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Morphology Features of Aluminum LFC Defects

Posted: 2014-12-24 04:41:00  Hits: 2910
Introduction
Lost foam casting process (EPC, LFC) for aluminum castings is used more and more widely in the automotive industry, especially in developed industrialized countries. EPC has been used for the production of automotive engine blocks, cylinder heads, transmission housing, water-cooled double exhaust pipes, automotive valves, intake and exhaust pipes, heat sink, gas nozzles and valves and other aluminum castings. In contrast, development of domestic EPC process is relatively slow, due to more complicated technical factors. Production equipment and raw materials fail to meet the requirements of the production process, resulting in difficulties for solving a number of casting defects. To improve the situation, the author analyses several common casting defects based on years of experience of aluminum alloy casting testing and production.
 
1. An EPC Technical Overview
Lost foam casting technology is a new precisely shaped technology. It adopts no binder dry sand and vacuum technology to form castings, which can be used for mass production of aluminum alloy cylinder block, cylinder head castings. Because of continuous improvement of mold materials, process modeling, coating technology and tooling equipment, LFC gets a broader range of applications. As the automotive, aviation, aerospace, shipbuilding and other industries shifting the requirements of aluminum alloy castings to thin wall, complex shape, high strength and high-quality, LFC casting technology has a more powerful competitiveness than traditional sand casting processes because of its high dimensional accuracy, smooth surface finish, less pollution and other obvious advantages.
 
2. Defect Analysis
2.1 Surface Pinholes and Slag Holes
Aluminum LFC applies dry sand molding and use thermal insulation coating to paint foam moulds, so its gasification needs to absorb a lot of heat. The furnace temperature and pouring temperature is higher than that of conventional sand casting, pressure die casting and gravity casting metal. Since aluminum element is relatively easy to be oxidized, the aluminum alloy liquid is easy to react with water vapor in the processing of melting and casting, producing oxide and hydrogen. Hydrogen contained in the molten aluminum alloy is likely to precipitate in the cooling process, thereby forming a dispersion of pinholes. Another reason is insufficient management and control to raw and auxiliary materials, smelting equipment and tools accordance with the requirements of production process, when it is not fully preheated, not strictly control to suction and oxidation of liquid aluminum, air entering mold due to improper operation, resulting in severe pinholes and slag holes.
 
Fig. 1 to Fig. 5 are common surface pinholes, needle holes, pin holes slag, slag hole and slag eye. Unreasonable production process is the main cause of these defects. Followings are preventing methods:
A. Change gating system and improve casting speed to ensure no turbulence when pouring.
B. Improve the permeability of coating layer and sand.
C. Control the density, moisture, foaming agent content of foam pattern.
D. Ensure the dryness of pattern and coating.
F. Control the amount and quality of binder on the premise of reliable bonding. The fewer, the better. Use low-fat volume of plastic material. 
 
             Fig.1 Pinholes                                           Fig.2 Gas and Slag Holes                                  Fig.3 Pinhole and Slag Holes
 
             Fig.4 Slag Holes                                                       Fig.5 Slag Inclusion                                      Fig.6 Air Hole
 
                      Fig.7 Slag Blowhole                                   Fig.8 Sand Inclusion                                     Fig.9 Wrapped Slag
 
2.2 Air Holes, Slag Holes, Sonim
Air holes refer to the smooth holes on the casting surface, formed by air volatilized from foam pattern or contained in aluminum liquid which is too late to expel during solidification. The main causes are excessive density of foam patter, uneven intensity, poor paint ventilation, insufficiently dry coating, low baked and pouring temperature, and unreasonable design of casting head system. The defect morphology is shown in Figs. 6 to 9
 
2.3 Sticky Sand, Nodulation
Sticky sand is mechanical sand mixture of liquid metal and molding sand when liquid metal infiltrates through molding sand. Sticky sand of aluminum castings are generally caused by insufficient filling of molding sand or too high vacuum. Morphology of various sticky sand defects is shown in Figs. 10 to 12.
 
                        Fig.10 Burnt-on Sand                            Fig.11 Nodulation                                      Fig.12 Surface Bonded Sand
   
Measures of preventing surface sticky sand is mainly controlling the quality of preparation and painting. Ensure uniform and integrity coating, especially for complex lumens and corners. In addition, adopt reasonable pouring temperature, casting speed and negative degrees.
 
2.4 Shrinkage
During EPC, the temperature and pressure of aluminum alloy molten liquid in dead head is low compared with sand casting due to large heat consumption of gasification. Feeding capacity of dead head is ordinarily not as good as that of sand casting, so it is more prone to shrink. The morphology defects are shown in Fig.13-Fig.15. Identify the causes by changing system design of casting head and improving its feeding capacity to prevent this case from happening.
 
                          Fig.13 Shrinkage                                     Fig.14 Dispersed Shrinkage                          Fig.15 Shrinkage  
 
2.5 Collapse Type (Sand Collapse)
The reason of sand collapse of lost foam casting defect is that aluminum liquid can’t totally replace foam pattern due to sanding movement, casting damage. As a result, we can’t get castings with full shape. See the morphology of the defect in Fig.16. This defect is often caused by too slowly aluminum alloy liquid filling and too low pouring temperature. The prevention method is generally improving mold strength, adopting reasonable modeling process and choosing proper casting speed and pouring temperature.
 
