Die compaction is a forming technique involving the application of uniaxial compaction and shaping of powders with small amounts of water and/or organic binders during compression in a die. Compaction processes include dry pressing – where water content is less than 2% and semidry pressing, with 5-20% water. The compaction force can be applied by mechanical or hydraulic means. Pressure ranges up to several tens of megapascals, and the product sizes range from 1 mm to 100 mm in linear dimensions.
The critical step in die pressing is filling the die uniformly, and the small particles/agglomerates cause problems due to increasing particle-to-particle contact and the entrapment of air. For such fine powders, it is first necessary to granulate the fine particles into larger agglomerates to increase the flowability. The relevant properties of granulates used for die compaction are mean granule size, the density of granules, flow properties of granules, internal powder friction, and external powder-wall friction. Binders and lubricants are often used to improve the mentioned properties. Lubricants are often used to facilitate the uniform transmission of force and reduce friction during compaction, while binders are added to provide compact strength for subsequent handling.
In general, the die compaction process is carried out in the following three steps – feeding of the powder or granulate into the die, compaction of the powder mass, and finally, the compact is ejected from the die.
In a single die compaction process, powders close to the punch and die walls experience better force than the center. This leads to density variations along the length of the sample. This non-uniformity can result in non-uniform properties of the sintered part.
Density variations are significantly reduced in a double-ended die compaction process. In this method, the powder experiences uniform pressure from both the top and bottom, resulting in minimal density variations. However, components with a high aspect ratio still experience high-density variations. Hence, components such as long rods and tubes cannot be produced by die compaction method.
Hot Isostatic Pressing
Hot isostatic pressing (HIP) is a manufacturing process used for reducing the porosity and increasing the densification of metal, ceramic and cermet powders with the application of pressure at elevated temperatures. The HIP unit consists of a pressure vessel, high-temperature furnace, pressurizing controls, and auxiliary systems (material handling, vacuum pumps, metering pumps).
Pressure vessels are usually constructed from low alloy steels, and the purpose of these vessels is to apply uniform gas pressure from all sides. Work zones inside the cylindrical pressure vessels range from 3 inches to 5 feet in diameter with heights from 5 inches to 10 feet. The operating pressures are usually set between 100-200 MPa. An inert gas, argon, is most frequently used as the pressurizing gas so that the material does not react with the gas at high temperatures. In the case of powdered materials, the powder is encapsulated in a container usually made from mild steel. Stainless Steel and glass are also used for constructing the powder container.
Once the container is filled with powder, the container is lowered into the system, and the system is vacuum pumped and sealed hermetically. The temperatures employed in HIP depend on the material powders, but usually, the temperature is in the range 1000-1200 OC. The furnace heats the powder container, while a pressurizing gas is let into the vessel, and the compressor is used to increase the pressure to the desired level. The application of high pressure at elevated temperatures results in densification due to plastic deformation, creep, and sintering.
The need for long-lasting, high-performance components exists in numerous manufacturing industries. The HIP is employed in major industries such as automotive, aerospace, military, heavy equipment manufacturing, industrial machinery, oil & gas, marine, and medical.
Materials most commonly benefitting from the HIP: Superalloy, high-steels, Stainless Steel, Titanium and Aluminum Alloys, Refractory Powder, carbide, advanced ceramics, and composite materials.
- Important parameters
Container materials and thickness are important parameters and must satisfy certain conditions. Firstly, the container must be strong enough to maintain shape and dimensions prior to and during the isostatic pressing. Secondly, the material must be soft, malleable at elevated temperatures, and it must be compatible with the powder being processed. It must not penetrate and react with the powder mass. Also, the container must be leakproof at high pressures.
The container shrinkage is not isostatic during the process, and it depends on parameters such as container material, geometry, thickness, container positioning, and powder fill density within the container..
- Benefits of HIP
- Densification of powdered metal parts to values close to theoretical density (~98-99%).
- Complete elimination of internal porosity in the castings.
- HIP results in highly efficient production as near net shapes are formed to precise tolerances with little or no secondary machining along with reduction in scrap loss.
- The HIP can also provide cost-effective diffusion bonding (cladding) of dissimilar metals.
