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 even cracks in castings during service. The inclusion defects 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 tool damage. Inclusion defects in epc castings are generally irregular clusters with uneven inclusion density 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 sand inclusion. The shape of defects is often diverse, the shape edge is irregular, size is different, in a cluster distribution, slag slag pores are generally black gray, color is different shades. There are powdery substances in the defect pit, irregular metallurgical slag in the metal liquid, solid residues the pyrolysis of similar materials, paint into irregular regular light gray powdery substances.
1.2 Inclusions prone parts of lost mold casting
Inclusions in lost mold castings are easy to occur, there are certain rules according to different casting structure, mold formation, production technology 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 the solidification process of casting. For thin-walled steel castings with heat resistance, wear resistance corrosion resistance, porosity slag hole defects are easy to occur, mainly in the parts connected with the gate 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 inclusion by blocked accumulation, molten steel cooling 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 solidification process. Generally, the filling time of small medium-sized castings is only a few seconds, more than ten seconds tens of seconds, the filling time of large castings is also completed in a few minutes. Different 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, 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 the gate, until "cavity" be filled. The boundary shape of the contact between liquid metal shape is related to the temperature of liquid metal, the properties of shape material the filling speed. If the temperature of liquid metal is higher, the shape density is smaller 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 shape can be divided into four models: contact mode, clearance mode, collapse mode involvement mode.
2.2 Turbulent morphology 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 stiffness to resist the impact buoyancy of liquid metal, so as to ensure the integrity effectiveness of the mold in the whole process of casting solidification, 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 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 gas in the liquid metal sources include several aspects, such as the appearance of the gasification residue pyrolysis products gas, liquid metal smelting process of pulp are gas, pulp are formed by oxidation of liquid metal oxide, there are certain gases in a high temperature of metal liquid to dissolve, 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 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 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 splash in the multi-bend 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 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 the "cavity" wall, causing splashing forming flow vortices that are easily involved in inclusions gases.
4.3 Prevent external debris entering the cavity
(1) appearance "cavity" dry sand seal: pouring system appearance casting appearance dry sand between the seal is 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 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 concave.
(3) as far as possible to use good foaming molding casting appearance 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 clean, when necessary, with filling paste to smooth local gaps pits.