Die Casting metal fluidity selecting alloys for complex geometry filling without defects

Content Menu

● Introduction

● Understanding Metal Fluidity in Die Casting

● Key Alloys Proven for Complex Geometries

● Factors That Determine Success or Failure

● Case Studies from Published Work and Production

● Practical Implementation Guidelines

● Conclusion

● Q&A

 

Introduction

In high-pressure die casting, the ability of a molten alloy to fill a die completely before solidification starts often decides whether a part is acceptable or ends up in the scrap bin. When the geometry includes thin walls, long flow paths, sharp corners, or large variations in section thickness, the demands on metal fluidity become extreme. A misrun in a 0.7 mm cooling fin, a cold shut across a sealing face, or gas porosity trapped in a deep rib can turn an otherwise perfect design into a production nightmare.

Over the past two decades, the trend toward lighter structural components and higher thermal performance has pushed wall thicknesses below 1 mm and flow-length-to-thickness ratios above 150:1 in many automotive and electronics applications. At the same time, cost and sustainability pressures have forced foundries to use more secondary (recycled) material, which can carry oxides and inclusions that hurt fluidity. Choosing the right alloy and controlling the process parameters that affect fluidity have therefore become central tasks for any die-casting engineer working on complex parts.

This article examines the practical side of fluidity in aluminum, magnesium, and zinc alloys for high-pressure die casting. It draws directly from published measurements of spiral fluidity length, vacuum filling tests, and industrial trials to show which compositions consistently deliver complete filling without defects. The goal is to give manufacturing engineers clear, actionable criteria for alloy selection and process setup when the part geometry leaves almost no margin for error.

Understanding Metal Fluidity in Die Casting

Fluidity is most often quantified as the maximum distance a molten alloy can travel in a standardized channel before the flow front freezes. In research laboratories the spiral test (typically a 10–15 mm wide, 5–8 mm deep Archimedes spiral cast in sand or permanent mold) remains the benchmark because it is simple and reproducible. In production environments, vacuum plate tests or stepped-plate castings better represent the rapid heat extraction found in steel dies.

The main variables that control fluidity are:

• Alloy composition (especially eutectic-forming elements)
• Superheat above the liquidus temperature
• Heat transfer coefficient at the metal–die interface
• Presence of oxide films and non-metallic inclusions

Silicon is the single most effective addition for improving fluidity in aluminum alloys. Each 1 wt.% Si above the eutectic composition lowers viscosity and extends the solidification range, allowing the metal to remain mobile longer. Strontium or sodium modification changes the Al-Si eutectic morphology from plate-like to fibrous or globular, which reduces the tendency for dendrites to interlock and block narrow sections.

Magnesium alloys show lower latent heat of fusion than aluminum, so they solidify faster for a given cooling rate, but their lower viscosity and surface tension give them excellent filling capability in vacuum-assisted or thin-wall applications. Zinc alloys, with melting points around 380–420 °C, require far less superheat and therefore suffer less from oxidation, making them forgiving for very detailed or thin features.

Key Alloys Proven for Complex Geometries

AlSi10MgMn (commercial name Silafont-36) has become the reference alloy for thin-wall structural castings. Measured spiral fluidity lengths exceed 750 mm at 690 °C pouring temperature, and industrial vacuum filling tests routinely achieve complete filling of 0.6 mm walls over 200 mm flow distance. The combination of 9.5–11.5 % Si, low iron (≤0.15 %), and manganese for die soldering resistance explains its dominance in European automotive chassis parts.

A383 (AlSi11Cu2Fe) and its low-iron variant A384.1 are widely used in North America when high thermal conductivity is not critical. Spiral lengths of 650–700 mm are typical, and the alloy tolerates higher copper levels for better machinability without sacrificing castability. Transmission valve bodies with intersecting oil galleries and 0.8 mm walls are routinely produced at scrap rates below 1.5 %.

A360 and A362 variants with controlled iron below 0.5 % remain the choice for heat-sink geometries that combine 0.4–0.6 mm fins with thick mounting bosses. The low iron prevents formation of β-Al5FeSi platelets that act as flow obstacles in narrow gates.

AZ91D and the newer high-fluidity variants AE42 and AE44 are selected when density below 2.0 g/cm³ is required. Vacuum-assisted die casting at 720 °C routinely fills 0.7 mm lattice structures without misruns. Calcium additions (0.2–0.4 %) further improve fluidity by reducing oxide thickness.

