• Metallic inclusions are the residues of metallic additions or improperly dissolved metallic additions
  or inoculants.

• Non-metallic inclusions occur in two types:
  A. Exogenous inclusions
  They are introduced from the outside. They can be the lining material of the furnace, ladle, or, mould material introduced as a result of erosion.
  B. Endogenous inclusions
  They are formed internally and occur due to liquid metal oxidation while pouring or as a by-product of metallurgical reactions. They are found, in particular, as oxides and sulphides.

The Effects of Inclusions in Castings

POOR SURFACE QUALITY - The inclusions, impair the appearance of the casting surface and need considerable machining tolerance. They even react with the mould material, increasing surface roughness.

DETERIORATION OF MECHANICAL PROPERTIES - The inclusions that form on the grain boundaries as films, damage large areas of metal structure. As a result mechanical properties like ductility and fatigue strengths will decline considerably.

POOR MACHINABILITY - The non-metallic inclusions are harder and therefore difficult to machine. This leads to the deterioration of the machined surface, reduced cutting performance and considerable reduction of tool life.

CASTING LEAKAGE - The inclusions in the castings act as conduits for pressure, which penetrates through the cast walls leading to leakage in pressure loaded castings.

GAS BUBBLES - The gas bubbles occur by the reaction of inclusions with the metal or can act as nuclei for bubble nucleation during solidification. This can be avoided by getting rid of the inclusions.

Mechanism of Filtration

The filtering of inclusions happens in three different ways. The inclusions larger than the pore opening of the entrance gets trapped there and in turn acts as a sieve for smaller particles. The finer inclusions that get through are retained in the void surfaces of the filter. The three filtering processes are:

SCREENING - The inclusions larger than the pore diameter are retained at the inlet of the filter. This filters only the large particles, particularly the ones that form films.

CAKE FILTRATION - The first large inclusions deposited on the filter surface (known as cakes) capture smaller inclusions. These inclusion layers increase, reducing the metal flow which ends up in the total clogging of the filter after some time. This helps in capturing the smallest of inclusions: particles as small as 1 to 5 microns.

DEEP BED FILTRATION OR DEPTH FILTRATION -The inclusions that pass through the inlet are captured on the filter walls inside the filter. This occurs effectively in Ceramic Filters where the metal has to pass through a zigzag path. Hence they come in contact with a larger surface area where the adhesive forces make the inclusions stick together and get fixed on the walls. Good wetability between the inclusions and the filter material leads to the effective trapping of inclusions.

In case of solid inclusions, when neither the filter material nor the solid inclusion is wetted by the melt, the inclusions are pushed towards the filter wall where it is held by adhesive forces. The lower the metal wetability of solid inclusions and filter, the better the trappings. The particles trapped in this manner stick to the wall even at places with intensive metal flow. The high efficiency depth filtration of Foam Filters makes them superior when compared to other types of filters. In foam filters, the whole depth of the filter participates in the filtration; the greater the filter depth, the more effective the filtration.

Selection of Ceramic Material

The filters are subject to highly stressful conditions when the molten metal oozes through them. The extreme conditions the filters go through are:

• Thermal shock on sudden contact with the molten metal
• The impact of molten metal due to direct impingement on the filter surface from a certain height
• Mechanical stresses at high temperatures causing creep
• Chemical corrosion of the filter due to action of slag and metal
• Erosion due to high temperatures and hydraulic forces

For Ceramic Foam Filters the following three materials are chosen:

ALUMINA (Al2O3) - Alumina based Ceramic Foam Filters are used for temperatures up to a limit of 11000C. They are most suited for the filtration of aluminium and its alloys.

SILICON CARBIDE (SiC) - Ceramic Foam Filters based on silicon carbide withstand temperatures up to 15000C and are used in the filtration of cast iron, copper and its alloys.

ZIRCONIA (ZrO2) - Ceramic Foam Filters based on partially stabilized Zirconia withstand temperatures up to 17000C and is used in the filtration of steel and super alloys.

Pictorial View of Castings

Cast Iron Filtration

Features that make Ceramic Foam Filters effective in Cast Iron Filtration:

• Filtration Efficiency is important. The slag and dross have to be removed from the molten metal to prevent them from entering the mould cavity.
• Metal Capacity must be adequate and consistent. The capacity must not vary from filter to filter, which could lead to premature blockage.
• Flow Rate must be high and consistent. Huge variations in flow rate could lead to mould-fill problems.
• This demands larger filters, which in-turn, increases cost and decreases yield.
• Dimensional Accuracy is crucial. The filters should fit into the print cavity precisely.
• Strength (hot or cold) is critical. The filters should be strong enough to withstand physical stresses during shipping and handling. They should also remain intact when the molten metal passes through them.

Specifications for Alumina based Ceramic Foam Filters

Flow Rates* for Alumina based Filters

Specifications for Silicon Carbide based Ceramic Foam Filters

The Importance of Yield

Yield is generally defined as the total weight of good, saleable castings expressed as a percentage of the total weight of the metallic materials melted to produce them.

Silicon CarbideFilter capacities and Flow Rates for Cast Iron Filtration

Energy represents one of the largest controllable costs in melting and improving yield and this is the best way to reduce it. Depending upon the type of casting produced and the grade of the metal used, yield varies from foundry to foundry. A typical grey iron foundry operates at an overall yield of 65%. Hence for every 100 tonnes of metal charged, the foundry produces 65 tonnes of saleable castings. Malleable iron castings produce 35 to 45% of yield, ductile iron 45 to 80%, while specialized grey iron castings produce yield as high as 90%.

In the casting process, defective castings are a critical issue that needs to be addressed. Reducing scrap has a two fold effect.

• The energy required for metal melting is low.
• Materials, consumables, and labour are also reduced (especially in molding, core making, and in the post casting processes). This increases the foundry's potential capacity.

Specifications for Zirconia based Ceramic Foam Filter

Guide to Zirconia Foam Filter selection for Carbon and Low Alloy Steels

Guide to Zirconia Foam Filter selection for High Alloy Stainless Steels

The most effective way of improving a foundry's yield is by using an efficient filtration system. Ceramic Foam Filters are the most effective filtering system available, and are used widely to reduce scrap and improve yield. Apart from capturing unwanted inclusions, they also reduce turbulent flow. The simple running systems developed with Ceramic Foam Filters provide extra castings, at times within a given mould box, which results in an improved box yield. The proper use of Ceramic Foam Filters in the runner system helps in reducing the length of the runner and gates, hence long runners and in-gates need not act as slag traps. The shorter runners and in-gates, in turn, help in increasing the yield significantly.