2.6 Cold Traps and Double-skin
Cold traps on lost foam casting surface are generally caused by low pouring temperature, slow pouring speed, low pouring pressure and irrational pouring system. Thin-walled aluminum alloy castings with complex shape are more prone to produce cold traps. Moreover, if the pressure during molten alloy filling process is too large, making part of molten alloy rise too fast along the shaped wall. When the rising speed of is faster than that of molten alloy in internal cavity, it is possible to form a thin layer on the mold surface of solidified shell (See Fig.17 and 18). Methods to prevent such defects are:
A. Choose special sand box which is conform to casting structure and process requirements and appropriately adjust the placement of foam pattern in sand box.
B. Improve gating system and raise the pouring temperature and speed.
C. Appropriately adjust the negative pressure and control the speed of gasification.
 
                     Fig.16 Sand Collapse                                               Fig.17 Cold Shut                            Fig.18  Double-skin
 
2.7 Inadequate Pouring
The molten aluminum alloy gasifies and fills the foam mold in the thin wall portion or away from the gate portion. Due to the narrow cavity, there is great resistance to the front gas flow, long flow distance, large heat loss of molten fluid and quick cooling speed, which may cause slow flow and decreased ability to filling, resulting in defects for inadequate pouring (See Fig.19 and 20)
 
Generally preventive measures of this defect:
A. Reduce the density of foam plastic mould, replace category of original beads and reduce the gas evolution.
B. Improve the pouring temperature and pouring speed of molten aluminum alloy.
C. Improve the permeability of foam plastic coating and reduce the gas resistance during filling process.
 
                   Fig.19 Inadequate Pouring                        Fig.20 Inadequate Pouring                           Fig.21 Holes
 
2.8 Surface Pores and Pits Defects
Under improper operation and relaxed control, LFC is prone to generating surface pores and pits defects, such as slag hole, sand holes, craters and others. In addition, due to poor quality foam plastics, there will be moire defects on casting surface, shown in Figure 21 to 23. The causes of these surface defects are more complex, so we need to analyze carefully and respectively, and take the following precautions: 
 
A. Strictly control the process operation, and ensure the quality of bonding and connection of casting head.
B. Improve the design and slag ability of casting head. Increase its feeding capacity by increasing the pouring temperature.
C. Ensure the dryness of foam plastic mould and coatings.
 
                    Fig.22 Surface Pores                                         Fig.23 Pits                                             Fig.24  Moire 
 
2.9 Carbon Defects (Wrinkled Skin)
Wrinkled skin is formed from the residue resolved by polystyrene during casting pouring. Polystyrene turns into viscous bituminous liquid at a temperature below 700 ℃. A portion of this liquid is absorbed by coating layer and the others formed a thin film between the molten aluminum alloy and the coating layer. Then it forms a crystalline carbon in a reducing atmosphere, which causes carbon defects on casting surface, shown in Figure 25.
 
To reduce such defects, we must strictly control production process of advance bubble and foam molding. Strictly control the quality of coatings, improve gating system and production technology, strict. Strictly control the quality of raw materials and aluminum smelting, improve the pouring temperature and casting speed.
 
2.10 Granular Surface and Surface Burnt
Granular surface defect is locally foam granulars on casting surface, shown in Figure 26. This defect will affect mechanics performance, destroy casting sealing and even lead to leakage and scrap. In addition, it will also affect the appearance of the casting, which users cannot accept it. This flaw is due to expansion of the beads and poor bonding, including:
A. Not enough content of beads foaming agent.
B. Low vapor pressure and short vapor pass time.
C. Poor quality of molding mold. 
 
Surface defects burnt are shown in Figure 27. 
The nature of such defect is substantially the same as that of granular surface. The latter is caused by insufficient vapor pressure and ventilation time, while the former is due to exceeding vapor pressure and ventilation time. The fundamental reason of surface burnt is poor casting mould. Only making necessary repairs can eliminate such defects.
 
               Fig.25 Carbon Defects                                        Fig.26 Granular Surface                             Fig.27 Surface Burnt
 
2.11 Uneven Surface 
The main reason of irregularities on casting surface is that the vent plug end face and the mold cavity is rough, the inlet feed piston rod position is not accurate, and there are damages or defects on the mold cavity surface. In order to prevent such small defects, we should strengthen regular inspection of the molds, identify and solve problems in time, and use moulds produced by professional factories. 
 
3 Conclusion
3.1 Aluminum breathing and oxidation is the biggest reason of casting defects. The prevention method is to strictly control various production processes links, including controlling selection of raw materials and operations of melting, deaeration, refining, deterioration, standing and pouring.
3.2 Carefully analyze a variety of casting defects, comprehensively consider various factors and take corresponding measures combined with existing conditions.
3.3 Different structural features of the casting process often have different requirements, as well as different defects. Analyze the reasons specifically and take appropriate measures to control and prevent.
3.4 Select suitable and qualified production and testing equipment is a prerequisite to prevent all kinds of defects.
 

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