Additive Manufacturing Material Science

Week 8

What we will be covering ½

• Lesson 1: Introduction to Additive Manufacturing
• Lesson 2: Additive Manufacturing Materials
• Lesson 3: Mechanical Properties
• Lesson 4: Physical Properties
• Lesson 5: Review - AM Materials
• Lesson 6: Polymers
• Lesson 7: Polymer Properties
• Lesson 8: Thermosets
• Lesson 9: Thermoplastics
• Lesson 10: AM Thermosets and Thermoplastics
• Lesson 11: Selecting Polymers
• Lesson 12: Review - AM Polymers (Quiz)

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Lesson 1/24: Introduction to Additive Manufacturing

• Additive manufacturing (AM) is a versatile field that encompasses a variety of methods, materials, and applications.
• Traditional manufacturing often uses processes, such as machining and grinding, to remove material.
• In contrast, additive manufacturing joins materials together, usually layer-by-layer, to create a final product. These products are built based off of a 3D computer model.

Lesson 2/24: Additive Manufacturing Materials

• All methods used in AM are constantly evolving as manufacturers develop new AM-compatible materials, making additive manufacturing a highly dynamic field
• Most AM processes use materials such as metal or polymer; however, some AM processes can use ceramics and composite materials
• Each material possess a unique characteristic which determines what AM process will suit it best

Lesson 3/24: Mechanical Properties

Tensile strength tests measure a material's ability to resist deformation

• Strength describes a material’s ability to resist various types of stress
• For example, compression strength refers to a material’s ability to resist being squeezed, and tensile strength refers to the materials resistance to being pulled apart by tensile stress. Yield strength is the highest amount of stress a material can withstand before permanent deforming.

Toughness tests measure a material’s resistance to sudden impacts

• Toughness describes a material's ability to absorb stress without breaking
• Impact toughness refers to the material’s ability to absorb a sudden blow
• This is tested by dropping a pendulum onto a material
• Damping toughness describes a material's ability to absorb and resist vibration

Lesson 4/24: Physical Properties

• Electrically conductive materials are used in electrical wires
• A materials electrical characteristics determine its reaction when exposed to electricity
• The more freely electrons can move in a material determines how conductive it is
• Magnetic materials attract or repel other materials
• A materials magnetism describes its ability to attract or repel other materials
• The charges of a materials electrons generate magnetic fields that flow from one end to the other
• The opposite charges attract while the same charges will repel each other
• Certain materials generate their own magnetic fields, others can magnetize when exposed to magnets, and some are not magnetic at all
• A materials reaction to extreme temperatures depends on its thermal characteristics
• A materials thermal characteristics determine its reaction when exposed to heat or changes in temperature
• Changes in temperature cause a different amount of thermal expansion in all materials
• Thermal stress is caused by a material expanding or contracting, and it can warp or distort a material
• Thermal conductivity is the materials ability to absorb heat, materials that are good thermal conductors, are also usually good electrical conductors
• Material corrosion occurs due to environmental exposure
• A materials chemical characteristics determine its reaction when exposed to environmental pollution, oxidation, and radiation

Lesson 5/24: Review - AM Materials

• Strength - A measure of a material’s ability to resist being deformed by various stresses
• Toughness - A measure of a material’s ability to absorb stress
• Magnetism - A measure of a material’s ability to attract or repel other materials
• Corrosion Resistance - A measure of a material’s ability to withstand exposure to environmental elements
• Hardness - A measure of a material’s ability to resist penetration
• Thermal Conductivity - A measure of a material’s ability to hold and transfer heat.

Lesson 6/24: Polymers

• Polymers consist of mer units, or monomers, which are numerous linked and repeating units of atoms
• These atoms bond together to form long chains of polymer molecules, called macromolecules
• 1 macromolecule of polymer material contains at least 100 mer units; however, most polymers contain 1,000 mers or more, and some may even contain more than even 10,000 mers per macromolecule
• Polymer molecules can form many different configurations, a polymers molecular structure determines its properties
• While there are natural polymers, most AM methods use various synthetic polymers, which include additives to alter their properties

Lesson 7/24: Polymer Properties

• Viscoelastic materials can stretch and return to their original shape
• Most polymers are viscoelastic materials, exhibiting both elastic and viscous characteristics
• Viscosity measures a material’s resistance to flow and is usually affected by temperature
• Elasticity measures a material’s ability to revert back to its original shape after being stretched
• Elastic materials will stretch or bend in response to stress, but will return to their original shape when the stress is removed
• Most polymers exhibit viscoelasticity because they can stretch and return back to their original shape in the short term, but will flow and lose their shape over time

