Scaling

Scaling in die casting simply refers to forming oxides on the metal alloy surface. The molten metal constituents react rather rapidly with the atmospheric oxygen to form oxides.

It creates a scale layer on the exposed surfaces, leading to eventual disintegration.Scaling may seem somewhat similar to soldering as both defects derive from molten alloys’ sticking to the mold. However, the mechanism and vulnerable casting spots deliver the distinction. Still, high-temperature elements involved in the act trigger scaling of die casting.

This article sheds light on the mechanism, causes, effects, and prevention of oxides on surfaces.

Causes of Scaling

Iron (Fe) contents in the alloys are the primary source of oxide formation. Some other active elements like Silicon (Si) to accompany the iron in alloys can also form oxides to limited extents. Extreme temperature condition provokes a fast reaction with the surrounding airborne oxygen.

The formed scale layer remains fairly thin, allowing further oxidation. Researchers found a relatively fast scaling at the initial stages, gradually becoming slower with time. Only a few specialized iron alloys and similar metals exhibit minimal scaling area or scaling parts.

High mechanical stress and rapid temperature change at elevated temperatures often contribute to accelerated scaling. The situation worsens with subsequent tensile stress, leading to multiple defects. It’s even possible to have the casting entirely damaged on scales.

Effects of Scaling

Scaling may seem a somewhat negligible surface irregularity at first. But it adversely affects the casting attributes in the long run. Some of the notable issues to counteract due to scales include –

1. 1. Reduced aesthetics with poor surface finish.
   2. Continued oxidation of the surface elements.
   3. Distortion of shapes and sizes to alter specs.
   4. Low outer adherence to affect permeability.
   5. Complexity in maintaining good workability.
   6. High susceptibility to other external defects.
   7. Additional costs and time to fix the damage.

Scaling or Oxidation Resistance

Adding certain alloy elements can uplift the existing scaling resistance in die casts. It requires an increase in the percentage of composition to trigger chemical inhibition. However, careful and calculated processing is necessary as mechanical properties may be altered considerably.

1. 1. • Silicon (Si) –Si delivers excellent results in improving scaling resistance. Any suitable percentage within the 3% limit showed a noticeable reduction in oxidation. Higher Si concentration pushes the resistance, specifically slowing down the oxidizing rate.
      • Nickel (Ni) or Copper (Cu) –Ni and Cu work sufficiently well on low-constituent cast iron alloys. These alloys should feature a nodular graphite composition for the optimum result. However, neither metal provides superficial reduction like silicon.
      • Zinc (Zn) –Zn also initiates a considerably high positivity in surging the oxidation resistance. Though it lacks full resistance, Zinc works like magic with aluminum die-casting alloys. But it isn’t recommended for cast iron alloys due to adverse effects.

Other heavier elements like Molybdenum (Mo) exert a similar influence to that of Nickel and Copper. However, only a few handful of alloys can show decreased scaling of die casting.

Preventing Scales

Though increasing a specific alloy constituent improves the resistance, there’s more to avoid scales. Preventive measures primarily concern proper implementation throughout the die-casting process.

1. 1. • Optimized Gating System –Ensuring suitably short-distant metal flow without any distant converging can optimize the interior gate functionality.
      • Proper Size and Proportion –Appropriate size, cross-sectional shape, uniform wall thickness, rounded corners, and slow runner can enable continuously smooth flow.
      • Accurate Mold Design –The designed mold must match the desired casting requirements with minimal deflection during the die-cast pressurization.
      • Perfect Rigidity of Mold –The desired strength of the mold components with sufficient support (high speed, immediate impact, less injection force) should bear optimum rigidity.
      • Cooling Water System –A closely placed waterway in the mold avoids excessive and superfast cooling. Adopting slag discharge can further help it reduce the scaling area.
      • Regular Tool Maintenance –Trimming the pressure chamber and cleaning the mold cavity at regular intervals can prevent many machining issues.
      • Precise Clamping Force –Perfectly adjusted clamping force is always preferable as any more or less can greatly affect the overall safety, quality, and serviceability.
      • Iron Content Control –Keeping the total iron percentage within 0.6% to 0.9% can deduct the overall sticking tendency of molten alloys with tools.
      • Suitable Release Agent –Increased demolding concentration, proper releasing agent, and sufficient spray amount can effectively diminish the tool-alloy sticking.

Modern die-casting manufacturers offer incredible precision to reduce the scaling of die casting. But special or custom die casts may experience this defect due to compromised parameter adjustments. Controlling the stress and high-temp situations using countermeasures is beneficial.