Laser metal deposition

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Laser metal deposition (LMD) is an additive manufacturing process in which a feedstock material (typically a powder) is melted with a laser and then deposited onto a substrate. [1] A variety of pure metals and alloys can be used as the feedstock, as well as composite materials such as metal matrix composites. [2] [3] Laser sources with a wide variety of intensities, wavelengths, and optical configurations can be used. While LMD is typically a melt-based process, this is not a requirement, as discussed below. Melt-based processes typically have a strength advantage, due to achieving a full metallurgical fusion.

Contents

Laser source

As with selective laser melting, the laser power does not have to be especially high as long as the laser energy is sufficiently concentrated. The achievable rate of material addition depends on both the amount of laser power applied, and the heat of fusion of the feedstock and substrate materials. As different materials absorb different wavelengths of light, it is important that the wavelength of the laser source is appropriately matched to the material's absorption spectrum, to ensure that the amount of energy absorbed by the material is maximised. For example, using LMD to deposit steel is efficiently performed using IR laser sources, while for copper-based alloys green lasers have better absorption. [4]

Types

Several different LMD processes exist, with both the feedstock and laser energy being delivered in different ways and at different locations.

Pre-placed powder

The simplest LMD technique involves pre-placed powders. A powder feedstock is placed onto the surface or a substrate, and a focused laser is then scanned or rastered over it, causing the feedstock to melt and fuse with the substrate. Typically an inert shielding gas is used to reduce the oxidation around the melt zone. This process is similar to selective laser melting, which involves a systematic layer by layer process building an object by selective laser fusion within a bed of powder.

Conventional

In conventional powder-fed LMD, a powder nozzle or nozzles are used, along with a focused laser source. The laser is focused onto the substrate to form a melt pool. Simultaneously, powder is sprayed out of the nozzle as a powder jet plume, directing material into the melt pool, where it melts. As the laser source moves away, the melt pool follows, with the material at the previous location solidifying. This process is typically achieved using a laser cladding head, which integrates the powder nozzles and the laser optics into one assembly, with both focused at a single target location. The size and area of the melt pool and the powder plume can vary widely, and may take on spot or line configurations, depending on the target application. As for powder-placed LMD, a shielding gas is typically used to minimise oxidation. The carrier gas used to deliver the powder is also typically a shielding gas. The LMD process can be used in many ways, such as by scanning over a wide surface to build up a thin (< 1 mm) coating (typically called laser cladding [5] [6] ) or by rastering over one particular area as an additive manufacturing process to build up objects in 3D layer by layer (sometimes referred to as directed energy deposition).

High speed

High-speed LMD (also known as EHLA [7] ) differs from conventional LMD in the focal point of the laser, and in the speed of the cladding process. For high-speed LMD, the focal point is located above the substrate. [8] [9] As powder is sprayed through the focal point, most of the laser energy is absorbed by the powder, where it melts in-flight. This results in molten powder feedstock impacting the substrate, where heat is transferred from the powder into the substrate. This typically results in a lower portion of thermal energy being transferred into the substrate, and as a result high-speed LMD produces a thinner weld bead deposit (typically < 0.5 mm per pass [10] ) with lower dilution and a thinner heat-affected zone compared to conventional LMD. [11] The speed of deposition (the velocity of the melt location on the substrate surface) is typically at least 10 times higher than the speed of conventional LMD, and the rate of material solidification is also faster. [4] The typical effect of these differences, compared to conventional LMD, is a deposit with smoother surface finish, finer grain microstructure, [12] improved corrosion resistance, [13] and higher hardness. [14] Both 2D coatings and 3D additive manufacturing are also possible using high-speed LMD. [15]

Wire feed

Similar to welding processes, LMD can be performed using a metal wire as the feedstock. [1] [2] This can be an advantage the avoids the cost and effort required to produce a feedstock powder.

Supersonic

Supersonic LMD is different from the other LMD processes in that the laser is not used to melt materials. Instead, this is primarily a modified cold spraying process, which is a type of solid-state deposition process involving deposition via a supersonic jet plume of powder. In Supersonic LMD a laser is used to pre-heat the substrate and the powder stream, in order to soften these materials. [16] By avoiding melting, and by operating at a lower temperature, this reduces the chance for oxidation of the feedstock and substrate materials to occur. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Powder metallurgy</span> Process of sintering metal powders

Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can reduce or eliminate the need for subtractive processes in manufacturing, lowering material losses and reducing the cost of the final product.

<span class="mw-page-title-main">Selective laser sintering</span> 3D printing technique

Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power and heat source to sinter powdered material, aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to selective laser melting; the two are instantiations of the same concept but differ in technical details. SLS is a relatively new technology that so far has mainly been used for rapid prototyping and for low-volume production of component parts. Production roles are expanding as the commercialization of AM technology improves.

<span class="mw-page-title-main">3D printing</span> Additive process used to make a three-dimensional object

3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer control, with the material being added together, typically layer by layer.

<span class="mw-page-title-main">Coating</span> Substance spread over a surface

A coating is a covering that is applied to the surface of an object, or substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. powder coatings.

<span class="mw-page-title-main">Superalloy</span> Alloy with higher durability than normal metals

A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Key characteristics of a superalloy include mechanical strength, thermal creep deformation resistance, surface stability, and corrosion and oxidation resistance.