• UV exposure can lead to thermal degradation
• Polymers are often more susceptible to heat damage than other materials
• UV ray exposure often leads to materials overheating and experiencing thermal degradation, during which polymer molecules can break down
• Over time, the polymer can discolor and fracture
• Most polymers have a high coefficient of thermal expansion, so they are more likely to change shape with higher temperatures
• UV ray exposure can also weaken a polymer’s atomic bonds, if these bonds break, then oxidation, another form of material breakdown, may occur

Lesson 8/24: Thermosets

• Polymer materials fall into one of two groups based on their molecular arrangement
• The first group consists of thermoset polymers, which are either liquid or solid at room temperature
• A thermoset consists of long monomer chains connected by a type of strong primary bond, called a covalent bond
• Manufacturers prepare a thermoset resin for manufacturing by providing reactive sites in two different areas of the monomer chain
• These reactive sites allow the thermoset resin to undergo curing, a process that uses heat, pressure, or ultraviolet radiation to initiate cross-linking between multiple molecule chains
• Cross-linking prevents individual chains from moving freely and independently from one another, upon cooling, the thermoset creates a permanently solid 3D structure
• Due to their cross-linking, thermosets can be heated, cooled, and formed into shape only once, any additional exposure to heat will char the thermoset part
• The degree of crosslinking determines a thermoset’s properties
• Increased crosslinking results in a stronger polymer that resists damage from many chemical and environmental elements, thermosets generally also have high levels of strength, rigidity, and heat resistance.

Lesson 9/24: Thermoplastics

• Thermoplastic molecules are held together by secondary bonds, which typically have very little cross-linking, and as a result, thermoplastic molecules move very freely when heated, and more space exists between the molecules as they untangle
• As they cool, the molecules form different arrangements
• Secondary bonds give thermoplastics a lower melting point, greater softness, and less rigidity than other materials, such as metals or ceramics
• Thermoplastics are easily shaped through molding or other shaping processes, and they retain their desired shape after they cool
• Branched Molecule Chain
• Thermoplastic molecule chains have two different arrangements: a linear or a branched arrangement, a linear consists of a long, but flexible, chain of monomers that connect end-to-end with each other
• Chain length affects a polymers strength, with longer chains resulting in stronger materials
• A branched arrangement consists of smaller chains of mer units that attach themselves to a longer polymer chain at irregular intervals, forming a tree-branch configuration
• A branched arrangement results in a thermoplastic with a lower density and melting point and melting point, but a higher viscosity and level of toughness than thermoplastics with a linear arrangement

• There are two groups of thermoplastics, the first group are the amorphous thermoplastics, which have amorphous regions consisting of molecule chains, and sometimes branches, that are usually large and complex
• These chains are randomly ordered and coiled, often intertwining with one another, which allows room for more heated molecules to move around, as a result, amorphous thermoplastics have a glass transition temperature rather than a true, defined melting point, so they soften gradually and become more pliable when heated and have a lower viscosity with higher temperatures, after cooling amorphous thermoplastics typically cool into a transparent metal
• Semicrystalline thermoplastics have both amorphous and crystalline regions
• A crystalline region consists of small and simple molecule chains that are usually structured and formed regular, repeating patterns
• Semicrystalline plastics have tightly packed molecules connected by stronger bonds that restrict molecular movement, and as a result these semicrystalline thermoplastics have a higher melting point rather than a glass transition temperature, additionally, due to the presence of crystalline regions, semicrystalline thermoplastics are higher in both strength and rigidity than amorphous thermoplastics

Lesson 9/24: Thermoplastics Cont.

• Thermosets recyclability
• Most thermoplastics have a high molecular weight, resulting in a strong and stiff material; however, high molecular weight can also result in a higher viscosity, decreasing the flow of a material during manufacturing
• After a thermoplastic cools and solidifies, manufacturers can heat and form the material for a secondary shaping process, this is a continuous cycle, so thermoplastics can be repeatedly heated and cooled with minimal effect on end-property performance, as a result, thermoplastics can be reused and recycled, extending their use

Lesson 10/24: AM Thermosets and Thermoplastics

• AM Photopolymer Part
• Most AM thermosets are proprietary photopolymer that mimic the specific properties of other polymers; for example, some proprietary thermosets possess the properties of polypropylene (PP), so they are economical, low-density polymers with high toughness, flexibility, and fatigue resistance
• Other proprietary thermosets mimic the properties of polyethylene (PE), resulting in relatively inexpensive polymers that have good ductility and impact strength as well as excellent chemical and electrical resistance
• Manufacturers design such proprietary thermosets with specific properties that are ideal for an array of AM applications

• AM Thermoset Part
• Other Am thermosets also include polyurethanes (PU), polyimides (PI), and ultraviolet-curable (UV-curable) epoxies
• Polyurethanes are wear-resistant materials that are used in a variety of applications, such as molds for both electrical and automobile parts
• Polyimides are lightweight, flexible materials that have excellent heat and chemical resistance, as a result, these are often ideal for use in AM electrical applications
• Finally, UV-curable epoxies are tough, heat-resistant materials with high chemical resistance
• AM manufacturers sometimes use UV-curable epoxies to create devices used in the aerospace, medical, and electronic industries

• AM Thermoplastics
• Acrylonitrile butadiene styrene (ABS) materials are versatile, lightweight thermoplastics that exhibit excellent toughness as well as resistance to heat, chemicals, and impacts
• ABS has poor electrical conductivity, so AM manufacturers can use it to build insulating parts, such as electrical enclosures as well as tools used for electronic assemblies

Lesson 10/24: AM Thermosets and Thermoplastics Cont.

