Shrink holes: Defect Pattern & Causes

The formation of shrink holes occurs in all technical casting materials, regardless of the mold or casting process. However, the pressure die-casting technique provides possibilities for preventing or minimizing the formation of shrink holes by creating high final pressure immediately after mold filling. This does not prevent gas porosity.

Depending on the manifestation of the solidification holes, there are closed shrink holes (interior shrink holes), open shrink holes (exterior shrink holes) and sink marks. The position of the shrink holes resulting from this is logical in that exterior shrink holes usually occur in the upper cast piece areas that solidify last, in thick-walled cast piece areas and close to the ingate and the gate. Interior shrink holes occur in larger wall thickness areas and at strong wall thickness transitions in particular; sink marks are likely to occur at cross-section transitions and on the exterior surfaces of relatively thick-walled cast parts. They often occur in combination with microporosity.

Manifestations of solidification holes

Materials with small solidification intervals and smooth-wall solidification, such as pure metals and eutectic or peritectic alloys, are particularly prone to solidification holes.

Solidification holes

(exterior shrink holes) are deep symmetrical cavities that generally have a funnel-shaped opening to the outside and sometimes continue into the interior as closed cavities. The walls of the cavities are mostly rough and frequently dendritic. Exterior shrink holes are clearly visible to the naked eye.

Interior shrink holes

have no connection to the outside and are thus located in the interior of the cast piece. Their shape is irregular and the walls are rough and often covered with dendrites. They become visible to the naked eye during a nondestructive test or during processing at the latest.

Sink marks are trough-shaped cavities in the surface of the cast piece that occur in the area of larger material accumulations. The surface of the sink mark does not differ from the surface of the remaining cast piece. Sink marks are also visible to the naked eye. If casting-related measures (directional solidification, feeding) do not succeed in shifting the shrink holes to areas outside the cast piece, this casting defect leads to rejection.

Contraction of the casting metals during solidification and cooling

The specific volume of the standard casting metals is larger in the liquid state than in the solid state. For this reason, these metals undergo contraction when solidifying and cooling. This leads to a volume deficit that manifests itself in the form of defects, such as shrink holes, sink marks, microporosity, etc. Shrink holes are thus the result of the interaction between the physical volume deficit during the solidification process and the possibility of compensating it through additional feeding.

The size of the technological volume deficit in conjunction with the specific volume is first and foremost a function of the casting material. Compared with the total volume deficit, its distribution in the cast body and to the specified volume defect types depends on the solidification procedure. Specific influencing factors here are the gas content of the alloy, the mold wall movement in the case of sandcasting and the graphite expansion during solidification in the case of cast iron alloys.

Shrink holes in cast iron alloys

Characteristic of the volume deficit of gray cast iron are the differences between the specific volumes of the individual microstructure components in the following table.

Specific volumes of individual microstructure components of cast iron

Microstructure
component
Specific volume
in m3/g
Ferrite 0.1271
Iron carbide 0.1303
Austenite (C-saturated) 0.1360
Graphite 0.4475

In eutectic solidification, the expansion of the graphite that is precipitated counteracts the solidification shrinkage of the austenite. With a certain graphite content, it is possible that the contraction occurring during solidification is compensated during the formation of the austenite. This means that depending on the chemical composition, the cooling conditions and the nucleation, "self feeding" may take place. If the volume of graphite precipitated eutectically is large, the volume expansion caused by the formation of the graphite may even be larger than the solidification contraction of the metallic phase, meaning that expansion takes place on the whole.

Cast iron with lamellar graphite shows a decreasing shrink hole tendency as the chromo-saturation level increases up to the eutectic point. The volume deficit is lowest there. This applies to both the size of the exterior shrink hole and the proportion of microshrinkage. In the hypereutectic range, the shrink hole tendency increases again.

At the same chromo-saturation level, the expected shrink hole volume decreases as the carbon content increases. Phosphorus increases the propensity for the formation of microporosity, particularly due to the formation of a phosphorus-rich residual melt for P contents > 0.3%.

Provided that the non-inoculated iron would also have solidified as a gray cast iron, the inoculation of cast iron with lamellar graphite also causes an increased propensity for shrink holes, porosity and sinking. A stronger "swelling" of the cast piece occurs through increased expansion due to heavy eutectic graphite precipitation and the solidification type also changes toward "pulp-like" solidification, meaning that the cast piece contour follows the cavity expansion based on the mold material more closely.

