Week 9: Metal AM Quiz

Additive Manufacturing - MEC454

• Metal Additive Manufacturing - Laser Powder Bed Advanced Topics, November 25th-26th 2024, taught by Prof Kamran Mumtaz.

Overview

• 1) Brief Metals Recap: This section will cover a brief review of metals.
• 2) Melt Pool Dynamics, Heat Flow & Defects: This section will cover the dynamics of the melt pool, heat flow, and resulting defects.
• 3) Residual Stresses & Support Structures: This section will focus on residual stresses and support structures used in the manufacturing process.
• 4) Microstructural Customisation: This section will explore how to customize the microstructure of the final product.
• 5) Multi-material: This will cover multi-material printing.
• 6) Productivity increase: Improved manufacturing speed will be discussed.

Part 1 of 6: Brief Metals Recap

• This section will cover a fundamental review of metal properties.

Metal AM Techniques

• Powder Bed Fusion: A process using thermal energy (laser or electron beam) to fuse metal powders layer by layer. It produces high-density, functional parts in a single step, with excellent mechanical properties and a wide range of materials.
• Powder/Wire Feed: Laser-based process that melts wire feedstock and deposits it to build a component.
• Sheet Lamination: Method for creating products by layering and bonding sheets.
• Binder Jetting: A technique using an inkjet print head to deposit a liquid binder onto a bed of powder, then selectively fusing the powder particles.
• Other: Alternative techniques.

PBF - Post-processing

• Removal of excess powder: Using techniques like tapping, compressed air, and ultrasonic methods.
• Thermal processing: Relieves stress and improves mechanical properties, including treatments to reduce pores and heal microcracks (e.g., furnace cycles or HIPING).
• Support removal: Often involves wire-EDM, band saws, or hand-finishing to remove support structures.
• Surface finishing: Operations like machining, shot-peening, tumbling, hand-benching, electro-polishing, and abrasive flow machining. Micro-machining (for fine features) and chemical reactions may also occur.

Process Control

• Material: Controlling material properties like morphology, particle size distribution, and layer thickness.
• Process parameters: Adjusting scanning speed, hatch distance, exposure time/dwell, thermal energy (power), and spot size.
• Environment: Monitoring inert gas/vacuum conditions and pressure.

Part 2 of 6: Melt Pool Dynamics, Heat Flow, Defects

• This focuses on the complex processes occurring during the laser melting stages.

LPBF - Lasers

• Materials are melted by absorbing laser energy.
• Typically fibre lasers are used.
• Lasers are versatile, accurate, and have a high energy density.

LPBF - Heat Transfer

• Portion of laser energy is reflected, absorbed, and transmitted by the material (transmission is generally negligible).
• Materials with high reflectance (aluminum, copper) are difficult to process.
• Surface roughness and oxides influence material reflectance.
• Material absorbance increases with temperature.

LPBF - Thermal Conductivity & Melt Energy

• Energy is transformed to heat and flows by conduction, affecting particle contacts.
• High thermal conductivity of some metals (like aluminium and copper) creates large melt pools, making control of part resolution more challenging.

LPBF - Energy Loss

• Heat loss is important after melt pool formation.
• Magnesium is hard to process due to close melt/vaporization temperatures.
• Energy losses include reflection, transmission, thermal diffusion, radiation from the melt pool, and vaporization losses.

LPBF - Melt pool

• The melt pool is complex, with many interacting factors like vaporization, heat radiation/convection/conduction, recoil pressure, and gravity influencing the formation, shape, and stability.

LPBF - Thermo-capillary Flows

• Non-uniform heating creates surface tension variations inducing fluid motion.
• Marangoni convection dominates the melt flow, affecting the melt pool topology.
• Surface tension decreases with increasing temperature (in pure liquid metals).

LPBF - Melt Pool Dynamics

• Wettability/wetting affects interlayer connection (porosity and strength).
• Surface free energies (liquid-vapour, solid-vapour, and solid-liquid) strongly influence wetting and, in turn, part quality.
• Oxide-film on metals often leads to poor wetting.

Capillary instabilities

• Liquid breakup reduces surface tension, this varies with temperature & uneven heat distribution.
• To reduce these instabilities, the length/diameter ratio of the melt pool should be kept small.

Melt Pool Ejection and Recoil Pressures

• Laser interaction with the vaporized material creates plasma plumes, causing shockwaves and pushing melt out of the pool due to recoil pressure.

Oxidation

• Oxidation is combining an element with oxygen; a metal's surface is continually oxidized when exposed to air.
• Metal powders have larger surface areas leading to faster oxidation.
• Aluminum forms very thick oxide layers, making processing more difficult (the laser must break down the oxides before melting).
• Oxidation potential increases with temperature, impacting processes needing an inert environment.

