Introduction to CNC Metal 3D Printing

 

CNC metal 3D printing combines the precision of computer numerical control (CNC) machining with the flexibility of additive manufacturing (AM). Unlike traditional subtractive methods, this hybrid approach involves building parts layer by layer from metal powders or wires and then refining them with CNC milling or turning to achieve a superior surface quality.

Historical context: Early AM technologies (1980s) focused on polymers, but advancements in laser sintering and direct energy deposition (DED) made it possible to print in metal. Today, industries use CNC metal 3D printing to produce high-strength, complex parts that cannot be milled from solid blocks.

How It Works

• Powder Bed Fusion (SLM/DMLS): A laser fuses metal powder layers in a controlled atmosphere. Post-build CNC machining ensures tight tolerances.
• Directed Energy Deposition (DED): A nozzle deposits molten metal, while a CNC arm shapes the part in real-time. Ideal for repairs and large components.

Example: A turbine blade printed via DED achieves near-net shape, then undergoes CNC finishing for aerodynamic precision.

Advantages

• Weight Reduction: Aerospace parts with lattice structures cut weight by 40% without sacrificing strength.
• Cost Savings: Boeing reduced material waste by 90% using AM for ducting systems.

Materials used in CNC metal 3D printing
A wide range of materials are supported by CNC metal 3D printing, each chosen for specific properties such as strength, heat resistance or biocompatibility.

Common metals and alloys:
Titanium (Ti-6Al-4V): Ideal for aerospace and medical implants thanks to its high strength-to-weight ratio and corrosion resistance.
Aluminium (AlSi10Mg): Used in automotive and lightweight structures thanks to its thermal conductivity and low density.
Stainless steel (316L, 17-4PH): Corrosion-resistant and durable; perfect for industrial tools and marine applications.
Inconel (718, 625): Superalloys for use in extreme environments (e.g. jet engines and the oil and gas industry).
Tool steels (H13, maraging steel): Ideal for moulds and high-wear components.Material selection criteria:
Mechanical requirements: Tensile strength and fatigue resistance.
Post-processing needs: – CNC machining compatibility
– Surface finish
Cost: Titanium is expensive, but this is justified for critical applications.

Industrial applications:
Aerospace & Defence:
GE Aviation: 3D-printed fuel nozzles for the LEAP engine reduced the number of parts from 20 to 1, cutting weight by 25%.
SpaceX: Uses DED to manufacture rocket engine components with internal cooling channels.
Automotive:
Bugatti: 3D-printed titanium brake calipers (lighter than aluminium) for the Chiron.
Local Motors: 3D-printed entire car chassis using hybrid CNC-AM systems.
Medical:
Custom Implants: Patient-specific hip replacements with porous structures for bone integration.
Surgical Tools: Lightweight, steriliseable instruments with complex geometries.
Energy:
Heat exchangers: Optimised internal channels for improved thermal efficiency.
Turbine blades: DED repairs extend the lifespan of high-value components.

Challenges and limitations
Despite its potential, CNC metal 3D printing faces hurdles.

Surface finish: As-printed parts often require CNC machining or polishing to achieve a Ra value of less than 0.8 µm.
Cost: There is a high upfront investment required for machines ($500K–$2M) and metal powders ($50–$500/kg).
Certification: Rigorous qualification is required in the aerospace and medical sectors (e.g. FAA, FDA).
Size constraints: Most powder-bed systems limit parts to <500 mm³, whereas DED allows larger builds, albeit with lower precision.

Future trends and innovations
AI and machine learning
Process optimisation: AI monitors laser power and melt pools in real time to reduce defects.
Generative design: Algorithms create lightweight, topology-optimised parts for AM.
Multi-material printing:
Graded alloys: Print parts with varying properties (e.g. a hard outer layer and a ductile core).
Embedded sensors: Printing with conductive materials enables real-time performance monitoring.
Sustainability:
Recycled powders: Up to 95% of unused powder can be reused.
Energy-efficient lasers: New systems reduce power consumption by 30%.

Case studies:
Case 1: GE Additive’s ATLAS Project
Goal: to print a one-metre aircraft engine bracket in titanium.
Result: 30% weight reduction and 20% cost savings compared to traditional forging.
Case 2: Siemens’ turbine blade repair
Process: DED + CNC machining to restore worn blades.
Savings: 60% cheaper than replacement, with equal performance.

CNC metal 3D printing bridges the gap between design freedom and industrial-grade precision. Although challenges remain, its presence in the aerospace, medical and energy sectors highlights its potential to transform manufacturing. As hybrid systems evolve, expect to see it adopted more widely across industries that are striving for agility, sustainability and cost efficiency.