Melting and pouring are two key steps in cast aluminum alloy production.
Good process control helps prevent pinholes, inclusions, cold shuts, underfilling, cracks, gas holes, and shrinkage defects.
Because aluminum melt absorbs hydrogen easily, oxidizes quickly, and can dissolve iron from tools, every step must be controlled carefully.
For buyers of aluminum alloy casting, melting quality directly affects casting density, machining stability, and final mechanical performance.
Raw Material Preparation
To produce high-quality castings, qualified raw materials must be selected first.
Raw materials should have correct chemical composition and clean structure. Alloy ingots should be checked for shrinkage cavities, fractures, severe porosity, and gas bubbles.
For Al-Si cast alloys, poor ingot quality can easily cause pinholes and large scrap rates. Sand castings are especially sensitive to this problem.
If ingots contain coarse primary grains, a lower mold casting temperature should be used to help them solidify faster and refine the grain structure.
Charge Pretreatment
Before charging, furnace materials should be cleaned.
Rust, oil, and surface contamination must be removed. Clean aluminum ingots and metal returns may not need blasting, but iron chips and magnetic impurities should be removed.
All charge materials should be preheated before melting. This removes moisture, shortens melting time, and reduces gas absorption.
A typical preheating temperature is about 350-450°C, with a holding time of more than 3 hours.
Furnace Material Storage
Furnace materials should be stored in a dry warehouse with small temperature changes.
Moisture can corrode aluminum materials and form aluminum rust. This moisture is difficult to remove by normal preheating.
Each batch should be marked by alloy type, composition, and grade. Chemical certificates should be kept with the materials.
Return materials such as runners and scrap castings should be stored separately by furnace batch.
Crucible and Tool Preparation
Crucibles and melting tools must be cleaned and dried before use.
New crucibles and long-unused crucibles should be sandblasted and heated at 700-800°C for 2-4 hours. This removes moisture and combustible material from the inner wall.
Before use, crucibles should be preheated to dark red, about 500-600°C, and held for more than 2 hours.
Tools such as ladles, skimmers, stirring spoons, pouring bags, and metal molds should also be preheated, coated, and fully dried.
For stable aluminum alloy melting, wet tools should never contact molten metal.
Melting Temperature Control
If the melting temperature is too low, alloying elements may not dissolve fully. Gas and inclusions are also harder to remove.
Low temperature can increase segregation, cold shuts, underfilling, and shrinkage problems.
If the temperature is too high, hydrogen absorption increases, grains become coarser, oxidation becomes more severe, and alloying elements burn off faster.
This reduces mechanical properties, casting performance, machinability, and melt treatment results.
In production, aluminum alloy melt should generally reach at least about 705°C and be stirred properly.
The melt should then be cooled to the correct pouring temperature after slag removal.
Melting Time Control
The melt should not stay in the furnace for too long.
Long holding time increases oxidation, gas absorption, and iron dissolution.
Typical control limits are:
- Sand casting: not more than 4 hours
- Metal mold casting: not more than 6 hours
- Die casting: not more than 8 hours
To speed melting, medium-sized low-melting charge and Al-Si master alloy can be added first. Larger return materials and pure aluminum ingots can then be added gradually.
Elements that oxidize or evaporate easily, such as magnesium and zinc, should be added near the end at lower temperature.
Transfer and Pouring
Aluminum oxide film is difficult to remove once it enters the melt.
Although solid aluminum oxide is denser than molten aluminum, it has a loose porous surface and strong adsorption ability. It can carry moisture and form pores or inclusions in castings.
The key during transfer is to reduce melt agitation and avoid contact with air.
When pouring from a tilting crucible, the ladle should be placed close to the crucible. The melt should flow along the ladle wall instead of hitting the bottom directly.
Before pouring, operators should check melt temperature, ladle capacity, coating dryness, and tool readiness.
Practical Pouring Rules
Good pouring practice is essential for high-quality castings.
Important rules include:
- Place the pouring cup 3-5 minutes before pouring.
- The pouring cup temperature should not exceed about 150°C.
- Do not pour under strong airflow.
- Skim oxide film and flux layers before taking metal.
- Keep the ladle steady and avoid lifting it too high.
- Keep the metal stream stable and continuous.
- Keep the distance between ladle and pouring cup as short as possible.
- Do not use melt close to the crucible bottom.
These rules help reduce oxide inclusions and improve cast aluminum alloy quality.
Melting Control for ZL104 Alloy
ZL104 is an Al-Si alloy containing about 8-10% Si, 0.17-0.3% Mg, and 0.2-0.5% Mn.
The key points are magnesium control and iron impurity control.
Magnesium has a low melting point and oxidizes easily. Small changes in magnesium content can clearly affect mechanical properties.
Iron is harmful in most aluminum alloys, especially Al-Si alloys. It forms coarse needle-like brittle phases that reduce plasticity and corrosion resistance.
Iron usually comes from charge materials, crucibles, and melting tools. Therefore, the iron content in furnace charge should be strictly limited.
Melting Control for ZL201 Alloy
ZL201 is an Al-Cu alloy. A practical composition is about 5% Cu, 0.8% Mn, 0.3% Ti, with aluminum as the balance.
Its melting control focuses on three points:
- Control titanium content and prevent titanium segregation.
- Strictly limit iron and silicon impurities.
- Strengthen stirring during melting.
Titanium can form TiAl3 particles and refine grains. However, if titanium content is too high or the melt is held too long, TiAl3 may grow and settle.
This reduces available titanium and lowers mechanical properties, especially ductility.
For stable aluminum alloy materials, composition control and melt stirring are both important.
Melting Control for ZL301 Alloy
ZL301 is an Al-Mg alloy containing about 9.5-11.5% Mg.
This alloy is very sensitive to temperature. Overheating increases oxidation and gas absorption, makes grains coarser, and causes serious shrinkage defects.
The melt temperature should be controlled carefully and should generally not exceed 700°C.
The full melting and pouring time should be as short as possible, usually not more than 5 hours.
ZL301 castings may develop black fracture defects when the melting temperature is too high, holding time is too long, or protection and refining are poor.
Iron and silicon impurities also reduce mechanical properties. Therefore, high-purity aluminum ingots and clean magnesium should be used.
Conclusion
Cast aluminum alloy quality depends heavily on melting and pouring control.
Clean raw materials, proper preheating, dry tools, controlled temperature, short holding time, and stable pouring all help reduce defects.
For industrial buyers, these controls are a practical way to judge supplier capability.
A supplier with strong melting discipline can deliver more reliable aluminum and magnesium alloy products for demanding casting applications.
