[Paper Review] Mechanism of keyhole pore formation in metal additive manufacturing

Lu Wang, Yanming Zhang+2 more|arXiv (Cornell University)|Sep 6, 2021

Additive Manufacturing Materials and Processes7 citations

TL;DR

This study proposes a multiphysics thermal-fluid flow model to simulate keyhole pore formation in laser-based metal additive manufacturing, revealing that pore formation stems from keyhole instability and bubble pinning at the solidification front. The key finding is that uneven recoil pressure and fluctuating energy absorptivity drive keyhole collapse and pore generation, while low ambient pressure stabilizes the keyhole and suppresses porosity by enhancing recoil pressure and reducing shape distortion.

ABSTRACT

Metal additive manufacturing has gained extensive attention from research institutes and companies to fabricate intricate parts and functionally graded materials. However, the porosity of the as-built part deteriorates the mechanical property and even hinders the further application of metal additive manufacturing. Particularly, the mechanisms of keyhole pores associated with the keyhole fluctuation are not fully understood. To reveal the mechanisms of the keyhole pores formation, we adopt a multiphysics thermal-fluid flow model incorporating heat transfer, liquid flow, metal evaporation, Marangoni effect, and Darcy's law to simulate the keyhole pore formation process, and the results are validated with the in-situ X-ray images. The simulation results present the instant bubble formation due to the keyhole instability and motion of the instant bubble when it pins on the solidification front. Moreover, the unevenly distributed recoil pressure on the keyhole surface is an important factor for keyhole collapse and penetration. Furthermore, comparing the keyhole pore formation under different laser scanning speeds shows that the keyhole pore is sensitive to the manufacturing parameters. The keyhole fluctuation features and energy absorptivity variation on the rear keyhole wall could be metrics to evaluate the likelihood of the keyhole pore formation. Additionally, the simulation under a low ambient pressure shows the feasibility of improving the keyhole stability to reduce and even avoid the formation of keyhole pores.

Motivation & Objective

• To understand the underlying mechanisms of keyhole pore formation during laser-based metal additive manufacturing, particularly under keyhole melting mode.
• To address the lack of direct observation and quantitative explanation for keyhole pore formation, especially the role of keyhole instability and dynamic forces.
• To evaluate the influence of manufacturing parameters—particularly laser scanning speed—on keyhole pore formation and stability.
• To investigate the feasibility of reducing or eliminating keyhole pores through low ambient pressure conditions.
• To validate simulation results against in-situ X-ray imaging data for accuracy and physical fidelity.

Proposed method

• Development of a multiphysics thermal-fluid flow model integrating heat transfer, incompressible Newtonian fluid flow, Marangoni convection, metal evaporation, and Darcy’s law for mushy zone resistance.
• Incorporation of laser ray-tracing to compute spatially varying energy absorptivity on the keyhole surface based on geometric shape and incident angle.
• Use of the Boussinesq approximation to model buoyancy forces in the molten pool and surface tension gradients due to temperature-dependent surface tension.
• Application of the Darcy-Forchheimer model to simulate resistance from solidifying dendrites in the mushy zone, influencing bubble motion and keyhole fluctuation.
• Simulation of keyhole dynamics under varying laser scanning speeds and ambient pressures (1 atm vs. 10⁻⁴ atm) to assess sensitivity and stability.
• Validation of simulated instant bubble formation and pinning behavior against in-situ X-ray imaging data to ensure physical accuracy.

Experimental results

Research questions

• RQ1 What are the physical mechanisms driving the formation of keyhole pores during laser-based metal additive manufacturing?
• RQ2 How does keyhole instability, particularly uneven recoil pressure distribution, contribute to keyhole collapse and pore nucleation?
• RQ3 What role does the molten pool flow and drag force from the mushy zone play in trapping bubbles and forming pores?
• RQ4 How do variations in laser scanning speed affect keyhole pore size, shape, and distribution?
• RQ5 Can low ambient pressure reduce or eliminate keyhole pore formation by stabilizing the keyhole geometry and enhancing recoil pressure?

Key findings

• Keyhole pore formation occurs in two stages: (1) instant bubble formation due to unbalanced forces on the rear keyhole wall from uneven recoil pressure, and (2) pinning of the bubble at the solidification front due to high downward flow velocity creating a vertical drag force.
• The drag force from the mushy zone is critical in determining keyhole fluctuation at the molten pool bottom, and a Darcy drag model incorporating grain morphology is essential for accurate simulation.
• Keyhole pore size decreases significantly with increasing laser scanning speed, and pores become more spherical and horizontally distributed at the molten pool bottom.
• Fluctuations in keyhole depth and energy absorptivity—especially on the rear keyhole wall—are strong indicators of pore formation likelihood, with lower standard deviations correlating with reduced porosity.
• Low ambient pressure (10⁻⁴ atm) increases recoil pressure on the rear keyhole wall, stabilizes the keyhole shape, and reduces energy absorptivity fluctuations, leading to no observed keyhole pores in simulation.
• The standard deviation of energy absorptivity is lower under low ambient pressure than under atmospheric pressure, indicating reduced keyhole distortion and improved stability.