Cold Isostatic Pressing
Cold Isostatic Pressing (CIP) is used to produce complex parts with very high consistency and uniform density, that is otherwise impossible to produce by other conventional methods.
An isostatic pressure reproduces the conditions found several thousand feet below the surface of the ocean in compressing any object placed within it.
In an isostatic press, hydraulic pressure is generated by pumps, and this pressure can be controlled accurately. A flexible mold designed to form an article is placed at the center of isostatic pressure. A membrane is used to keep the flexible mold dry, and this assembly is contained within the pressure vessel. Two sealing caps, one at top and the other at the bottom, are positioned to retain the mold assembly securely in the vertical plane. A measured amount of powder is fed into the mold cavity over the bottom cap, and when the cavity has been filled, the top cap comes into place to seal it.
The fluid is pumped around a flexible mold assembly and is pressurized. Water or oil is usually used as the pressurizing medium, and applied pressure ranges from 100-400 MPa. As the pressure increases, the loose powder is compacted into a solid shape. The amount of compaction depends on the amount of pressure applied and the material powder used. After the compaction, the pressure is released in a controlled manner. The speed of decompression is changed to suit different materials and compacted shapes. The loose, free-flowing powder is now a solid shaped compact with a uniform density throughout. The flexible mold moves away from the compacted part and resumes its original shape, ready for the next cycle. When the pressure is zero, and the bag has totally regained its shape, the contact can be removed. The top tool is moved upwards in preparation for the next fill of powder, and the bottom tool is moved downward, taking the compact with it, and the compact can be removed from the tooling before it returns to the pressure vessel for the next cycle.
Powder flow: The powder flow is an essential characteristic of this process. The powder must be able to flow quickly and evenly into the cavity. If it does not, the compact will have inconsistent weight and dimensions. If the powder clogs or bridges on the way into the cavity, no contact will be produced. Different size powders compact in different amounts resulting in different sized components. Depending on the natural characteristics of the powder, there may be a need to add binding agents to increase the component strength after pressing.
Applications of CIP: Hydraulic aircraft fittings using titanium powder and aluminum-vanadium master alloy powder, high-speed tool steels, compacts with internal threads, long hollow cylinder filters, and tungsten and molybdenum slabs for further forging or/and rolling.
- Benefits of CIP
- Programmable pressure distribution with good accuracy and reduced mechanical stresses of processed materials.
- The ability to process large, complicated, and near-net shapes, saves time and cost during secondary machining processes.
- Capability to produce large aspect ratios (2:1) with uniform densities.
- Green strength allows in-process handling and treatment and lowers the production cost.
Slip casting is utilized for compacting metal and ceramic powders to produce large, complex shapes. A ‘slip’ can be defined as a suspension of metal or ceramic powder in a soluble liquid, which is poured into a mold, dried, and sintered.
Slip is usually made of a dispersion agent to stabilize the powder, a solvent to control slip viscosity and facilitate casting, a binder for providing green strength to the cast, and a plasticizer which modifies the properties of the binder.
Formation of an appropriate and consistent slip is necessary for a successful slip casting process, which is achieved by controlling the particle size, size distribution, and the order of component addition, their respective mixing time, and the addition of proper deflocculant.
Water is most widely used as the suspending medium in this process, but alcohol and other organic liquids can also be used. Additives like alginates, ammonium, and sodium salts of alginic acids serve three functions of deflocculant, suspension agent, and binding agent.
The cast obtained in the form of powder suspension in a suspending medium and the slip should have low viscosity and low rate of setting so that it can be readily poured. The slip-cast should be readily removable from the mold.
Steps involved in slip casting are preparation of assembled plaster mold, filling the mold, absorption of liquids from the slip into the mold, removal of the cast from the mold, and trimming of finished parts from the mold.
Slip casting offers certain advantages as the process does not require expensive equipment, and the products that cannot be made from pressing can be made from slip casting. However, slip casting is a slow process and has limited applications commercially. Slip casting is used in the manufacturing of tubes, boats, crucibles, cones, turbine blades, rocket guidance fins.
