Key Takeaways
- Inconel 625 combustion nozzle printed by DMLS for defence.
- Consolidated several parts into one, removing welds.
- Survives extreme heat and pressure.
- Superalloy additive for mission-critical components.
The Challenge
A Hyderabad-based defence systems supplier manufacturing a gas generator unit for a pyrotechnic actuation system had a combustion nozzle assembly consisting of seven discrete components: a main body, three injector inserts, two retaining rings, and a convergent-divergent nozzle exit cone. The assembly required brazing and Inconel welding at four joints, creating 12 individual braze/weld quality points that each required NDT certification.
The procurement cycle for all seven components — machined from Inconel 625 billet — was 18–22 weeks. The brazing and welding operation added another 4–6 weeks. Total lead time from order to certified nozzle assembly: 24–28 weeks. The programme was supply-chain limited: nozzle assembly delivery was the longest-lead item in the entire actuation system.
The programme office tasked the supplier's engineering team to investigate whether additive manufacturing could compress the lead time while maintaining qualification equivalence to the existing design. Layer X in Ahmedabad was engaged for a DMLS design and build feasibility study.
Technical Requirements
- Material: Inconel 625 (AMS 5666 chemistry, ELI requirements for trace elements)
- Operating temperature: 650°C continuous, 900°C peak (2-second burn events)
- Pressure: 180 bar operating, 270 bar proof test
- All braze joints eliminated — equivalent strength through monolithic construction
- NDT: Radiographic (X-ray) inspection of all walls > 1.5 mm; dye penetrant on all internal surfaces
- Qualification: Mechanical properties (tensile, fatigue) to match AMS 5666 Inconel 625 bar requirements after heat treatment
Part Consolidation Design
The seven-component assembly was redesigned as a single monolithic DMLS part. Key design decisions:
Injector Geometry Integration
The three injector inserts (each separately brazed into the main body in the legacy design) were redesigned as integral internal passages with helical swirl geometry for combustion mixing. Internal helical channels at 35° swirl angle improved mixing efficiency by an estimated 12% versus the legacy straight-bore injectors — a benefit that emerged from the geometry freedom of DMLS that was not achievable in the brazed machined assembly.
Convergent-Divergent Nozzle Integration
The exit nozzle (previously a separate turned component welded to the main body) was printed integrally with the main body. The throat diameter (critical dimension: 8.5 mm ±0.05 mm) was printed at 8.8 mm and EDM wire-cut to final dimension — a hybrid machining approach that provided tighter tolerance control than the legacy turned-and-welded joint.
Lattice-Reinforced Wall Sections
The main body walls between the injector passages — a structurally complex region with competing stress fields from pressure load and thermal gradient — were redesigned with Altair Inspire topology optimisation. The resulting lattice web structure reduced mass in this region by 28% while maintaining equivalent structural performance under 180 bar pressure loading.
DMLS Build and Post-Processing
Inconel 625 was built at 20 µm layer height for maximum detail resolution in the internal injector passages. Build time: 31 hours. The build produced two units (one flight candidate, one destructive test article).
Post-processing sequence:
- Stress relief: 870°C for 1 hour in argon (per AMS 2801)
- HIP (Hot Isostatic Pressing): 1,175°C at 103 MPa for 4 hours — closes any sub-surface porosity to achieve >99.8% density
- Solution anneal: 1,150°C for 1 hour, water quench
- EDM wire-cut nozzle throat to 8.500 mm ±0.02 mm
- Electropolish internal injector passages (Ra from 12 µm to 1.8 µm)
- CMM inspection: 28 critical dimensions, all within drawing tolerances
- Radiographic inspection: No indications
- Dye penetrant inspection: No linear indications
- Tensile testing on companion test bars (from same build plate): UTS 860 MPa, Yield 510 MPa — both exceed AMS 5666 minimums
Results
| Metric | Legacy 7-part assembly | DMLS monolithic (Layer X) |
|---|---|---|
| Component count | 7 | 1 (monolithic) |
| Braze/weld joints | 12 | 0 |
| NDT points requiring certification | 12 joints + volume | Volume only (no joints) |
| Lead time | 24–28 weeks | 8 weeks (design + build + HIP + test) |
| Mass | 1,240 g | 1,060 g (15% reduction) |
| Cost (per unit, 10+ run) | ₹1,85,000 | ₹1,42,000 (23% reduction) |
| Programme critical path | Nozzle assembly on critical path | Removed from critical path |
The monolithic DMLS nozzle passed all qualification tests (proof pressure, hot-fire testing at 180 bar for 10 burn cycles) and was accepted into the qualification database for the programme. The supplier reduced their nozzle assembly supply chain risk by eliminating four external suppliers (three machining vendors + one brazing vendor) and replacing them with a single Layer X production order.
Key Learnings
Part consolidation is the most powerful economic and risk-reduction lever in DMLS. Every eliminated joint eliminates: a separate machined component, a brazing or welding operation, an NDT inspection point, and a supplier qualification requirement. For assemblies with complex internal geometry and high-temperature requirements, Inconel 625 DMLS consistently delivers consolidation ratios of 3–8× with lead time reductions of 60–75%.
Contact Layer X Ahmedabad to discuss part consolidation opportunities in Inconel, Ti-6Al-4V, or stainless steel for your aerospace, defence, or energy application.