Cast iron with nodular graphite displays a greater shrink hole tendency than cast iron with lamellar graphite. If a gray iron melt is treated with magnesium, the shrink hole volume increases from 0.5 cm3 to 5 cm3.

Unlike the endogenous shell-forming solidification of cast iron with lamellar graphite, the solidification of cast iron with nodular graphite is endogenous and pulpy. As well as the liquid contraction, which can be controlled by the feed and gating system, and a secondary shrinkage, an expansion occurs during the solidification process.

Based on the different time phases of solidification, graphite can take effect during its precipitation in the peripheral shell and bulge or expand it if it and the mold material yield to this pressure from inside. If the liquid metal sinking in the cast body is not topped up, this alone can cause shrink holes to form. Later on in solidification, the graphite is precipitated in a larger area of the cast body.
As the peripheral shell can already shrink, the conditions for self-feeding become more favorable. If the peripheral shell is also still in a state of eutectic solidification, increased expansion is to be expected.

Provided the mold is rigid, the exploitation of this expansion phase leads to a compensation of the secondary shrinkage and thus to cast pieces without shrink holes and with a high yield.

Significantly hypo- and hypereutectic compositions, missing inoculation, excessive casting times at temperatures that are too high and magnesium contents that are too high foster the shrink hole susceptibility of cast iron with nodular graphite. Alloyed cast iron types generally also show a higher shrink hole tendency, as do increased pig iron additives in the charge make-up. However, this latter impact can always be seen in conjunction with the melting process.

Shrink holes in aluminum alloys

Here, too, the total volume deficit depends primarily on the alloy composition and the reference temperatures (cooling conditions), and its formation is controlled to a great extent by the solidification process.
The carrying capacity of the peripheral shell and the feeding capacity, i.e. the conditions for melt transport in the solidifying cast body, play a major role.
In general, the following progression of shrinkage behavior can be specified depending on the alloying element content: Based on the pure material, the volume of exterior shrink holes decreases roughly up to the alloy with the largest temperature interval of solidification and then increases again toward the eutectic. The opposite applies to the volumes of the sink marks and microporosity. This state of affairs corresponds to the change of the solidification process. As more copper, magnesium and silicon are added, the overall volume of shrink holes decreases as compared to that of the aluminum itself; silicon exerts the strongest influence. As the content of Cu or Si increases, the solidification shrinkage of Al-Cu and Al-Si alloys decreases. However, the formation of shrink holes increases significantly in the presence of contamination due to iron, for example.
Exterior shrink holes and sink marks in aluminum alloys can also be caused by a sand form that was not compacted properly or, in permanent mold casting, by ingot mold temperatures that are too high. Since the total volume deficit can be between 2% and 7%, depending on the composition, measures for directional solidification and sufficient feeding at the cast piece are always necessary.

Refinement with sodium or strontium causes the amount of exterior shrink holes to increase significantly in sand and permanent mold casting; sink marks and interior shrink holes (microporosity) decrease. In real cast parts, this can lead to a change of the solidification process from exogenous to endogenous in certain cross-sectional areas. In this case, the effects overlap, i.e. the proportions by volume of exterior shrink holes and porosities are in the central areas.
During refining, it must be taken into account that the aluminum-silicon eutectic can adjust itself to either lamellar or granular, depending on the phosphorus content. In permanent mold casting, both modifications display roughly the same total volume deficit; in sandcasting, the granular material is slightly below the lamellar material.

In both cases, the distribution of the volume deficit changes once just 0.02% sodium is added. In the case of the granular and lamellar material, the volume of exterior shrink holes increases, whereas the number of sink marks decreases. After this, further additions of sodium only cause slight changes to the volume of exterior shrink holes. In the case of permanent mold casting, sodium has almost no influence on the lamellar type. In the granular alloy, the tendency to form exterior shrink holes is noticeably lower. As more sodium is added, the volume of exterior shrink holes increases and the volume of sink marks decreases. Further, adding more sodium in sandcasting causes the branching of the external solidification hole to grow.

 

Source: "Guss- und Gefügefehler" (casting and structural defects) by Stephan Hasse