How to Measure and Control Metal Quality

In part 1 of our blog, we discussed the 3 most prevalent impurities in aluminum that can affect the quality of your castings.  In part 2, learn how a die caster can monitor and control these impurities to ensure the highest quality castings.

How to “clean” a furnace filled with molten aluminum?

A die caster controls aluminum impurities in the furnace using robust furnace cleaning and maintenance systems.

1. Flux- Flux is used to remove oxides and impurities from molten aluminum. Flux is injected below the surface of the metal using nitrogen gas.  It reacts with aluminum oxides and breaks the bond, freeing the aluminum.  These oxides will then float to the surface of the bath. This light, grey ashy material is called dross.  Dross is then skimmed from the surface and removed from the furnace.

Flux can either be raked into the surface of the aluminum or be injected beneath the surface.  Either can be effective, however, Injecting flux with nitrogen has the added benefit of removing hydrogen from the molten aluminum. 

2. Mechanical scraping of the furnace walls- This is another important step in cleaning a furnace. Cleaning tools are used to scrape aluminum oxide build up from the walls of the furnace.  Oxide growth on the furnace walls must be removed regularly or they will continue to grow.

Regular cleaning using flux, nitrogen and properly designed tools is the first step towards metal quality.

3. Furnace Maintenance-A robust furnace maintenance program is also crucial to create the highest quality metal. Gas powered furnaces use high flow air blowers to generate the heat necessary to melt and hold molten aluminum. Gas burners must be tuned properly to the proper air and gas ratio. A burner which is lean (more air than gas) will provide excess oxygen and promote aluminum oxide growth.

The temperature of the outside shell of a furnace should be monitored as part of the maintenance program.  A high temperature on the shell is an indication of a crack in the refractory wall of the furnace.  The furnace is less efficient when heat is escaping through cracked walls.  The heat source will have to generate more heat to maintain the metal temperature. This added heat will promote aluminum oxide build up inside the furnace.

Furnaces must be regularly emptied, cleaned, inspected and patched as part of their maintenance schedule.  Sediment of sludge and oxides on the floor of the furnace will be removed during cold cleaning. 

How do you know if your furnace is clean?

Two common tools used to evaluate metal quality are the K-Mold and Reduced Pressure Tester. 

1. K Mold
2. Reduced Pressure Tester

Picture of a K mold with test castings.

A K-Mold is a small mold used to cast a rectangular bar.  This bar is designed to allow easy fracturing.  The K-Mold casting can be easily broken to inspect the fractured surface. Visible oxides on these surfaces is an indication of oxides in the furnace.

Pictures show reduced pressure testing samples showing hydrogen content before and after furnace cleaning.

A Reduced Pressure Tester is used to measure the amount of hydrogen in the aluminum bath.  A small disc is poured and solidified under a vacuum.  This disc is sectioned and examined for the size and quantity of gas pores.  This is an excellent tool for ensuring cleaning practices are providing the desired results.

Quantifying metal quality can be challenging. The best method to determine if high-quality metal is being used in your castings is to evaluate your caster’s metal quality procedures. If a caster has a detailed furnace cleaning and maintenance program that is being followed, then you can expect consistent high-quality metal.