Causes of gravity casting defects of aluminum alloy cylinder head

The causes of casting defects are all from the poor casting process design and structural design of the mold; The manufacturing accuracy of the mold is poor; The technological equipment in the casting process does not match; The implementation of process discipline and process specification in the casting process is not in place, etc.:

1. Causes of “gap” casting defects

Causes of “gap” casting defects: the casting defects are “cold shut” + “air gap” composite casting defects, and the structural analysis is insufficient: before the development and design of the mold, there is no detailed work on “casting process analysis of product structure”. The product wall thickness is the “fire path” in the mold filling process. It is necessary to “process correct” the casting wall thickness according to the weight of molten aluminum, the mold filling flow path of molten aluminum, the heat loss in the mold filling process and other factors.

·Low pouring temperature: the casting workshop is used to pouring the pouring temperature of small castings (the pouring weight is generally about 15kg). For the castings with the pouring weight of 29kg, if the pouring temperature of 720 ℃ is used without adjusting the “mold temperature” and “pouring speed”, due to the large temperature loss of molten aluminum in the filling process, The front edge of multi strand aluminum water confluence is nearly semi-solid, and a “cold shut” casting defect will be formed at the confluence.

Low mold temperature: in order to ensure the secondary dendrite spacing on the combustion chamber surface of the casting, the cooling mold experience of small castings with fast “pouring beat” is used. Imagine using low mold temperature, and long-term cooling water is used to reduce the mold temperature. However, this cooling mode leads to the aggravation of the temperature loss of mold filling aluminum water, which is not conducive to improving the “cold shut” casting defects, but also seriously affects the feeding effect of the riser (the consequence is that the camshaft hole produces “micro porosity” and even “shrinkage porosity” casting defects).

·Improper design of pouring speed:

In order to reduce the temperature loss of mold filling molten aluminum, the rapid pouring of about 9 seconds is adopted, and the pouring speed is about 3.20kg/s. With such a fast pouring speed, the filling “fire path” of the mold design is not enough. The molten aluminum will not enter the mold cavity completely according to the designed path, and some molten aluminum will diffuse from other parts. Irregular filling mode will produce “eddy current” and “air entrainment”, and the residual air will form “air gap” casting defects.

In order to improve the tendency of “air gap” casting defects, we try to pour slowly for about 15 seconds. The pouring speed is about 1.93kg/s. Slow pouring does reduce the length and width of “gap”, but it still exists, that is, the tendency of air gap decreases and the tendency of cold shut increases due to the increase of temperature loss.

2. Causes of “porosity” casting defects on the machined surface

Causes of “porosity” casting defects on machining surface: the casting defects are “micro porosity” and “shrinkage porosity”

·The casting wall thickness is not “process corrected” and the process parameter design is unreasonable, resulting in large temperature loss of molten aluminum during mold filling and insufficient feeding capacity of riser;

·No feeding channel is designed for local “hot joint”;

·The design of sand core is wrong, and the internal local aluminum water channel is insufficient.

3. Causes of out of tolerance of inlet geometry and size

·Deformation of inlet sand core: we adopt the “hot core box” process for core making. The sand core at about 250 ℃ will deform due to the influence of thermal stress during cooling;

·The clamping gap of the core box is out of control: due to the poor clamping parallelism of the core shooting machine, the clamping gap of the core box changes, and the size of some sand cores is too large;

·The surface of the sand core, especially the casting defects such as surface looseness and damage near the measuring point, has “bumps” at the corresponding part of the product after pouring. If these “bumps” happen to be at the measuring point, the detection data will be out of tolerance;

·The positioning gap between sand core and pouring die is too large.