DJM Technical Introduction

    The casting shrinkage hole and shrinkage porosity formation during the casting process of metal,

    The casting shrinkage hole and shrinkage porosity formation during the casting process of metal, 
    prone to shrinkage, shrinkage and how to identify? 
    How to distinguish shrinkage? 
    What casting alloy is easy to produce the shrinkage and solidification process of castings?

    If there is no reasonable control, easy casting shrinkage hole, shrinkage a loose 1 casting solidification solidification mode: casting solidification process, the cross section is generally divided into three regions: 1 - 2 - 3 solid - liquid phase solidification zone on the solidification zone of influence is the width of the coagulation zone, so divided layer: the solidification mode of pure metal solidification, eutectic alloy no solidification zone during solidification of liquid solid two phase section, by a well-defined separate solid layer decreased with the temperature increasing, the liquid layer decreases, direct crystallization center mushy solidification temperature of alloy solidification in a wide range Within a certain period of time, there is no solid casting surface layer, solidification zone throughout the entire section, first paste after curing. The solidification between solidification layer by layer between most alloy solidification and paste solidification temperature range of crystallization of 2 factors affecting the alloy casting solidification mode of the small range of solidification zone is narrow, the more inclined to layered solidification. Such as: sand casting, low carbon steel layer by layer solidification, high carbon paste solidification 2) crystallization temperature gradient alloy castings at a certain temperature range, the temperature gradient solidification zone width depending on the inner and outer layer. The temperature gradient is small, the wider the solidification zone. (inside and outside temperature, fast cooling, narrow solidification zone) two alloy the contraction of liquid alloy from the process of pouring temperature to cooling to room temperature, the volume and size decrease. Many cast defects (shrinkage, shrinkage, crack, deformation, residual stress) The basic reason of the 1 stages. The liquid shrinkage Shrinkage from molten metal is poured into the mold to start before solidification. The temperature difference of the coagulation temperature is proportional to the.2 liquid contraction to reduce the volume and quality of casting temperature start) solidification shrinkage: from beginning to end. Solidification of the same kind of alloy solidification, solidification temperature range is greater. Solidification shrinkage rate is high. Such as: 35 steel, the volume shrinkage rate of 3%, 45 4.3%3): steel solid shrinkage after solidification to room temperature. Solid shrinkage casting size, the linear shrinkage. 2 factors affecting the shrinkage of 1) chemical composition: Iron promotes graphite formation elements increase, reduce shrinkage. Such as: gray C, Si = s, close down, up close up. Due to large volume of graphite, volume expansion, partially offset by solidification shrinkage.2) pouring temperature: temperature = = 3) liquid contraction casting structure and casting conditions of casting in casting shrinkage by casting and type The core obstacle. The actual shrinkage is smaller than the free shrinkage. Have good deformability dry mold. 3 shrinkage formation in the casting and finally solidified place some of the holes, centralization and shrinkage. The pure metal eutectic composition, easy to produce shrinkage. The basic reasons for the shrinkage of castings in the solidification period, liquid and the metal solidification shrinkage and shrinkage is far greater than the solid. 4 factors affecting the shrinkage of volume (add) liquid contraction, solidification shrinkage increase shrinkage volume increase; 2) during solidification, solid shrinkage shrinkage decrease, pore volume decreases; 3) lower casting speed shrinkage volume down; 4) increase casting speed of liquid up 5 easy generation shrinkage shrinkage porosity formation due to shrinkage of casting solidification area finally failed to get up, or, since the alloy mushy solidification, small liquid area is dendritic crystal separated feeding is difficult to get to. 1) macro shrinkage Loose visible, often appear in the hole near the center. The castings or non eutectic composition, the crystallization range is wide, it is easier to form shrinkage 2) micro porosity during solidification, solidification - small holes area formed between grains, forming the first dendrite to molten metal is divided into many small isolated part, condensation contraction, formation of intergranular micro holes. The wider the solidification zone, it is easier to form micro porosity, little harm to the cast, so as not to defect, but the air tightness and mechanical performance requirements of high casting, we must try to reduce the solidification shrinkage. (before than after the solidification of the small because, after the solidification of the liquid, coagulation, solid three contraction, the first coagulation flocculation, solid two contraction zone ---- this is also the formation of micro shrinkage basic reasons. Loose and shrinkage formation) 6 basic reasons for similar shrinkage, shrinkage and prevent the basic principle: to develop the rational process of feeding, shrinkage Loose into shrinkage. Sequential solidification: riser feeding and solidification: Cold - iron thickness. Reducing the thermal stress, but the porositydefect, it is used to shrink the alloy. Placement of riser, the implementation of the order of solidification, can prevent shrinkage effectively, but the riser waste metal, waste time, casting the cost increase. Moreover, increase the casting stress, easy to deform and crack. Acupuncture is mainly used for solidification shrinkage, crystallization interval small alloy. Non eutectic alloy, the first crystalline dendrites, hinder the metal flow, very small. For the crystalline alloy riser temperature range is wide, the tendency to paste solidification. Crystallization began, developed dendritic shaped bone covered the entire section of the riser of the road is blocked, so hard to avoid the microporosity. Obviously, alloy casting with near eutectic composition and crystallization of narrow range is appropriate.

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