Sintering is the process of heating the powder compact to elevated temperatures but below their melting point such that powder particles coalesce into a solid due to diffusion, not due to melting. Sintering is performed in order to impart strength and integrity to the powder compact, to remove porosity, and to increase the degree of densification.
The kinetics of the sintering process is split into three stages:
Initially, the particles that are in contact form grain boundaries at the point of contact due to diffusion, and the increase in the initial density of compaction results in a higher degree of coherency in the material. In this stage, necks begin to form at the point of contact between two adjacent particles, and therefore, this stage is referred to as the neck growth stage. There are no changes observed in dimensions in this stage, and the porosity also does not decrease. The driving force for the neck formation is the energy gradient resulting from different curvatures of the particles and the neck. In the initial stages, surface diffusion is usually the dominant mass-transport mechanism, as the compact is heated to the sintering temperature.
In this stage, adjacent necks begin to impinge on each other, resulting in densification and grain growth. The packing density of the green packing is critical as high packing density produces relatively few pores in the final object. The intermediate stage results in pore channel closure, where interconnected pore charnels are closed off isolating porosity. Pore channel closure occurs due to neck growth and due to the formation of new contact points by pore shrinkage. Initially, the pores form a network of interconnected cylindrical pores broken up by necks. As the process continues, the pores are smoother and become isolated from each other. In the intermediate stages, bulk transport mechanisms such as grain boundary diffusion and volume diffusion, dominate the sintering process.
In this stage, most of the pores are closed. As sintering proceeds, the pores which formed a network in the intermediate stage, are isolated from each other. The third stage is the slowest stage of sintering. The pores break away from the grain boundaries and become spherical as the grain size increases. Pore shrinkage is the most crucial stage in sintering, and in this stage, solids are transported into the pores, and the gas in the pores escape to the surface. As a result, there is a decrease in the volume of the sintering mass, and smaller pores are eliminated, whereas the larger pores might shrink or grow. In some cases, pore growth during the final stage of sintering leads to a decrease in density because gas pressure in the larger pores inhibits further densification.
Vacuum Induction Melting (VIM)
Melting of the metal achieved via electromagnetic induction carried out under vacuum condition is described as Vacuum Induction Melting. The furnace has a crucible that is refractory lined and is surrounded by an induction coil inside a vacuum chamber. An AC power source is connected to the furnace, which supplies the power with a frequency depending upon the size of the furnace and the material being melted. Under vacuum conditions, the refining is done so as to achieve the desired melt chemistry. Impurity elements are removed using chemical reactions, flotation, volatilization techniques. Then a preheated refractory tundish is inserted through a valve, and the molten metal is then poured into the desired molds.
Advantages of this process are close control of chemical analysis, excellent process control, slag free melt.
Generally, Vacuum Induction Melting process is used to make stainless steel, superalloys, battery, and electronic alloys.
Vacuum Arc Remelting (VAR)
Vacuum Arc Remelting is a secondary melting process that is usually adopted for more refined melts. Vacuum induction or standard air-melted ingots are generally used as feed for the furnace. A consumable electrode made out of the metal that is being remelted is used under a high vacuum. A small amount of the metal being remelted is placed in the bottom of the crucible. DC power is supplied to initiate the arc between the electrode and the base plate. The crucible is designed in such a way that it has a larger diameter than that of the electrode so as to prevent the arcing between them. Water jackets around the crucible are useful for the cooling and solidification processes. The quality of the melt is dependent on the cooling rate, power supply frequency.
This process is generally adopted in order to make superalloys, titanium alloys, zirconium alloys where more homogeneity and cleanliness of the melt is essential.
Electron Beam Melting is a process similar to that of laser melting, which uses an electron beam instead of a laser for melting. Using CAD software, the desired model is designed and then sent to the slicer software, where it cuts the model to successive physical layers of deposited material and sends it to the 3D printer, which initiates the manufacturing process. A layer of the metal powder is distributed on the build platform, and an electron beam is injected, which melts the metal powder. This process repeats as the platform is lowered for the next layer of the metal powder to be coated on the top of the previous one. Therefore, the parts are built as the layers are coated one after the other under vacuum. Once everything is complete, the part is removed, and the unmelted powder is removed with a blowgun. Then the parts are sent for post-processing such as polishing, matching the surfaces, heat treatment to release stresses if necessary.
