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The reasons for the porosity of the aluminum piston and the defects of the shrinkage cavity are not to be seen.

Abstract: Aiming at the common air volume in the production process of cast aluminum piston, the shrinkage defect types were analyzed briefly, reveals the main causes of the defects and solutions. precision sheet metal.

For the piston production enterprise, casting rejection rate is one of the key factors restricting the production efficiency and cost of the piston, the piston for some special structure, the proportion of casting scrap sometimes as high as 40% ~ 50%. in view of this, targeted to take some measures to improve, reduce the piston casting scrap, has been working piston manufacturing enterprises one of the key. precision sheet metal.

The 1. main types of casting defects

The production enterprises in the aluminum piston blank, using gravity casting forming. precision sheet metal. The aluminum liquid in the filling process due to the influence of some factors, the final casting blank will often appear all sorts of macroscopic or microscopic defects. After the statistics, there are two kinds of casting defects in the highest proportion, its value even accounted for comprehensive the rejection rate of 50%, the two kinds of common casting defects is porosity, shrinkage porosity (shrinkage), the simple introduction of the two kinds of defects.

(1) stomatal is a kind of the most common casting defects of aluminum piston, the appearance of mainly two forms: one is located in the inner and outer surfaces of castings of different sizes of spherical bubble, alone or together, the hole wall is smooth; the other one is dispersed in the castings, the bee nest small pinhole there, the pinhole is generally less than 1mm, mostly concentrated in the thickness of the casting section or part of the slow cooling rate, pinhole around finishing. precision sheet metal.

Mainly produced under the blowhole pin hole, and below the top of the casting ring, this is because the core pin and ring prevent gas float, and the top hole is because gas float to the top of the casting, the gas can not be discharged smoothly from the top, especially in the middle of the casting riser is more prone to the defects.

(2) the shrinkage and shrinkage of piston is another common casting defects form, appearance of surface of casting microstructure is not close, there are many small holes in the casting section. This defect exists mainly in the piston wall thickness at or near the junction of the ingate. Shrinkage holes caused by the irregular geometry of pine the surface is rough, especially internal hole, shrinkage of the section is very loose, coarse grain, spongy. precision sheet metal.

Analysis of 2. causes and ways of improvement

(1) air from the causes are divided into two types: precipitate pores and blowhole. precision sheet metal. Precipitate pores mainly refers to the refining of aluminium liquid is not complete or re pouring time inhale, excess gas during solidification (mainly hydrogen) separated from the liquid aluminum can not be successfully discharged after casting porosity caused by the blowhole is mainly due to the unreasonable structure of the gating system, the liquid aluminum involved in large amounts of gas in the filling process, solidification is not smooth discharge cavity caused.

To precipitate pores, first we should pay attention to raw material storage environment. v. Due to various metal and non-metal materials, will be prepared by casting Aluminum Alloy such as silicon, copper, magnesium, modifier, slag cleaning agent and refining agent, the material storage environment if the humidity is too high, the moisture would increase after melting the refining process of hydrogen content exceed the standard risk, when pouring excess hydrogen, if not timely discharge will appear in the metal hole parts. So the raw material storage environment must be cool, dry, according to past experience data in piston industry, metal material storage humidity should be less than 80%, non metal storage humidity the material is generally not more than 60%.

Key in the process of refining is caused by pores. The purpose is to purify the refining Aluminum Alloy melting after the completion of the removal of hydrogen and oxidation in liquid aluminum inclusions, if the refining effect is poor, easily caused by liquid aluminum finally higher hydrogen content turn, pouring waste. To vent piston enterprises different, because refining process is not the same, this is no longer to elaborate on the specific parameters of the refining. precision sheet metal. But no matter how refined, finally takes as a quantitative index to measure the hydrogen content in aluminum liquid refinement, our company production proved long time, that after the completion of the hydrogen content in aluminum refining control within 0.15mL/100g is preferred.