<span class="mw-page-title-main">Powder coating</span> Type of coating applied as a free-flowing, dry powder

Powder coating is a type of coating that is applied as a free-flowing, dry powder. Unlike conventional liquid paint, which is delivered via an evaporating solvent, powder coating is typically applied electrostatically and then cured under heat or with ultraviolet light. The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a thick, tough finish that is more durable than conventional paint. Powder coating is mainly used for coating of metal objects, particularly those subject to rough use. Advancements in powder coating technology like UV-curable powder coatings allow for other materials such as plastics, composites, carbon fiber, and MDF to be powder coated, as little heat or oven dwell time is required to process them.

Titanium powder metallurgy (P/M) offers the possibility of creating net shape or near net shape parts without the material loss and cost associated with having to machine intricate components from wrought billet. Powders can be produced by the blended elemental technique or by pre-alloying and then consolidated by metal injection moulding, hot isostatic pressing, direct powder rolling or laser engineered net shaping.

Spray forming, also known as spray casting, spray deposition and in-situ compaction, is a method of casting near net shape metal components with homogeneous microstructures via the deposition of semi-solid sprayed droplets onto a shaped substrate. In spray forming an alloy is melted, normally in an induction furnace, then the molten metal is slowly poured through a conical tundish into a small-bore ceramic nozzle. The molten metal exits the furnace as a thin free-falling stream and is broken up into droplets by an annular array of gas jets, and these droplets then proceed downwards, accelerated by the gas jets to impact onto a substrate. The process is arranged such that the droplets strike the substrate whilst in the semi-solid condition, this provides sufficient liquid fraction to 'stick' the solid fraction together. Deposition continues, gradually building up a spray formed billet of metal on the substrate.

<span class="mw-page-title-main">Thermal spraying</span> Coating process for applying heated materials to a surface

Thermal spraying techniques are coating processes in which melted materials are sprayed onto a surface. The "feedstock" is heated by electrical or chemical means.

<span class="mw-page-title-main">Cold spraying</span> Coating deposition method

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<span class="mw-page-title-main">Sputter deposition</span> Method of thin film application

Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV. The sputtered ions can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber. Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film.

Solution precursor plasma spray (SPPS) is a thermal spray process where a feedstock solution is heated and then deposited onto a substrate. Basic properties of the process are fundamentally similar to other plasma spraying processes. However, instead of injecting a powder into the plasma plume, a liquid precursor is used. The benefits of utilizing the SPPS process include the ability to create unique nanometer sized microstructures without the injection feed problems normally associated with powder systems and flexible, rapid exploration of novel precursor compositions.

Electron-beam additive manufacturing, or electron-beam melting (EBM) is a type of additive manufacturing, or 3D printing, for metal parts. The raw material is placed under a vacuum and fused together from heating by an electron beam. This technique is distinct from selective laser sintering as the raw material fuses have completely melted. Selective Electron Beam Melting (SEBM) emerged as a powder bed-based additive manufacturing (AM) technology and was brought to market in 1997 by Arcam AB Corporation headquartered in Sweden.

<span class="mw-page-title-main">Ultrasonic nozzle</span> Type of spray nozzle

Ultrasonic nozzles are a type of spray nozzle that use high frequency vibrations produced by piezoelectric transducers acting upon the nozzle tip that create capillary waves in a liquid film. Once the amplitude of the capillary waves reaches a critical height, they become too tall to support themselves and tiny droplets fall off the tip of each wave resulting in atomization.

<span class="mw-page-title-main">Selective laser melting</span> 3D printing technique

Selective laser melting (SLM) is one of many proprietary names for a metal additive manufacturing (AM) technology that uses a bed of powder with a source of heat to create metal parts. Also known as direct metal laser sintering (DMLS), the ASTM standard term is powder bed fusion (PBF). PBF is a rapid prototyping, 3D printing, or additive manufacturing technique designed to use a high power-density laser to melt and fuse metallic powders together.

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<span class="mw-page-title-main">Fused filament fabrication</span> 3D printing process

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<span class="mw-page-title-main">Detonation spraying</span> Method of thermal spraying

Detonation spraying is one of the many forms of thermal spraying techniques that are used to apply a protective coating at supersonic velocities to a material in order to change its surface characteristics. This is primarily to improve the durability of a component. It was first invented in 1955 by H.B. Sargent, R.M. Poorman and H. Lamprey and is applied to a component using a specifically designed detonation gun (D-gun). The component being sprayed must be prepared correctly by removing all surface oils, greases, debris and roughing up the surface in order to achieve a strongly bonded detonation spray coating. This process involves the highest velocities and temperatures (≈4000 °C) of coating materials compared to all other forms of thermal spraying techniques. Which means detonation spraying is able to apply low porous and low oxygen content protective coatings that protect against corrosion, abrasion and adhesion under low load.

Cold spray additive manufacturing (CSAM) is a particular application of cold spraying, able to fabricate freestanding parts or to build features on existing components. During the process, fine powder particles are accelerated in a high-velocity compressed gas stream, and upon the impact on a substrate or backing plate, deform and bond together creating a layer. Moving the nozzle over a substrate repeatedly, a deposit is building up layer-by-layer, to form a part or component. If an industrial robot or computer controlled manipulator controls the spray gun movements, complex shapes can be created. To achieve a 3D shape, there are two different approaches. First, to fix the substrate and move the cold spray gun/nozzle using a robotic arm; the second one is to move the substrate with a robotic arm, and keep the spray-gun nozzle fixed. There is also a possibility to combine these two approaches either using two robotic arms or other manipulators. The process always requires a substrate and uses only powder as raw material.

References

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