• AM Thermoplastic Part
• Polycarbonate (PC) materials are lightweight thermoplastics that offer excellent heat resistance and impact strength
• As a result, AM manufacturers use polycarbonates to build parts that offer protection from exposure to high temperatures and impacts, such as medical-device casings and safety helmets

• AM Dental Application
• Polyamide (PA), or nylon, materials are exceedingly tough thermoplastics that exhibit high levels of ductility, impact strength, and wear resistance
• Additionally, polyamides are biocompatible materials that are able to build parts with excellent tolerances and surface finish quality
• As a result, AM manufacturers can use polyamides in dental and medical applications

Lesson 11/24: Selecting Polymers

• Selecting Polymers
• Since polymers were the first materials used in AM processes, there is a large and diverse body available to caboose from when selecting a polymer for AM use
• Additionally, manufacturers are constantly developing new polymers that possess unique, and advanced properties
• Selecting an appropriate polymer first depends on the type of AM method with which it will be used, due to the nature of the processes, material extrusion and powder bed fusion exclusively use thermoplastic filaments or pellets
• On the other hand, material jetting and vat photopolymerization, both of which use curing processes, exclusively rely on thermosets
• Polymer selection also depends on the properties required for the finished part
• For example, polymer selection should take into account the material’s tensile strength, moisture resistance, and flammability
• Additionally, selection can further be narrowed down using a material’s optical properties, such as its level of transparency or opacity, its color, and its index of refraction, or the amount of light the polymer reflects
• Prior to making a selection, always refer to material data sheets and test reports provided by the manufacturer to determine its appropriateness for use in an AM process

Lesson 12/24: Review - AM Polymers (Quiz)

1. Thermoplastic molecules are held together by secondary bonds, which are weaker than the covalent bonds that hold thermoset molecules together. (T/F)
2. Thermoset plastics can be divided into amorphous and semicrystalline groups depending on the formation of their molecules. (T/F)
3. Polymer atoms bond together to form macromolecules, which are long molecular chains. (T/F)
4. A very small amount of polymers are available to use in additive manufacturing processes. (T/F)
5. Thermosets and thermoplastics can be used interchangeably between additive manufacturing processes. (T/F)
6. Repeatedly heating a thermoset will damage the material. (T/F)

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Lesson 12/24: Review - AM Polymers (Answers)

• Thermoplastic molecules are held together by secondary bonds, which are weaker than the covalent bonds that hold thermoset molecules together. (T/F)
• Thermoset plastics can be divided into amorphous and semicrystalline groups depending on the formation of their molecules. (T/F)
• Polymer atoms bond together to form macromolecules, which are long molecular chains. (T/F)
• A very small amount of polymers are available to use in additive manufacturing processes. (T/F)
• Thermosets and thermoplastics can be used interchangeably between additive manufacturing processes. (T/F)
• Repeatedly heating a thermoset will damage the material. (T/F)

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Lesson 13/24: Metals

• Several metal groups, including ferrous metals, nonferrous alloys, and superalloys, are compatible with AM processes.
• A metal consists of atoms connected by a strong primary bond, called a metallic bond.
• Most metals are polycrystalline
• A Face Centered Cubic Crystal (FCC) Structure consists of one atom at the center of each of the cubes six sides. As well as an atom in a corner of each side. FCC metals, like aluminum are often ductile.
• A Body Centered Cubic Crystal (BCC) One atom at the center of each side and an atom in each corner. BCC metals, like Chromium, are often hard.
• A Hexagon close-packed (HCP) crystal structure has atoms closely packed together in a hexagon pattern. HCP metals, like titanium are often brittle.

Lesson 14/24: Metal Properties

• Heat treatments can adjust some metals mechanical properties
• Copper is used in many electrical applications due to its high conductivity
• More ductile metals such as copper are easily shaped without breaking, while most steels are relatively hard and can withstand denting and scratching
• Manufacturers can use heat treatment processes, such as annealing, quenching, and tempering, to adjust the hardness of certain metals.
• Most metals conduct heat well, allowing heat to move through them freely
• Metals also have good electrical conductivity
• Some metals have strong magnetism and can act as electromagnets when exposed to electricity or other magnets.
• Metals with low corrosion resistance can be alloyed or coated to resist damage.

