The forming process of inclusion defect in epc casting

1 Inclusion defects in epc castings


Inclusion defects in epc castings are very common. Inclusion defects in epc castings often greatly reduce the properties of castings. At the same time, due to the irregular shape of inclusion, it may cause cracks or even cracks in castings during service.  The inclusion defects not only reduce the casting properties, but also improve the local hardness of the casting due to surface defects, which can easily lead to rough machining surface, affect the cutting efficiency and tool damage.  Inclusion defects in epc castings are generally irregular clusters with uneven inclusion density and porosity, also known as slag porosity.


1.1 The form of inclusion defects in epc castings


The inclusion defects of epc castings are slag, slag porosity and sand inclusion.  The shape of defects is often diverse, the shape edge is irregular, size is different, in a cluster distribution, slag and slag pores are generally black and gray, color is different shades.  There are powdery substances in the defect pit, or irregular metallurgical slag in the metal liquid, or solid residues from the pyrolysis of similar materials, or paint into irregular or regular light gray powdery substances.


1.2 Inclusions prone parts of lost mold casting


Inclusions in lost mold castings are easy to occur, and there are certain rules according to different casting structure, mold formation, production technology and conditions.  The reason of formation is related to the content of residue in liquid metal, the process of negative pressure pouring, the flow state of liquid metal and the solidification process of casting.  For thin-walled steel castings with heat resistance, wear resistance and corrosion resistance, porosity or slag hole defects are easy to occur, mainly in the parts connected with the gate or riser.  Parts that are connected to the casting casting filling process, the flow for a long time, to keep the heat time is longer, the liquid steel to cast material overheating, partial melting casting material, absorb more gas in the liquid steel and inclusion by blocked accumulation, molten steel cooling and solidification shrinkage, easy to cause these parts after cooling solidification form hole, shrinkage porosity, slag mixed defects.


2 Filling process of lost die casting


Casting defects are formed at the moment of casting filling and solidification process. Generally, the filling time of small and medium-sized castings is only a few seconds, more than ten seconds or tens of seconds, and the filling time of large castings is also completed in a few minutes.  Different from ordinary cavity casting, the particularity of mold filling in epc casting is the main cause of inclusion defect formation.


2.1 Filling form of epc casting


About the lost foam casting liquid metal filling process, liquid metal filling morphology, from the gate into the casting, after the “cavity” metal fan form forward frontier, under the effect of gravity, the liquid metal filling frontier downward deformation, but the overall trend is advancing in the direction of the far away from the gate, until “cavity” be filled.  The boundary shape of the contact between liquid metal and shape is related to the temperature of liquid metal, the properties of shape material and the filling speed. If the temperature of liquid metal is higher, the shape density is smaller and the filling speed is faster, the overall advancing speed of liquid metal is faster.  For aluminum alloy without negative pressure pouring, the interface between liquid metal and shape can be divided into four models: contact mode, clearance mode, collapse mode and involvement mode.


2.2 Turbulent morphology and wall attachment effect of liquid metal filling


In the production process of lost mold casting, the dry sand mold is tightened by applying negative pressure to the dry sand mold during the casting process, so that the mold has enough strength and stiffness to resist the impact and buoyancy of liquid metal, so as to ensure the integrity and effectiveness of the mold in the whole process of casting and solidification, and obtain the complete structure of the casting.  The negative pressure allows the black alloy castings to be cast in lost mode. The dry sand mold has sufficient strength and stiffness to allow the casting process to proceed without increasing the height of the sand box.


3 The source of inclusions in liquid metal


Inclusion and gas in the liquid metal sources include several aspects, such as the appearance of the gasification residue pyrolysis products and gas, liquid metal smelting process of pulp are and gas, and pulp are formed by oxidation of liquid metal oxide, there are certain gases in a high temperature of metal liquid to dissolve, and the debris into the sealed cavity “, etc.


4 Ways to reduce inclusions in lost die castings


4.1 Reduce the original inclusions in liquid metal


Reducing inclusions in liquid metal before pouring is the main way to reduce inclusions in steel casting parts.  There are many ways to purify liquid steel. The metal liquid purifier can be used, that is, the slag collecting material can be used. The inclusion of smaller particles can be adsorbed on the larger particles of the inclusion by relying on the adsorption of the inclusion to form larger inclusion particles, which is beneficial to improve the dynamic conditions of its floating.


4.2 Process measures should be taken to reduce the inclusion in the metal liquid and strengthen the removal of inclusion


(1) Reasonable design of pouring riser system: as far as possible to use less than one box casting, as far as possible to reduce the time of metal liquid in the pouring system, that is, to reduce or cancel the runner;  Multiple casting in one box will inevitably make the pouring system too long. When the metal liquid passes through the pouring system, it is easy to produce turbulence and splash in the multi-bend and variable section channel of the pouring system, which reduces the temperature of the metal liquid, leads to the oxygenation of the metal liquid, scour the side wall of the spate and increases the original inclusions in the metal liquid.


(2) Appropriate reduction of negative pressure: negative pressure is an important reason for increasing turbulence caused by metal liquid filling.  Increased turbulence causes molten metal to wash away the gating system and the “cavity” wall, causing splashing and forming flow vortices that are easily involved in inclusions and gases.


4.3 Prevent external debris from entering the cavity


(1) appearance “cavity” and dry sand seal: pouring system appearance and casting appearance and dry sand between the seal is not easy to cause dry sand into the “cavity”, sealing mainly depends on the coating, coating performance to meet the requirements, uniform thickness, especially in the appearance of the inner corner, to prevent coating accumulation too thick caused by cracking or falling off.


(2) reduce shape bonding joints: too many shape bonding gaps, easy to cause too much change in the amount of glue used in the gap, resulting in adhesive joints convex or concave.


(3) as far as possible to use good foaming molding casting appearance and casting system appearance: if because of the production batch can only use the plate cutting appearance, the appearance of the surface should also be properly treated, polished and clean, when necessary, with filling paste to smooth local gaps and pits.


Post time: Sep-14-2021