Current technologies for aluminum castings and their machinability

USE OF REAL TIME MICROFOCUS RADIOSCOPY FOR INVESTIGATION OF LIGHT METAL CASTING PROCESSES INTRODUCTION

In the light metal casting the state of art is radiographic testing of castings made in the solidified state after the casting process. More detailed the quality of the products in a fabrication process is controlled at the end of the production line after several mechanical treatment procedures are done. The range of casting quality is found out by radiography with conventional film technique by the human testing specialists.
The disadvantage of the test procedure after production is first the delay in time between the production process and the test procedure. Second no insight can be gained into the production process itself. It should be necessary to observe the time-dependent changes of the density profiles of the solidifying casting inside the die, noticing the formation of typical defects like porosities, shrinkages, thermal cracks, inhomogenities and contour-errors, Fig. 3. The present state is that the casting parameters being responsible for lower casting quality, for instance die-temperature, temperature of the liquid metal, casting velocity and others cannot be changed and optimized within a short time. The consequence for the light metal products is manufaturing sometimes more than 50 per cent scrap. Additionally the conventional radiography does not harmonize with the concept of modern fabrication, where computer supported automatic production lines with robots are manufacturing. In accordance to this modern idea quality control must be automated. Due to this postulations an IN-LINE-PRODUCTION-CONTROL SYSTEM has been developed to investigate and control non destructively the detailed course of the light metal casting process.

Vaporisation

When melting and casting metals, temperatures are often sufficiently high that some alloys' components will be evaporating all the time. The evaporation of elements from melts can be severe, and has consequences of which it is useful to be aware. Although examples will occur repeated throughout the book, several instances are gathered here to illustrate how common the effect is.
Figure 1.6 illustrates how volatile sodium is, so that additions to Al-Si alloys to modify the eutectic are only short-lived, because of the sodium evaporating off from the melt within 15 or 20 min.

Zinc similarly evaporates from Cu-Zn alloys, and can oxidise, creating the familiar zinc flare. The wind of vapour blowing away from the melt seems responsible for the lack of gas porosity problems in the zinc-containing brasses because gases such as hydrogen in the environment are continuously flushed away. This interesting and useful phenomenon is dealt with in more detail in Section 6.4.
When melting and holding magnesium alloys in a low-pressure casting machine, it is essential to suppress the evolution of vapour by the presence of some air, or other actively protective gas or flux above the melt. The experimental use of an inert gas, argon, above the melt to avoid oxidation led to the atmosphere becoming high in magnesium vapour such that when the unfortunate operator opened the door to charge more ingots, admitting air, he was killed in the explosion. When vaporisation is properly suppressed, the use of magnesium alloys in low-pressure casting machines is perfectly safe.
In the production of ductile iron, magnesium is actually above its boiling point when added to cast iron. The addition reaction is therefore so energetic that the boiling action requires special techniques often involving special reactor vessels to prevent the melt erupting out of the container.
Alloys of copper and iron that contain lead are a particular problem because of the toxicity of lead. Thus leaded copper alloys and leaded free-cutting steel are now being phase out. The casting of these alloys into sand moulds led to increasing amounts of lead that had condensed in the sand, causing the contamination of nearly everything in the foundry.
Manganese vapour from manganese steels similarly condenses in the moulding sand and is thought to be responsible for the enhanced wetting and penetration of moulds by manganese steels.
Evaporation is a particular problem for alloys melted in vacuum. The charge make-up has to allow for the losses by evaporation. Furthermore, the condensation of the metal vapours on the cold walls of the vacuum chamber usually ignite when the chamber is opened to the air; the fine black metallic dust then burns, with a flame that licks its way around the chamber walls. If not burned on each occasion, the dust can accumulate to a thickness that might become dangerous. Even when the dust is oxidised, it is black and dirty and can be a health hazard. Thus vacuum melting and casting requires special personal care. Melting and casting in an inert gas such as argon greatly reduces the rate of evaporation and consequently reduces these problems.