinkage, solidification shrinkage and solid shrinkage will occur, which will bring porosity and shrinkage defects to castings. In fact, it occurs during liquid and solidification shrinkage, that is, the so-called volume shrinkage, which is mentioned in this article α V 。 For example, the volume shrinkage of carbon steel (with carbon content of 0..0%) α V is 10.5% - 14%. Press to analyze. We also assume that there is a carbon steel part with a carbon content of 0.4%, and its volume shrinkage is α V=11.3%, take a cross-sectional area △ f=25mm2 and substitute it into formula (1):
this shows that the metal shrinkage required for this section is 3.5 mm (2). This proves that this formula is of practical significance. According to this formula, we can further deduce that if there is a cylinder, as shown in figure (I), a feeding riser is set from the low level to the top of the gate, and the top heat is high. Therefore, during the condensation process of the casting, it solidifies from the top of the gate from bottom to top, and the casting receives feeding from bottom to top, In other words, the existing thermosetting composites (epoxy or polyester) are difficult to recover and transport K (shrinkage supplement) from top to bottom at the end of their service life. Since each section of the casting needs its own shrinkage supplement K from bottom to top, and at the same time, it is necessary to force the casting to change from a cylinder to a truncated cone, as shown in figure (II), in order to create a way to meet the above sequential shrinkage supplement requirements, which forms R1 in figure (II), And then it came into being“ θ "Angle. This can definitely have the advantage of high service life. According to the advantages, all kinds of metals (alloys) only need α If the V value is different (although the shape and size of the parts are the same), the angle will be reduced“ θ "The value is different. In order to confirm the above theory, we also derive" θ "For the calculation formula of angle value, we integrate the shrinkage supplement required for all sections of the whole part in figure (I):
in order to meet the shrinkage supplement requirements of the casting, its shape is shown in figure (II). It is a truncated cone. According to the volume formula of the truncated cone:
to calculate R1 to obtain the shrinkage supplement angle." θ "Value.
example 1): take 0.1% carbon steel shrinkage α V＝10.5% ?
calculate the feeding angle of a casting with a diameter equal to 50mm and a height of 50mm“ θ "Value?
then the feeding angle" θ "Equal to
example 2) next, we use the above formula to calculate the feeding angle of the hollow cylinder
see figure (III)" θ "Value.
in figure (III), h=50mm, r=26 r small =20
carbon steel with carbon content of 0.1% α V = 10.5%
calculate the feeding angle“ θ "Value
calculate its sectional area first
the above formula is not only applicable to metal (alloy) gravity casting, but also low-pressure castings. As long as the sectional area of various shapes of castings is calculated first, then the R1 value is calculated and substituted into formula (3), and the corresponding H value is found, it is not difficult to calculate the feeding angle. (end)
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