Comparative Study of Compositional Dilution between Laser Cladding and Weld Overlay Layers

Research Context and Motivation

Dilution — the mixing of base metal into the cladding or overlay layer — is one of the most critical factors governing the final composition and performance of cladded components. In corrosion-resistant and wear-resistant overlay applications, excessive dilution can compromise the intended alloy chemistry, reducing corrosion resistance, hardenability, or wear resistance below acceptable levels. Conversely, insufficient dilution may lead to poor metallurgical bonding between the overlay and the substrate.

The literature under review conducts a systematic comparative study of dilution behavior between laser cladding (LC) and conventional weld overlay processes, including submerged arc welding (SAW), gas metal arc welding (GMAW), and plasma transferred arc (PTA) powder cladding. The study employs both experimental and numerical approaches to quantify dilution rates under various process parameters and substrate configurations.

Core Findings on Dilution Mechanisms

Dilution Rate Comparison Across Processes

The study reports the following typical dilution rate ranges:

The fundamental reason for the lower dilution in laser cladding is the highly concentrated energy density of the laser beam (10⁶-10⁷ W/cm²), which creates a shallow melt pool with limited interaction volume with the substrate. In contrast, arc welding processes have lower energy densities (10⁴-10⁶ W/cm²) and deeper penetration, resulting in greater substrate melting and higher dilution.

Numerical Modeling of Dilution

The study employs finite element thermal modeling to predict melt pool geometry and dilution rates. The key governing equations include:

• Heat conduction equation with moving heat source (Goldak double-ellipsoidal model for arc processes; Gaussian model for laser processes)
• Liquid-solid interface tracking via enthalpy-porosity method
• Mass balance for dilution calculation: Dilution = V_base_melted / (V_base_melted + V_fill_material)

The model predictions show good agreement with experimental dilution measurements (within ±5 percentage points), validating the approach for process parameter optimization.

Effect of Process Parameters on Dilution

For laser cladding, the dilution rate is primarily governed by the linear energy input (E = P/v, where P is laser power and v is scan speed) and the powder/wire feed rate. Increasing laser power increases dilution, while increasing scan speed or feed rate decreases it. The study identifies an optimal operating window where dilution is minimized while maintaining adequate bond strength:

• Laser power: 2-4 kW
• Scan speed: 0.2-0.8 m/min
• Powder feed rate: 10-30 g/min
• Resulting dilution: 8-18%

For PTA cladding, dilution is controlled by the plasma arc current (typically 100-300 A) and the powder feed rate. The study recommends a powder feed rate of at least 50 g/min for dilution below 20%.

Metallurgical Consequences of Dilution

Phase Composition and Microstructure

The dilution level directly affects the phase composition of the overlay layer. For a Ni-Cr alloy overlay (e.g., Stellite 6) on a carbon steel substrate:

The δ-ferrite phase, which forms when dilution introduces excess iron into the Ni-based overlay, is brittle and reduces toughness. In the context of pressure vessel cladding, this is particularly concerning because δ-ferrite can initiate cracking under thermal cycling.

Bond Strength and Interface Quality

Lower dilution in laser cladding does not necessarily mean weaker bonding. The study demonstrates that laser cladding achieves bond strengths of 400-600 MPa (measured by push-out testing per ASTM G119), which is comparable to or exceeds that of arc weld overlay (350-550 MPa). The key factor is not dilution per se but the metallurgical compatibility and the absence of defects at the interface.

Laser cladding interfaces are characterized by:

• Full metallurgical bonding without unmelted powder particles
• Minimal intermetallic compound formation (e.g., Fe-Ni intermetallics limited to <5 μm thickness)
• Smooth transition from overlay microstructure to substrate HAZ

Process Selection Guidelines

Based on the dilution analysis, the following selection criteria are proposed:

1. For corrosion-critical applications (e.g., nuclear reactor components, chemical processing vessels): Laser cladding or PTA is preferred to maintain overlay composition within tight specifications (dilution < 15%).
2. For wear-resistant applications (e.g., pump shafts, valve seats): Dilution is less critical, and SAW or GMAW may be acceptable due to equipment simplicity and cost.
3. For thick overlay requirements (>5 mm): Multi-pass laser cladding or PTA is recommended; SAW may be more economical for very thick builds.
4. For field repair applications: GMAW or SMAW with pre-positioned cladding strips offers practical advantages despite higher dilution.

Engineering Practice and Quality Control

In pressure vessel fabrication, dilution control is implemented through the following quality assurance measures:

• Chemical analysis of the overlay layer at multiple depths (per ASTM E135 or E146) to verify composition
• Hardness profiling across the overlay thickness to detect composition variations
• Metallographic examination of the overlay-substrate interface to assess bonding quality
• Intergranular corrosion testing (ASTM A262 Practice E) for stainless steel overlays to confirm dilution has not compromised corrosion resistance

A practical challenge in multi-pass overlay is the progressive reduction of dilution with each subsequent pass. The first pass typically has the highest dilution (30-50% for arc processes), while the final pass may have only 5-15% dilution. This results in a composition gradient through the overlay thickness, which must be accounted for in performance prediction.

Study Insights and Implications

The comparative study provides a clear quantitative basis for process selection in cladding applications. The dilution advantage of laser cladding (5-25%) over conventional arc welding (20-60%) is substantial and has direct implications for alloy design freedom — lower dilution means the as-deposited composition more closely matches the intended alloy specification, reducing the need for post-deposition heat treatment to correct phase balance.

However, the study also highlights a practical trade-off: laser cladding requires higher capital investment, more precise process control, and greater surface preparation requirements. For large-scale pressure vessel fabrication, the throughput advantage of arc welding processes often outweighs the dilution disadvantage, provided that consumable selection compensates for expected dilution levels.

The most actionable insight for practicing engineers is that dilution must be treated as a design variable, not merely a process outcome. When specifying overlay materials, the expected dilution rate should be incorporated into the alloy selection, and the resulting composition should be verified through chemical analysis during fabrication. This approach ensures that the as-built overlay layer meets the intended performance requirements regardless of the cladding process employed.