How Aluminium Fittings Are Made: Die Casting vs CNC Machining

Two aluminium fittings can sit side by side in a catalogue, look nearly identical, carry the same nominal specifications, and have fundamentally different internal quality levels — because they were made by completely different processes. The manufacturing route determines porosity level, dimensional consistency, thread quality, pressure capability, and ultimately, whether the fitting performs as described or fails unpredictably in service.

Understanding the difference between die casting and CNC machining in aluminium gives you the tools to ask the right questions and make better procurement decisions.

Process 1: High-Pressure Die Casting (HPDC)

In die casting, molten aluminium alloy is injected into a steel die (mould) at high pressure — typically 700–1,200 bar injection pressure. The metal fills the cavity in milliseconds and solidifies rapidly against the cold die surface. The part is ejected in seconds, complete in its near-net shape.

What die casting does well:

• Complex three-dimensional shapes in a single shot — internal passages, external ribs, bosses, and features that would be impossible or prohibitively expensive to machine
• Very high production rates — a large HPDC machine can produce hundreds of fittings per hour
• Consistent external geometry — die cavities reproduce shape with high repeatability
• Low cost at high volumes — the tooling cost is high, but the per-part cost is very low once amortised

What die casting does poorly:

• Internal porosity — gas and shrinkage porosity is inherent in the die casting process. Turbulent metal flow traps gas; rapid solidification causes shrinkage voids. In a pressure-bearing fitting, porosity is a direct failure risk — a pore at the fitting wall creates a leak path.
• Mechanical properties — die cast aluminium (typically A380 or A360 alloy) has lower ductility and fatigue resistance than wrought aluminium. Ductility of A380 is approximately 3% elongation at break; 6061-T6 wrought is 12–17%.
• Surface finish on critical sealing surfaces — die cast surfaces have a "skin" that is relatively good, but machined sealing faces are always more reliable than as-cast faces.

Addressing Die Casting Porosity

The industry has developed solutions for porosity in die cast aluminium:

Vacuum die casting: The die cavity is evacuated before injection, reducing trapped gas. Significantly reduces gas porosity but adds process complexity and cost.

Impregnation: After casting, parts are placed in a vacuum/pressure chamber with a liquid sealant (typically anaerobic resin). The sealant fills porosity and then cures. This is a standard process for pressure-critical die cast parts and is widely used in automotive and pneumatic fitting production. Impregnated die cast fittings can reliably hold pressure that untreated castings cannot.

Solution heat treatment: T6 heat treatment of die cast aluminium improves mechanical properties but can cause blistering at subsurface porosity — a process limitation that means not all die cast alloys can be heat-treated.

Process 2: CNC Machining from Wrought Bar or Plate

CNC-machined aluminium fittings start with wrought aluminium — bar stock or extrusion — that has been hot-worked and heat-treated to the T6 condition. CNC lathes and machining centres then machine the fitting to its final geometry.

What CNC machining does well:

• Excellent mechanical properties — wrought, heat-treated aluminium (6061-T6, 6082-T6) has superior strength, ductility, and fatigue resistance compared to die cast alloys
• No porosity — wrought bar has effectively zero internal porosity
• Tight dimensional tolerances — CNC machining holds ±0.01–0.05mm routinely; die casting achieves ±0.1–0.3mm before machining
• Excellent thread quality — cut threads in 6061-T6 are clean, consistent, and strong
• Full traceability — wrought bar has material certifications and heat number tracking

What CNC machining does poorly:

• Complex internal geometry — you can only machine what a cutting tool can reach. Die casting can produce internal passages that machining cannot
• Cost at low complexity — for a simple threaded port or elbow, machining from bar produces significant waste material (chips) and is slower than casting

The Practical Decision

When in doubt: ask the manufacturer which process produced the fitting, ask for the material certification, and for pressure-critical applications, ask whether porosity testing or impregnation was performed. A manufacturer who can answer these questions clearly is one who understands their process and its limitations. That is the supplier you want.