Lesson 15/24: Steels

• Steels are popular commercial metals that consist of iron and carbon as well as a small amount of other alloying elements.
• The most common AM steels are tool steels, stainless steels, and maraging steels .
• Tool steels are extremely strong due to high carbon levels, they are wear resistant, hard, and tough.
• Nickel, Manganese, and nitrogen, stainless steels contain high levels of chromium, which provides superior corrosion resistance.
• Stainless steel exhibit excellent strength and ductility
• Maraging steels are low carbon steels, exceedingly strong, tough, hardenable, and wear resistant.
• Maraging steels are nickel, cobalt, molybdenum, and titanium.

Lesson 16/24: Nonferrous Alloys

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• Adding alloying materials, such as copper, magnesium, manganese, silicon, and zinc to plain aluminum increase strength, electrical conductivity, and corrosion resistance
• Titanium alloys are strong, lightweight, and have an excellent strength-to-weight ratio and low coefficient of thermal expansion.
• Often used in medical implants and aerospace applications
• Copper alloys include various bronzes
• Copper offers high ductility, thermal and electrical conductivity, and corrosion resistance. Tin is the most common alloying element in bronze although aluminum and silicon may be used.
• These alloys provide greater strength, toughness, stiffness, and fatigue resistance

Lesson 17/24: Superalloys

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• A superalloy is a complex metal with superior hardness and strength as well as exceptional thermal, corrosion, and creep resistance
• Most are proprietary materials that manufacturers designed for specific, advanced applications in aerospace, biomedical, or nuclear power industries
• Most common AM superalloys are cobalt-chromium based which may contain combinations of titanium, molybdenum, aluminum, tungsten, tantalum, and niobium. Cobalt-Chromium
• Nickel-based superalloys provide a good balance of tensile strength, creep strength, and rupture strength

Lesson 18/24: Metal Powdering Processes

• Water atomization is the least expensive process because of water’s relatively low cost, availability, and recyclability
• It can be used for most AM metals except for reactive metals
• Compared to gas-atomized powders, water atomization often produces tough metal powders, that have a larger, irregular shape
• Manufacturers use gas atomization for reactive metals and superalloys
• Gas atomization produces spherical metal powders that are more uniform in size with smaller diameters and have a lower surface-oxygen content than water-atomized powders
• Gas atomization is a more expensive process due to the additional equipment necessary to control gas pressure as well as safely store pressurized gases

Lesson 19/24: Selecting Metals

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• As with polymers, only certain AM processes can use metals.
• Powder bed fusion, binder jetting, and sheet lamination processes can all utilize metal powders, while directed energy deposition (DED) processes can use either metal powder or wire
• Higher levels of surface porosity reduce a metal part’s fatigue resistance since defects, such as cracks, often begin to form at these areas of surface porosity
• DED systems are able to utilize metal powers that have a more irregular shape and size, it is more likely that a finished part may not meet its specifications when using a powder that is not high quality

Lesson 20/24: Review AM Metals

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Review: AM Metals

• Water: A process that produces tough metal powders with a large, irregular shape and size
• Gas Atomization: A process that produces spherical-shaped metal powders with a relatively uniform size
• Super alloys: A group of metals with superior hardness, strength, and resistance to heat, creep, and corrosion
• Maraging steels: a group of metals that gain strength and hardness due to metallic precipitates
• Crystal Structures: Arrangements that are regular, repeating, and uniform atomic patterns in a metal

Lesson 21/24: Ceramics

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• A ceramic is an inorganic compound consisting of metallic and nonmetallic atom
• Common AM ceramic materials include metal oxides, nitrides, carbides, graphites, and glasses
• Most ceramics are exceedingly high levels of hardness, surpassing even those of steel. Despite their increased hardness, ceramics often weigh less than most metals
• Ceramics are poor conductors, but offer excellent resistance to wear and corrosion , and many can withstand extreme temperatures and adverse environments

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Lesson 22/24: Ceramics

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• A composite is a hybrid material that consists of at least two materials, each from a different material group
• Because a composite is a mixture of two different materials, it frequently has properties that are superior to those for each individual material
• Many composites possess a metal’s strength, a polymers light weight, and a ceramic’s rigidity.
• Composites also often have a high strength-to-weight ratio, excellent corrosion and fatigue resistance

Lesson 23/24: Selecting Ceramics and Composites

• Both ceramic and composite materials can be used in powder bed fusion and binder jetting AM processes, while composites can only be utilized in material extrusion, material jetting, and vat photopolymerization processes
• Primary considerations with composite and ceramic materials would be the finishing process
• For example long, fiber reinforcements help to increase a composite materials tensile strength, hardness, and rigidity