Internal casting defects of die casting (part 1)

1 Casting porosity

(1) Features and inspection methods:

After dissection, appearance inspection or flaw detection showed that the pores had smooth surface and round shape.

(2) Causes:

① The introduction direction of the alloy liquid is unreasonable or the flow speed of the metal liquid is too high, resulting in jet; ② the exhaust channel is blocked too early or the front side impacts the mold wall to form a vortex to enclose the air, which is mostly produced in the poor exhaust or deep cavity; ③ because the furnace charge is not clean or the smelting temperature is too high, so that more gas in the metal liquid is not removed, which is precipitated during solidification, and can not be fully discharged; ④ coating The gas volume is large or used too much, and it is not poured completely before pouring, which makes the gas involved in the casting. This kind of gas is mostly dark gray surface; the high-speed switching point is wrong.

(3) Treatment method:

① Adopt clean furnace charge, control smelting temperature and exhaust treatment; ② select reasonable process parameters, injection speed and high-speed switching point; ③ guide the pressure balance of liquid metal and fill the cavity in order to facilitate gas discharge; ④ exhaust tank and overflow tank shall have sufficient exhaust capacity; ⑤ select coating with small gas output and control exhaust volume.

2 Shrinkage cavity and porosity

(1) Features and inspection methods:

① Through dissection or flaw detection, the shape of the hole is irregular, unsmooth and the surface is dark; ② the large and concentrated hole is small and scattered hole is loose.

(2) Causes:

① During the solidification process of castings, there are cavities caused by shrinkage without metal compensation; ② the pouring temperature is too high and the distribution of mold temperature gradient is unreasonable; ③ the injection ratio is too low and the pressurization pressure is too low; ④ the inner gate is thin, the area is too small and the solidification is too early, which is not conducive to pressure transmission and metal liquid feeding; ⑤ there are hot spots or section changes sharply on the casting structure; ⑥ the metal liquid pouring The injection quantity is too small and the residual material is too thin to play the role of feeding.

(3) Treatment method:

① Lower pouring temperature and reduce shrinkage; ② increase injection specific pressure and boost pressure to improve compactness; ③ modify the inner gate to make the pressure transfer better, which is conducive to the feeding effect of liquid metal; ④ change the casting structure, eliminate the metal accumulation part, and make the wall thickness as uniform as possible; ⑤ speed up the cooling of thick parts; ⑥ thicken the material handle 15-30mm to increase the feeding effect.

3 Casting inclusions

(1) Features and inspection methods:

The metal or non-metal impurities mixed into the die casting can be seen with irregular state, different size, color and height points or holes after processing.

(2) Causes:

① The furnace charge is unclean and too much material is returned to the furnace; ② the alloy liquid is not refined; ③ the ladle is used to take the liquid and pour it into the slag; ④ the graphite crucible or coating contains graphite falling off and mixed into the metal liquid; ⑤ the temperature is high and the duration is long during the heat preservation.

(3) Treatment method:

① Clean the alloy materials, especially the dirt on the returned materials; ② refine and degas the alloy molten liquid to clean the slag; ③ when pouring with the ladle, carefully remove the liquid level to avoid mixing the slag and oxide skin; ④ clean the mold cavity and pressure chamber; ⑤ control the temperature of heat preservation and reduce the time of heat preservation.

4. Casting brittleness

(1) Features and inspection methods:

The grain size of the casting base metal is too large or very small, which makes the casting easy to break or break.

(2) Causes:

① The impurities of zinc, iron, lead and tin in aluminum alloy exceed the specified range; ② the alloy liquid is overheated or the holding time is too long, resulting in coarse grains; ③ the severe supercooling, resulting in fine grains.

(3) Treatment method:

① Strictly control the impurities in the metal; ② control the smelting process to reduce the pouring temperature; ③ increase the mold temperature.

5 Casting leakage

(1) Features and inspection methods:

The pressure test of the die casting has resulted in air leakage and water seepage.

(2) Causes:

① Secondly, internal defects such as porosity, shrinkage cavity, slag hole, crack, shrinkage porosity, cold insulation and pattern;

③ The design of pouring and exhaust system is not good; ④ die casting punch is worn and injection is not stable.

(3) Treatment method:

① Improve the specific pressure; ② take corresponding measures for the internal defects; ③ improve the pouring system and exhaust system; ④ carry out infiltration treatment to make up for the defects; ⑤ replace the pressure chamber and punch.

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