ZA-8 and ZA-12 continue to be used for small, highly detailed components such as lock bodies and connector housings. Their low melting temperature and near-zero superheat requirement give spiral lengths above 800 mm even in air-melt conditions.

Factors That Determine Success or Failure

Superheat is the easiest lever in production. Raising pouring temperature from 680 °C to 720 °C in AlSi9Cu3(Fe) typically increases spiral length by 120–180 mm, but each additional 20 °C above 730 °C sharply raises dissolved hydrogen and oxide content. Most high-volume plants therefore lock superheat between 60 °C and 90 °C and rely on alloying or vacuum to gain the remaining fluidity.

Grain refinement with AlTi5B1 and eutectic modification with AlSr10 master alloys are now standard practice. Measured improvements of 80–120 mm in spiral length are common, and the reduction in hot-tearing susceptibility often eliminates the need for higher pouring temperatures.

Oxide films are the silent fluidity killer. A single folded bifilm can reduce local flow distance by more than 50 %. Rotary degassing to hydrogen levels below 0.15 ml/100 g and ceramic foam filtration (30–50 ppi) are the most reliable countermeasures when using secondary alloys.

Gate velocity must stay in the 30–50 m/s window. Below 30 m/s the flow becomes laminar and traps air against the die surface; above 60 m/s atomization creates oxide networks that later appear as porosity bands.

Case Studies from Published Work and Production

A 2020 study on A356 with 0.2 % scandium addition showed spiral fluidity increasing from 520 mm to 610 mm and hot-tearing susceptibility dropping by 40 % in a constrained bar test. The parts were aerospace brackets with 0.7 mm ribs and 18:1 flow ratios.

Industrial conversion from A380 to A383 for a North American transmission case reduced cold shuts from 7.8 % to 0.9 % while maintaining the same cycle time. The only process change was switching to strontium-modified ingot.

A European battery housing with 0.65 mm walls and 280 mm flow length required vacuum die casting and Silafont-36. First-article rejection rate fell from 34 % (with standard A365) to under 1 % after the alloy change and addition of 60 ppm Sr.

Practical Implementation Guidelines

1. Run spiral fluidity tests on every new ingot lot; reject material below the agreed minimum length.
2. Maintain superheat within ±8 °C of target using automated temperature control on holding furnaces.
3. Use 40–50 ppi ceramic filters and rotary degassing on every melt when secondary content exceeds 50 %.
4. Verify modification level weekly with thermal analysis; aim for depression below 6 °C.
5. Simulate every new die with accurate alloy data (liquidus, solidus, viscosity curves) before cutting steel.

Conclusion

Achieving defect-free filling of complex die-casting geometries is no longer a matter of trial and error. Alloys such as Silafont-36, A383, low-iron A360, and vacuum-capable magnesium compositions have demonstrated spiral fluidity lengths well above the thresholds needed for 0.6–0.8 mm walls and flow ratios exceeding 150:1. When these materials are combined with controlled superheat, consistent grain refinement and modification, and clean melt practice, misruns, cold shuts, and excessive porosity essentially disappear from production.

The economic impact is direct: scrap rates drop from double-digit percentages to below 2 %, die life improves because of lower soldering, and downstream machining yields rise. As vehicle electrification and lightweight design continue to drive part complexity higher, the ability to select and control alloys for maximum fluidity will separate leading die-casting operations from the rest.

Q&A

Q1: Which aluminum alloy gives the highest fluidity for 0.6 mm walls?
A: Silafont-36 (AlSi10MgMn) consistently shows spiral lengths above 750 mm and is the current benchmark for structural thin-wall castings.

Q2: Does secondary aluminum lose fluidity compared to primary?
A: Only if oxide and inclusion levels are not controlled. With proper filtration and degassing, fluidity within 5 % of primary material is routine.

Q3: How much does strontium modification improve filling?
A: Typical gains are 80–150 mm in spiral length and a marked reduction in hot tearing in thin sections.

Q4: When should magnesium be considered instead of aluminum?
A: When part weight is critical and vacuum die casting equipment is available; AZ91D with 0.3 % Ca fills 0.7 mm lattices reliably.

Q5: What gate velocity range avoids both misruns and excessive porosity?
A: 35–50 m/s is the safe window for most aluminum and magnesium alloys in complex dies.