LPBF - Part Properties

• Complete melting produces parts at full density which allows for matching/exceeding the properties of cast and wrought approaches.
• Lower than full density compromises fracture toughness and fatigue properties, potentially leading to premature failure.
• The properties of toughness and fatigue are critical for aerospace industry applications, and similar implants.

LPBF - Porosity and Cracks

• Lack of fusion porosity involves irregular-shaped pores. Correct energy density in the system overcomes these defects.
• Gas occluded porosity is spherical and can be corrected by allowing more time for gas to escape.
• Pores within the powder are also typically spherical and can be addressed with improved powder quality.
• Solidification cracking is due to thermal gradients and rapid solidification causing uneven expansion/contraction of the material, and is addressed with corrective measures to reduce thermal gradients or modify material compositions to increase lattice stress.
• Controlling these defects is crucial for obtaining a high-quality 3D printed part.

Part 3 of 6: Residual Stresses and Support Structures

• Metals expand on heating, contract on cooling.
• Uneven cooling can lead to distortion (and witness marks) requiring mitigation.
• Standard practices include heat treatment to dissipate stresses.

Stresses

• Stresses and Metal Processing: Most metals expand on heating and shrink when cooled. Controlled cooling is needed to avoid distortions and witness marks in the manufactured parts. Mitigation/management systems are used for manufacturing good-quality parts.
• Background: Massive internal tensile stresses are generated during in-built materials processing which causes these stresses to dissipate; the geometry shows the result of cutting the material in the as-built condition, without heat treatment.
• Effects of Residual Stress: Includes part distortion, plate distortion, cracking (with or without applied load), and decreased mechanical properties (visible only when tested).

ALM specific quality challenges

• Distortion can occur in parts with rapid cross-sectional changes.

Cross section change distortion

• Peripheral steps or witness marks are commonly formed due to the non-uniform shrinking of the molten portion.
• Solidified areas tend to shrink proportionally to their lateral spans.
• Alternative geometries can exacerbate the distortion effects.
• The effect is considered when larger cross-sections form/bridge smaller ones during builds—smaller regions will shrink more than the larger regions.
• This phenomenon is consistent across similar geometries, though visual examples may show more or less significant distortion effects.

Support Structures

• Thermal warpage caused by rapid heating/cooling and large thermal variations may occur, necessitating supports/anchors, which limit geometric freedom and increase post-processing, cost, time, etc.
• Supports are also needed for parts that cannot be easily stacked.
• (Polymers vs metal-based SLM methods)

Overhang length

• The length of overhangs can range from 1mm to 15mm.

Warp Measurement

• Measuring warp height (open and closed end) and warp length.

EBM Overhang and Effect of Thickness

• The influence of thickness on overhang capability (measured by the height of warping) is discussed.

LPBF Overhang

• The influence of bed temperature on warping is presented across different thicknesses of the overhang.

How to eliminate supports?

• In conventional processes, reducing thermal gradients in powder beds and enhancing knowledge of processing parameters help reduce residual stress.
• A novel approach of the Anchorless Metal Additive Manufacturing method is discussed; this is an approach to eliminate the need for supports in the build.

Anchorless Selective Laser Melting

• Certain combinations of metals form eutectic alloys with low melting points, allowing for liquid/mushy states that enable easier manufacturing strategies. This specific method will be a discussion point.

ASLM

• The method involves heating powder beds.
• The laser melts the metals together, and the mixture is held at the temperature until full solidification, enabling better part geometries without needing anchors/supports.

Oxidation

• The combination of an element with oxygen.
• Metal surfaces continuously oxidize when exposed to air.
• Powders have a large surface area, resulting in quicker oxidation.
• Aluminum forms a thick oxide layer, making processing difficult (laser must break down the oxides before melting).
• Oxidation risk rises with temperature.

Additional topics discussed in the presentation include:

• Microstructural customisation - controlling microstructural characteristics through varied processing parameters to manufacture custom structures,

• Multi-material integration - different materials can be incorporated in the printing process to achieve desired design and properties.

• Productivity improvements—including the use of more lasers or other technological advancements to increase the process speed.

###General Summary of the Presentation Session

• The overall presentation session dives into the detailed mechanics of laser powder bed fusion (LPBF), along with advanced approaches and techniques.
• It explores different variables and techniques for designing, processing, controlling, and optimizing the manufacturing process itself.
• Various troubleshooting solutions are also provided.
• The session ultimately aims to equip students with the knowledge to perform highly-skilled additive manufacturing processes.

Test your knowledge on the characteristics and processing of eutectic alloys in additive manufacturing. This quiz covers key concepts such as selective laser melting, microstructure, and the effects of temperature on metal properties. Perfect for students and professionals in materials science and engineering.