Ductile iron castings have been widely used in the field of large-scale mold casting and are one of the commonly used production processes for rough parts. With the rapid development of the automotive industry, the demand for equipment molds has been increasing year by year, and the impact of casting defects has gradually become prominent. Common defects include wrinkling, deformation, shrinkage, sand inclusion, and carbon deposition. This article mainly studies the shrinkage defects of ductile iron castings.

1. Formation and hazards of shrinkage cavities

(1) The mechanism of shrinkage formation is that the volume of liquid alloy iron decreases during the process from liquid to solid, and it undergoes three shrinkage processes: liquid shrinkage, solidification shrinkage, and solid shrinkage.

When the liquid shrinkage and solidification shrinkage are greater than the solid shrinkage, shrinkage cavities will occur, with extremely irregular shapes, rough pore walls, and dendritic crystals. Shrinkage cavities are divided into concentrated shrinkage cavities (referred to as shrinkage cavities) and dispersed shrinkage cavities (referred to as shrinkage porosity).

(2) Shrinkage characteristics: Shrinkage is mainly concentrated in the upper and solidified parts of the casting, as well as in areas with significant wall thickness differences, small concave corner radii, and near the inner runner where solidification is late or slow (referred to as hot spots). There are four main forms of shrinkage, namely open shrinkage, angle shrinkage, core surface shrinkage, and internal shrinkage, as shown in Figure 1.

(a) Open shrinkage hole (b) Angle shrinkage hole (c) Core surface shrinkage hole (d) Internal shrinkage hole

Shrinkage form

(3) The harm of shrinkage in molds mainly lies in the following four aspects: firstly, reducing the effective load-bearing cross-sectional area of castings, and even causing stress concentration, greatly reducing the physical and mechanical properties of castings; Secondly, the continuity of the casting is disrupted, significantly reducing its air tightness, corrosion resistance, and other properties; The third reason is that the surface roughness of the castings increases after processing, resulting in roughening of the parts; Fourthly, shrinkage cavity accounts for a large proportion of defects in ductile iron, often becoming an irreparable defect, directly leading to the scrapping of castings and bringing huge economic losses to enterprises.

2. Location of shrinkage defects

Through statistical analysis of past casting failure phenomena, it was found that shrinkage defects in high grade ductile iron often occur in the following parts: the hot spot and solidification area of the casting; The load-bearing area or the use surface area; Areas below 10mm on the surface.

3. Cause analysis

(1) Hot spot of casting and shrinkage cavity of solidification part Hot spot of casting mostly occurs in the included angle and corner of three sides of the casting, small diameter casting hole and wall thickness difference part of the casting. Heat is slowly dissipated or concentrated to a certain point. The outer layer of molten iron has solidified, but the hot spot position is still in liquid state. The solidification layer gradually forms dendritic crystals and grows constantly to divide the remaining molten iron into several different molten pools, As the temperature decreases, the hot spot position begins to shrink and the volume becomes smaller. At this time, the iron liquid cannot be replenished, and the solidified pore wall is rough and filled with loose pores of dendrites, forming a large number of dispersed shrinkage pores.

Ductile iron undergoes a paste like solidification process from liquid to solid, during which eutectic transformation occurs and graphite is precipitated. The specific volume of graphite is greater than that of molten iron, resulting in volume expansion. At this time, the thin solidification layer on the surface of the casting causes the mold to move outward, and the internal space cannot be supplemented by molten iron. Irregular concentrated shrinkage cavities are formed at the solidification site. Therefore, the solidification characteristics of ductile iron itself make it highly susceptible to shrinkage defects.

(2) In order to ensure the improvement of the appearance quality of castings, casting factories that currently use more load-bearing parts or facial positions often shape the casting surface (processing surface) upwards during process design. During the pouring process of molten iron, some substances such as insufficient gasification or entangled sand particles will accumulate in the upper layer of the mold surface. This method mainly considers the ability of the mold surface to remove surface impurities during the later precision machining process. When the machining allowance of the casting surface is insufficient, some casting defects will remain on the use surface or even important load-bearing parts. Increasing machining allowance in mold design will increase the cost of the mold, and often control shrinkage through the casting process.

(3) Through collecting carbon equivalent values from 16 production shifts on the surface below 10mm for process capability analysis, it was found that the adjustment of carbon equivalent values met the process requirements, but the overall value was concentrated at around 4.4%, close to the lower limit. For castings without riser design, the carbon content was relatively low, the eutectic expansion force was insufficient, and the self compensation ability was poor, making it easy for internal shrinkage cavities to appear below 10mm on the surface.

4. Shrinkage control measures

(1) The riser process design can adopt a reasonable riser design for concentrated shrinkage defects, and the role of the riser in the pouring system is to compensate for the volume changes caused by shrinkage. The process design should strive to achieve a high temperature of the molten iron at the riser during pouring, a low temperature away from the riser, and the solidification of the riser itself, achieving the effect of sequential solidification. Therefore, the riser shape design should make the ratio of volume to cooling surface area reach a large value, and the riser height is greater than the diameter. At the same time, the insulation riser can be selected to ensure reasonable temperature distribution. The holes caused by shrinkage during the solidification process of molten iron will be continuously supplemented by the molten iron at the riser, resulting in a casting with good density.

(2) The diversity of cold iron placement in the appearance of automobiles determines the complexity of automobile molds. Therefore, during the casting process, there are often thick and large parts, wall thickness differences, and hot spots. These positions are difficult to supplement with the riser and pouring system. The application of local quenching can effectively control the generation of shrinkage holes. Cold iron is currently a widely used method, mainly divided into external cold iron and internal cold iron. External cooling iron is mainly used in thick and large areas, where the cooling speed is slow and the positions are concentrated. When placing external cooling iron, attention should be paid to the sand separation thickness, and generally the cooling effect is best between 15-30mm. Although the greater the thickness of the cold iron itself, the better the cooling effect, in order to avoid overcooling, the thickness of the cold iron is generally 70% of the wall thickness at the cooling position. When placing the cold iron, the main consideration should be to control the distance between the cold iron at 20-25mm, forming a temperature gradient. Internal cooling iron is commonly used at concave corners or inside concave cores. Attention should be paid to the rust and moisture removal of internal cooling iron, otherwise fusion with the casting after pouring will affect the performance of the casting.

(3) The traditional riser design method for pouring temperature control has increased the usage of molten iron and increased production costs. Some production enterprises have started to adopt non riser process design, which successfully avoids shrinkage. The condition for this method is to achieve expansion greater than contraction from pouring to solidification. By controlling the pouring temperature, molten iron is introduced from thin and thin parts. The inner runner adopts a flat trapezoidal cross-section, which can solidify and seal the channel as soon as possible after pouring. The eutectic transformation and precipitation of graphite inside the casting occur, and the specific volume of graphite is greater than that of molten iron, resulting in volume expansion. The strength of the sand box and sand mold causes internal pressure to form self feeding, thereby avoiding internal shrinkage. Recommended pouring temperature: 1420~1450 ℃ for small thin-walled parts, 1400~1420 ℃ for medium wall thickness parts, and<1380 ℃ for thick and large parts.

(4) The control of carbon equivalent, whether it is a casting process with or without a riser, can increase the carbon equivalent or set an upper limit value while applying the quenching method. With the increase of carbon equivalent, the amount of graphite precipitation will increase, promoting graphitization expansion to strengthen shrinkage.