[Paper Review] The influence of divergent laser beams on the laser powder bed fusion of a high reflectivity aluminium alloy

Sagar Patel, Haoxiu Chen+2 more|arXiv (Cornell University)|May 17, 2021

Additive Manufacturing Materials and Processes64 references4 citations

TL;DR

This study proposes using a divergent laser beam to stabilize conduction-mode melting in laser powder bed fusion (LPBF) of high-reflectivity AlSi10Mg, achieving over 99.98% density with near-zero subsurface porosity. The approach suppresses melt pool instabilities by avoiding transition and keyhole modes, with a critical aspect ratio threshold of ~0.4 for the conduction-to-transition mode transition.

ABSTRACT

The laser powder bed fusion (LPBF) of aluminium alloys is associated with numerous challenges when compared to other commonly used alloys (e.g., steels and titanium alloys) due to their higher reflectivity and thermal conductivity. This leads to a higher defect density in the final parts, commonly related to melt pool instabilities in the transition and keyhole melting modes. In this work, a laser beam defocusing strategy using a divergent beam is proposed to achieve a stable conduction mode microstructure in AlSi10Mg, a eutectic Al composition that is most studied in LPBF. The effects of conduction, transition, and keyhole melting modes on the final part are studied in detail using processing diagrams, metallography, and X-ray computed tomography. The conduction mode LPBF of AlSi10Mg leads to parts with densities of over 99.98%, with close to no porous defects in the subsurface regions, which are known to directly affect the fatigue life of the final parts. The threshold between conduction and transition mode melt pools is also observed to be at a melt pool aspect ratio (ratio of melt pool depth to width) of about 0.4, which differs from the conventionally assumed 0.5. Additionally, a significant difference is observed in the standard deviation of the melt pool depths for the transition and keyhole mode melt pools, when compared to the conduction mode melt pools. This points to the large differences in laser absorptivity between melting modes, that are exaggerated by the onset of vaporization expected for transition and keyhole mode melting during LPBF of high reflectivity aluminium alloys.

Motivation & Objective

• Address the challenge of melt pool instability in laser powder bed fusion (LPBF) of high-reflectivity aluminium alloys like AlSi10Mg.
• Overcome the high reflectivity and thermal conductivity of AlSi10Mg, which lead to defects and poor processability in conventional LPBF.
• Stabilize conduction-mode melting by using a divergent laser beam to avoid transition and keyhole modes.
• Quantify the threshold between conduction and transition mode melt pools in terms of melt pool aspect ratio.
• Minimize subsurface porosity to enhance fatigue life of LPBF-fabricated AlSi10Mg components.

Proposed method

• Employing a divergent laser beam to increase beam diameter and reduce power density, promoting stable conduction-mode melting.
• Using processing diagrams to map the transition between conduction, transition, and keyhole melting modes based on laser power and scan speed.
• Conducting metallographic analysis to evaluate microstructure and porosity in the final parts.
• Applying X-ray computed tomography (XCT) to quantify and localize porosity, especially in subsurface regions.
• Measuring melt pool aspect ratio (depth/width) to identify the critical threshold between conduction and transition modes.
• Analyzing standard deviation of melt pool depth across different melting modes to assess process stability and absorptivity variation.

Experimental results

Research questions

• RQ1 What is the critical melt pool aspect ratio that distinguishes conduction mode from transition mode in LPBF of AlSi10Mg?
• RQ2 How does the use of a divergent laser beam influence the formation of porosity and microstructure in LPBF-fabricated AlSi10Mg?
• RQ3 What are the differences in melt pool depth variability between conduction, transition, and keyhole modes in high-reflectivity aluminium alloys?
• RQ4 To what extent does the transition to keyhole mode increase laser absorptivity and process instability in AlSi10Mg during LPBF?
• RQ5 Can conduction-mode LPBF with a divergent beam achieve near-theoretical density in AlSi10Mg parts?

Key findings

• Conduction-mode LPBF using a divergent beam produced AlSi10Mg parts with a density exceeding 99.98%, indicating near-theoretical density and minimal porosity.
• The threshold between conduction and transition mode melt pools occurs at a melt pool aspect ratio of approximately 0.4, challenging the conventional assumption of 0.5.
• Significantly higher standard deviation in melt pool depth was observed in transition and keyhole modes compared to conduction mode, indicating greater instability.
• The increased standard deviation in transition and keyhole modes is attributed to higher and more variable laser absorptivity due to vaporization onset.
• Subsurface porosity, a major factor in fatigue life reduction, was nearly eliminated in parts processed under conduction-mode conditions.
• The divergent beam strategy effectively suppresses keyhole formation and associated defects by maintaining stable conduction-mode melting.