The refined aluminium liquid qualified into pouring process immediately after casting, not for a long time. Because of the high temperature molten aluminum contact with air will produce hydrogen and hydrogen and water vapor in the air reacts, will continue to produce hydrogen dissolved in liquid aluminum at high temperature, so the casting production process is the crucible furnace the aluminum liquid suction process. At the same time as the continuous 19 times of hydrogen solubility in high temperature aluminum liquid is about casting, if the aluminum liquid stored for a long time will cause multi variable hydrogen content in liquid aluminum, hydrogen can release the aluminum liquid solidification in the cavity when increased. When the hydrogen content in aluminum liquid reaches a certain a limit state after casting precipitation of hydrogen will not be completely discharged, the remaining hydrogen forming pores in the casting surface will be placed inside and outside. precision sheet metal. Therefore, the time not too long refining qualified aluminum liquid in the air, generally less than 4H when the aluminum liquid put is appropriate. After the time is over 4h, the refining process must be retreated.

The blowhole, solutions are to increase exhaust function of metal type, vent groove and vent plug, tilt casting, improving pouring system size and proportion, to set up reasonable position. In the pouring riser before the start, we tend to metal type inclined at an angle alpha, to a certain time after metal casting at the same time for gradually reduction; sprue first contact with the aluminum liquid, we will set a vertical gradient of beta at design time, generally not less than 5 degrees. These two kinds of technological gradient has two main purposes: one is to reduce the vertical drop casting aluminum liquid, prevent the flow process of molten aluminum roll roll gas, ensure the aluminum liquid gently into the runner; two is suspended bubble tilt pouring can inlay ring below the erosion, the floating ring and the metal insert through the void type to the top. The runner into metal cavity The stage size should be gradually narrowed, reaches the minimum in the junction of the runner and the blank, so the aluminum liquid into the cavity can maintain a steady flow, and can prevent the air entrapment in the liquid aluminum runner. The aluminum liquid into the cavity, in order to make the cavity can be discharged smoothly in the filling process with the gas in liquid aluminum, usually with the exhaust slot in the sprue type on the side face and the pin core and the outer mold surface provided with a depth of no more than 0.2mm. When the exhaust rises to the top limit position of aluminum liquid in the cavity, in order to make the gas can be discharged smoothly metal type, general the top die is provided with vent plug and open top riser. Top riser should be opened in the top of the casting most thick parts, not only because the thick part of the final cooling, most exhaust gas volume, prone to stomatal defects. precision sheet metal.

( 2 ) The shrinkage and shrinkage of the shrinkage cavity are mainly caused by the different solidification speed of the casting due to the difference of the wall thickness during the solidification process of the casting , and the defect generation part is concentrated and regular . However , because of the casting structure , the solidification method of the alloy is usually solidified and solidified in sequence . Therefore , the problem is solved . It is necessary to strengthen the cooling of overheated parts . For manual casting of metal type , it is necessary to add a certain thickness of cold iron or inlaid metal material such as pure copper to the large part of the casting . In the same condition , the thermal conductivity of pure copper is about 8 times that of cast iron and steel . For mechanical casting metal type , the order of cooling of pure copper is about 8 times of that of cast iron and steel . In addition , in order to accelerate the cooling effect of the overheated part , a V – shaped groove with a depth of 1 – 1.5 mm is usually formed in the overheated part , so as to strengthen the heat radiation effect at the overheating of the casting . in addition to strengthening the cooling on the overheated part , it is also possible to enlarge or open a certain feeding riser to the overheated part , so as to strengthen the repairing effect of the part .

3 . Conclusion Based on the basic principle of piston casting , it is difficult to find out the main cause of the casting defects , so as to reduce the casting scrap rate . In the case of piston enterprises , it is very important to analyze the casting defects deeply and to reduce the casting scrap rate .

References :

[1] He Yongdong, Zhang Xinming. Mechanism of hydrogen formation in electrolytic aluminum liquid [J]. Journal of Central South University, 2007,12 (6):1055-1062.

[2] Liu Daochun. A brief analysis of the casting defects of aluminum alloy and the prevention of [J]. die engineering, 2013 (9):82-87.

[3] Liu Fumei. Casting aluminum piston shrinkage and shrinkage production and prevention and control of [J]. internal combustion engine parts, 1999 (5):19-23.

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