The advantages of this process include its high density, superior mechanical properties than those achieved by laser melting due to its low thermal stresses.
The drawbacks of this process are it is an expensive process with lots of post-processing requirements. Surface finish is not good, and production is limited to certain metals.
Cold Crucible Induction Levitation Melting
Cold crucible induction levitation melting uses a water-cooled copper crucible that does not react with the melt under vacuum in an induction coil. An electromagnetic field produced by the induction coil goes through the crucible to produce eddy currents that helps heat and melt the metal. The eddy current produces Lorentz force, which helps levitate the liquid metal, which produces zero contaminated highly homogeneous melts.
The advantages of this process are products have negligible crucible contamination, with melts having precise composition and homogeneity.
Refractory metals and alloys adopt this process for the production of titanium, few rare earth metals, superalloys, etc.
Sol-Gel process is the settling of (nm sized) particles from a colloidal suspension onto a pre-existing surface, resulting in ceramic materials. Metal alkoxides are the most widely used precursors in the sol-gel process. Metal alkoxides are the members of the family of metalorganic compounds, which have an organic ligand attached to the metal or the metalloid atom. The most thoroughly studied metal alkoxide is tetraethyl orthosilicate. Metal alkoxides are popular precursors because of their ability to hydrolyze and polymerize easily. Since water and alkoxides are immiscible, it is necessary to use a common solvent to bring them into the solution, and the most commonly utilized solvents are alcohols.
Initially, the metal alkoxide is hydrolyzed, and depending on the amount of water and catalyst present in the solution, the reaction can stop partially or go to completion. Partially hydrolyzed species are accompanied by condensation reactions. Generally, the processes of hydrolysis and condensation polymerization are difficult to separate. The hydrolysis of the alkoxide need not be complete before condensation starts.
As the hydrolysis and condensation polymerization reactions continue, viscosity increases until the solution ceases to flow. The time required for gelation to occur is an important characteristic that is sensitive to the chemistry of the solution and the nature of the polymeric species. This sol-to-gel transition is irreversible, and there is little if any change in volume.
The drying process involves the removal of the liquid phase; the gel transforms from an alcogel to a xerogel. Low-temperature evaporation is frequently employed, and there is considerable weight loss and shrinkage. The drying stage is a critical part of the sol-gel process. As evaporation occurs, drying stresses arise that can cause catastrophic cracking of bulk materials.
The final stage of the sol-gel process is densification. At this point, the gel-to-glass conversion occurs, and the gel achieves the properties of the glass. As the temperature increases, several processes occur, including elimination of residual water and organic substances, relaxation of the gel structure, and, ultimately, densification.
- Benefits of Sol-Gel process
- Versatile – better control of structure, including porosity and particle size, the possibility of incorporating nano-particles and organic materials..
- Extended Composition Ranges – allows the fabrication of any oxide composition, but also some non-oxides.
- Better Homogeneity – high purity, due to mixing at a molecular level.
- Less Energy Consumption – no need to reach the melting temperature as the network structure can be achieved at low temperatures.
Direct Fluoridation Method (DFM)
A specialized process involving a plasma phase reaction to increase the purity, viscosity, and consistency of the material over HF-reaction methods. Then, secondary melting in fluorine gas atmosphere compensates for fluorine stoichiometry deficiencies. Used for military-grade (MIL-SPEC compliant) fluoride production. Read more
Liquid Phase Formulation (LPF)
A liquid-solid solution process used to purify undesired elements out of the compound. For example, it used to separate zirconium out of HfO2 and iron out of Ta2O5 to achieve a very low impurity profile.
High precision machining
Waterjets, lathes, mills, surface grinding, and unique precision-machined part production, including complex components such as dental and bone implants.
Best Effort Basis: Employed for certain new custom or engineered chemicals that we have reservations about producing in one run. Under this arrangement, the customer will be required to pre-pay 70% of the list price. In return, LTS will attempt the production of a material up to three times. After each, if the material is deemed unacceptable by the customer, LTS will refine the process based on the analysis of the previous attempt and try again.