Analysis and control of shrinkage cavity defect in spheroidal graphite casting

Spheroidal graphite casting has been widely used in the field of large-scale Mould Casting, and it is one of the most commonly used production processes for blank. With the rapid development of the automobile industry, the demand for equipment mould increases year by year, and the influence of casting defects is gradually prominent. The common defects are wrinkling, deformation, shrinkage, sand inclusion, and carbon deposit, etc.

Formation and damage of shrinkage cavity

(1) the mechanism of shrinkage cavity, the volume of liquid alloy liquid iron decreases from liquid to solid, and the liquid shrinkage, solidification shrinkage, and solid shrinkage occur.

When the shrinkage of the liquid state and solidification is larger than that of solid-state, the shrinkage cavity will be produced. The shape of the shrinkage cavity is very irregular.

(2) shrinkage cavity characteristics, shrinkage cavity mainly concentrated in the upper part of the casting and the last solidification location, as well as the casting wall thickness gap, concave corner radius small and inner gate near the late or slow solidification location (called hot spot) . There are 4 types of shrinkage cavity, namely, open shrinkage cavity, angle shrinkage cavity, core surface shrinkage cavity, and internal shrinkage cavity.

(3) the damage of shrinkage cavity in the mould mainly has the following four aspects: First, reduce the effective bearing section area of the casting, even cause the stress concentration and greatly reduce the physical and mechanical properties of the casting; second, the continuity of the casting is destroyed, it makes the airtightness and corrosion resistance of the castings decrease obviously, the roughness of the surface of the castings after processing increases, which leads to the roughening of the castings, and the shrinkage cavity occupies a large proportion in the defects of Ductile Iron and often becomes an irreparable defect, directly caused the casting scrap, brought the huge economic loss to the enterprise.

Location of shrinkage defect

Based on the statistical analysis of the failure phenomena of the past castings, it is found that the shrinkage defects of nodular cast iron mainly occur in the following parts of High-Grade Ductile Iron: The hot spot and the final solidification part of the castings, the bearing part of the use face part; 10 mm below the surface.

Analysis

(1) the shrinkage cavity of the hot spot and the last solidifying part of the casting, the hot spot of the casting often appears in the corner, the corner, the small diameter of the casting hole and the wall thickness difference part of the casting, the heat is diffused slowly or concentrated to a certain point, the outer layer of the molten iron has been solidified, but the hot spot is still in a liquid state, the solidification layer gradually forms dendrites and grows to divide the remaining molten iron into several different molten pools. As the temperature decreases, the hot spot begins to shrink, and the volume becomes smaller, at this point can not be added to the Molten Iron and solidification of the hole wall rough, full of dendrites loose hole, forming a large number of dispersed shrinkage.

Nodular cast iron changes from a liquid state to solid-state in the form of a chip. During the solidification process, the eutectic transformation will occur, and graphite will precipitate out, at this point, the thin solidification layer on the surface of the casting makes the mold move outwards, the inner space can not be supplemented by liquid iron, and irregular concentrated shrinkage cavity is formed at the final solidification place. Therefore, the solidification characteristics of nodular cast iron itself make it easy to appear shrinkage defects.

(2) in order to ensure the improvement of the appearance quality of the casting, the casting surface (processing surface) is usually molded upward when the casting process is designed, materials such as partially gasified sand or entrained sand will accumulate in the upper layer of the surface, which is mainly due to the ability of the surface to remove surface impurities during the finishing process. When the machining allowance of the casting surface is insufficient, some casting defects will remain on the service surface or even the important bearing part. Mold Design to increase the machining allowance will increase the cost of the mold, often through the casting process for shrinkage control.

(3) the process capability analysis of the surface below 10 mm by collecting the equivalent carbon values of 16 shifts in the production site (see figure 5) shows that the adjustment of the carbon equivalent values is in line with the process requirements, but the overall values are concentrated around 4.4, close to the lower limit, for the castings without riser design, the carbon content is low, the EUTECTIC expansion force is insufficient, the self-compensation ability is poor, easy to appear the inner shrinkage hole below 10mm surface.

Shrinkage control measures

(1) Riser process design, for the centralized shrinkage defects, can be taken reasonable riser design, the role of the riser in the gating system is to compensate for shrinkage caused by volume changes. The process design should strive to achieve the highest molten iron temperature at the riser, the lowest temperature away from the riser, and the final solidification of the riser itself, to achieve the effect of sequential solidification. Therefore, the riser shape design should make the ratio of the volume to the cooling surface area reach the maximum, the riser height is larger than the diameter, at the same time, the insulation riser can be selected to ensure a reasonable temperature distribution. The holes due to shrinkage in the solidification process of liquid iron will be continuously replenished by liquid iron at the riser so that the castings with better compactness can be obtained.

(2) the placing of chill and the variety of automobile shapes decide the complexity of automobile mould, so the compact structure, the position of wall thickness, and the hot spot often appear in the casting process, these positions are difficult to make up for the riser and gating system. Local cooling can effectively control the shrinkage. Chill is a widely used method, which can be divided into the external chill and internal chill. The outer chill is mainly used in the thick part, where the cooling speed is slow, and the location is concentrated. The sand barrier thickness should be paid attention to when the outer chill is placed. To avoid supercooling, the thickness of the chill is 70% of the wall thickness in the cooling position, and the distance of the chill should be controlled at 20 ~ 25 mm in a cooling position to form a temperature gradient. The inner chill is mainly used in the concave corner or the inner side of the concave core. The rust removal and dehumidification of the inner chill should be paid attention to.

(3) pouring temperature controlling, the traditional riser design method has increased the usage of molten iron and increased the production cost, and some production enterprises have started to adopt riser-free process design, the condition of avoiding shrinkage cavity is that the expansion is larger than the shrinkage from pouring to solidification. Through controlling the pouring temperature, the molten iron is introduced from the thin part, the inner runner adopts the flat thin trapezoid section, and the passage can be solidified and closed as soon as possible after pouring, and the eutectic transformation in the casting can precipitate graphite, the volume of graphite expands because its specific volume is larger than that of liquid iron. The strength of the sandbox and sand mould makes the inner pressure form self-feeding, thus avoiding the inner shrinkage hole. The recommended pouring temperature is 1420 ~ 1450 °C for thin-walled small parts, 1400 ~ 1420 °C for medium-walled thick parts, and < 1380 °C for large thick parts.

(4) the control of carbon equivalent, whether with or without risers, can increase the carbon equivalent or set the upper limit at the same time as the quenching method is used. The graphite precipitation will increase with the increase of carbon equivalent, promote graphitization expansion to enhance feeding.