Die casting is a highly efficient and less-cutting metal forming precision casting method that is developing rapidly in China. Compared with other casting methods, it is not only green and environmentally friendly, but also has advantages of short production process, simple process, high yield rate of die casting process, high precision, low surface roughness and less machining. It is widely used in production of spare parts in various industries. However, due to characteristics of "high speed and high pressure", die casting will inevitably lead to casting defects such as porosity and shrinkage. It is of great significance to study how to control porosity, occurrence position and number in casting. This article takes a model aluminum alloy left box of our company as an example to analyze and solve blowhole defects in bearing mounting hole position.

Certain type of aluminum alloy left box is an important part of our company. External dimensions are 423*298*246mm, maximum wall thickness is 21mm, minimum wall thickness is 4.0mm, and product weight is 6.43kg. Wall thickness of bearing mounting hole location (see Figure 1) is thick, which is very different from surrounding wall thickness, and is ring-shaped. When aluminum liquid is filled, gas entrapment and turbulence are easy to occur, resulting in a large number of air holes and shrinkage holes in thick part. One of quality requirements of product is that box is placed in sink, it is clamped with an air pressure device to inflate box, so that inside of box reaches 0.1-0.2Mpa. After inflation is completed, pressure is maintained for 10s and box has no leakage. Left box of our company requires YL113 GB / T15114-2009 as material.

 

Left box uses high-pressure casting,"high speed and high pressure" of die casting process makes aluminum liquid prone to air entrapment and turbulence when filling cavity, so that there are often air holes in die casting parts. In small batch trial production of aluminum alloy left box, through X-ray non-destructive testing, it was found that there are a large number of air hole defects inside location of bearing mounting hole, number and distribution can be seen (see Figure 2). Blowhole defect cannot be observed by naked eye inside die casting parts, and is exposed after machining (see Figure 3), which makes casting pass rate lower.

 

 

Main reasons for defects are: in first aspect, due to thick wall thickness of bearing mounting hole of casting, wall thickness varies greatly, shape of ring is difficulty of process control; second aspect, due to unreasonable design of mold gating system and slag collection, inlet direction and distribution of designed gate affect exhaust effect at the location of bearing mounting hole. When aluminum water is filled, slag package at this place is filled first (see Figure 4), so that slag bag designed at this position cannot play a good role in collecting slag and gas; third aspect, in-gate speed is too high, turbulent flow is generated, fast position is not set properly, injection temperature is high, and specific pressure is small, etc., which easily leads to generation of pores.

 

Since castings have their own characteristics, strong practicality and experience, entrance direction of aluminum liquid must ensure that liquid flow is filled along mold wall as far as possible, which is beneficial to exhaust. Distribution method of runner needs to consider which distribution can be filled more orderly and not hold back.. Therefore, left box mold was modified: an additional runner was added to fixed mold gating system of left box mold (see Figure 5), which increased cross-sectional area of inner runner and reduced filling speed of inner gate. Aluminum liquid can be filled along mold wall to increase amount of aluminum liquid filling position of casting bearing mounting hole.

 

Figure 5 Newly added sprue after modification of fixed mold

 

By modifying slag pack at mounting hole position of left box dynamic die bearing, slag pack volume is increased (see Figure 6), so that slag pack can accommodate gas wrapped by aluminum liquid when filling bearing mounting hole position, thereby reducing occurrence of air holes at this position.

Through computer mold flow analysis, it was found that after modifying mold gating system and slag collection bag, aluminum liquid first filled annular hole of left box movable mold bearing installation (see Figure 7), then filled slag collection bag, and discharged gas to slag collection bag. At the same time, because slag arrives at the place after aluminum water arrives, solidification is slower than bearing installation around annular hole, which is conducive to gas and slag collection.

 

Figure 7 Mold flow analysis results after mold repair

For gating system of this product, in order to prevent machined surface air holes at the far end of gate, slow and fast transition points are adjusted to reduce gating temperature and increase boost pressure for control. It has been proved by practice that second fast position is set at 485mm compared with original second fast position 470mm, which improves fullness of pressure chamber, soup spoon is repaired to stabilize amount of soup. Flow rate of pressurization is increased from 60% to 80%, which meets effect of pressurization and shrinkage after rapid completion. Molten aluminum gating temperature is adjusted from 650 ° C to 630 ° C, and pore defect is effectively controlled.

(1) Modify gating system, increase cross-sectional area of inner runner, and modify size of slag package to effectively solve problem of stomatal defects in production process.
(2) Take appropriate measures to adjust slow and fast transition points, increase boost pressure, reduce gating temperature and other measures can effectively solve occurrence of pores in similar products.
(3) Effectively control internal pores of castings during production process, improve product quality, and save production costs, which has important reference significance for production practice.