Elimination of Gas Porosity in Gravity Die Casting

In regular technological production, the casting process and the quality of castings are influenced by numerous factors, such as melt preparation, ** mould and core preparation, ** mould temperature, ** casting temperature and pressure**, etc. The results of a series of test castings confirmed our assumption, therefore measures were proposed for further prevention of gas porosity, namely: The case under consideration once again confirms that the foundry process is a complex field requiring extensive theoretical and practical knowledge, as each casting with its specific problems represents a chapter in itself, despite being based on the same theoretical foundations.

Solution and Method

In regular technological production, the casting process and the quality of castings are influenced by numerous factors, such as melt preparation, ** mould and core preparation, ** mould temperature, ** casting temperature and pressure, etc. To ensure ** casting quality according to customer requirements, despite the high degree of production automation, ** good knowledge of the process** and ** professional coordination** of all participants are essential. It was established that even ** minor deviations** from the prescribed technological documentation ** represent a significant risk** and ** increase non-conformity** in the process, consequently resulting in negative economic indicators for the company.

The diploma thesis presents the comprehensive technological sequence of the gravity die casting process for aluminium alloy AlSi7Mg using the example of a turbocharger housing casting, from the perspective of quality assurance according to customer requirements. The issue of gas porosity in castings is highlighted, which is most readily recognised by its shape. Based on prior X-ray and microscopic analysis as well as mechanical processing of castings, ** the presence of gas porosity was confirmed**.

Subsequently, it was determined that the defect originates from poorly manufactured cores, due to insufficient heating temperature of the core box.

Results

The results of a series of test castings confirmed our assumption, therefore measures were proposed for further prevention of gas porosity, namely: The case under consideration once again confirms that the foundry process is a complex field requiring extensive theoretical and practical knowledge, as each casting with its specific problems represents a chapter in itself, despite being based on the same theoretical foundations. Even the most thorough knowledge of metallurgy and all metallurgical processes is not sufficient for solving all problems that arise during casting development or when defects appear on it.

The opinions of those unfamiliar with the field suggest that for work in a foundry, a specialist in metallurgy alone is sufficient, however, upon closer examination of the foundry, the work, machinery and functions within it, we quickly realise that without mechanical engineers, electrical engineers and hydraulic engineers, it cannot function. Mechanical engineers must ensure very demanding details. Through mould design, they ensure minimal dimensions, thereby consequently saving enormous amounts of energy consumed for heating to the appropriate temperature, later for cooling and maintaining optimal temperature.

By reducing mould weight, the closing and opening force of the mould is reduced, whilst simultaneously they must ensure sufficient rigidity for the moulds to withstand all acting forces. Through the method of mould clamping, engineers ensure the foundryman an uninterrupted process of casting the alloy into the mould, whilst simultaneously allowing the mould the possibility of thermal expansion during temperature changes of heating and cooling. With the correct closing force, the mould must seal 100%, otherwise flash occurs.

With the correct position of ejectors, optimal lifting of the casting from the mould is determined, otherwise deformation of the casting occurs. By determining the method of separating the casting from the gating system. Electrical engineers add their contribution to mould manufacturing, more precisely in mould heating.

With precise determination of locations for heaters and their power, less energy is consumed during casting. The task of electrical engineers also includes determining the power of heaters and their distribution throughout furnaces for the melt, which must maintain constant temperature throughout the entire withdrawal period. The contribution of hydraulic engineers is evident in mould cooling with water, as they must determine flow rates and pressures of the cooling liquid.

Their task also encompasses the procedure for determining the speed of mould movement from the position for manual pouring to the casting position. From all the above, we can see that a good foundry requires many specialists ** from various fields** to be able to cast the most demanding products without interruption, which are currently required by the automotive industry.

• appropriate repair of gas burners for core box heating, to ensure the temperature of both core box parts is between 220–260 °C, which ensures minimal gas release from the core upon contact with the melt;
• preventive cleaning of air channels ** for the removal of released gases** from the mould cavity during casting itself. This measure was chosen with the aim of additionally ensuring adequate gas removal on the assumption that minor deviations might accidentally occur during the core manufacturing process itself.