Spray Forming: The Hidden Manufacturing Process Behind Advanced Materials

Spray forming can achieve cooling rates of up to 10^7 K/s during solidification, which is significant as it surpasses those of conventional casting methods. Professor Singer developed this remarkable metallurgical process at the University of Swansea in the late 1960s that creates near-net-shape components with uniform microstructure and minimal segregation. Metal droplets produced through this technique range from 20-200 μm and reach speeds of 50-100 m/s while they cool mid-flight.

Manufacturing capabilities have transformed through the spray forming process, especially when you have spray steel production needs. Material densities between 96-99.5% can be achieved through spray metal techniques with substantially reduced macrosegregation, unlike traditional methods. Osprey Metals Ltd commercialized their spray forming process, which has become invaluable for manufacturing components from challenging materials like nickel-based superalloys and high-speed steel. Steel billets weighing over 1 ton, nickel superalloy ring blanks up to 500 kg, and aluminum alloy extrusion billets reaching 400 kg are now possible through spray forming steel.

This piece examines how spray forming combines traditional Powder Metallurgy and metal injection molding concepts to create advanced materials with improved properties. The technology's step-by-step process and its expanding applications in various industrial sectors are also explored.

 

Evolution of Spray Forming from Traditional Metallurgy

The develoPment of spray forming marks a breakthrough in metallurgical processing that started in the late 1960s. This fresh approach created a link between conventional casting and powder metallurgy that offered unique advantages over both methods.

Singer's Osprey Spray Forming Breakthrough

Modern spray forming technology came from Professor Singer's pioneering work at Swansea University in the 1970s. He created a new method that directed a high-pressure gas jet against a stable stream of molten metal to create atomization. The droplets collected on a target and formed near-dense billets with near-net shapes. This breakthrough led Singer's team to create Osprey Metals Ltd., which became the first company to use this process commercially.

The original goal was to create an alternative way to produce thin metal sheets directly from molten material. This would replace the traditional metallurgical methods that needed casting and forming of large slabs. The Osprey process, as people now call it, has sparked industry interest in the last 35 years despite various technical challenges. Today, about 25 licensees work worldwide, from small research labs to large commercial plants.

Transition from Powder Metallurgy to Spray Metal Techniques

Spray forming bridges the gap between traditional powder metallurgy and direct casting methods. The materials made through spray forming don't have macrosegregation and prior-particle boundaries, unlike conventional powder metallurgy products. This hybrid method blends gas atomization with the creation of integrated blocks or individual elements in one step.

The process saves money by converting molten alloy into semifinished products in a single step. Rapid solidification leads to better properties through microstructural refinement and removes macrosegregation. To name just one example:

• Companies can make spray-formed aluminum alloys at lower costs than powder metallurgy options, with prices close to direct chill cast alloys when produced in large volumes
• These materials show better fatigue life and fracture toughness than powder metallurgy products because they have less oxide contamination
• The refined grain structure creates materials that perform better and have improved isotropy

Integration with Metal Injection Molding Concepts

Spray forming combines elements from both traditional metallurgy and powder-based techniques. Like metal injection molding, it creates near-net-shape components and skips several processing steps. The difference lies in spray forming's continuous operation versus metal injection molding's multi-stage process.

The real-life process starts when metallurgically prepared metal melt moves from a melting crucible through a tundish into the atomization area. Several melting methods have emerged:

• Bottom pour induction units sit right above the atomizer head, using ceramic nozzles to feed from the furnace to the atomizer
• Tilt-pour furnaces use an induction furnace that tilts to pour melt into a conical tundish, which sends molten metal to the delivery nozzle
• Complex systems for nickel superalloys mix vacuum induction melting, electroslag re-melting, and cold hearth crucibles to manage impurity levels

This 35-year-old technology now competes with and wins markets from existing methods like casting, ingot metallurgy, electrode remelting, and powder metallurgy. Knowing how to create specialized alloys with better properties has opened doors to making materials that were hard to produce through conventional methods.

 

Step-by-Step Breakdown of the Spray Forming Process

Spray forming is a sophisticated metallurgical process that merges atomization, rapid solidification, and deposit consolidation into one operation. The process shows its technical complexity as we look at each stage carefully.

Melt Preparation and Inert Gas Control

The process starts with careful preparation of the metal alloy in a crucible using induction heating. The melt stays 50-150°C above the alloy's liquidus temperature to minimize thermal gradients. The melt chamber needs thorough purging with inert gasses like nitrogen or argon to prevent oxidation. A slight overpressure helps maintain this protective atmosphere. Specialized applications such as nickel superalloys need sophisticated melting setups that use vacuum induction melting, electroslag re-melting, and cold hearth crucibles to manage impurity levels.

Tundish Flow and Nozzle Configuration

The liquid metal flows into a conical tundish after preparation and moves to the melt delivery nozzle. This setup separates the melting process from spraying operations, which allows better optimization of each stage. The tundish helps stabilize metal flow and serves as a buffer before atomization. Current systems use either bottom-pour configurations with ceramic nozzles (3-5mm diameter) or tilt-pour systems where an induction furnace tilts to deliver metal into the tundish. Primary gas jets work at intermediate pressures (2-4 bar) to protect the melt stream from disruption in the turbulent spray chamber.

Gas Atomization and Droplet Formation

High-velocity gas jets (250-350 m/s) at high pressure (6-10 bar) hit the metal stream during atomization. These secondary atomizer jets form either an annulus around the melt delivery nozzle or appear as discrete jets in symmetrical positions. This powerful interaction breaks the melt stream into droplets between 10-500 μm in diameter. The gas-to-metal mass flow ratio (GMR) ranges from 1.5 to 5.5 and affects droplet size distribution, cooling rate, and process yield. Higher GMR creates finer droplets but reduces overall process yield from 75% to about 60%.

In-flight Cooling and Droplet Acceleration

Droplets speed up and cool rapidly during flight, reaching speeds of 50-100 m/s. Cooling happens mainly through convection and radiation. Small droplets (<50 μm) become fully solid before landing, medium ones (50-200 μm) reach a semi-solid state, and larger ones (>200 μm) stay mostly liquid. Computer models reveal cooling rates from 10³ to 10⁶ K/s, with 60-80% of latent heat disappearing within milliseconds after atomization. Droplet diameter, initial axial gas velocity, melt flow rate, and initial superheat play crucial roles in flight dynamics.

Deposition and Billet Formation

Droplets merge into a solid deposit with 96-99.5% density on reaching the substrate. The billet surface keeps a solid fraction between 0.3-0.6. The process needs careful balance between incoming droplets' heat and heat loss through conduction, convection, and radiation from the billet. Engineers can adjust this balance through spray height, atomizer gas pressure, melt flow rate, and atomizer setup. Modern techniques use programmable oscillating atomizer drives to make shapes more consistent.

Microstructure Development and Solidification

Final solidification happens at much slower cooling rates (1-20 K/s) compared to in-flight cooling. The microstructure forms from a mix of solid, semi-solid, and liquid droplets. Residual liquid exists as a continuous network marking polygonal grain boundaries at the billet surface. Rapid solidification creates very fine, equiaxed grain structures without macrosegregation. This unique process combines quick cooling of small droplets with moderate final solidification to produce materials that have better microstructural uniformity than conventional casting methods.

Material Capabilities and Alloy Compatibility

Spray forming technology can process many metal alloys that are hard to manufacture through conventional methods and gives them improved properties.

Spray Forming Steel and High-Speed Tool Alloys

High-speed steels made through spray forming show remarkable improvements compared to their cast counterparts. The process creates finer primary carbides that spread more evenly throughout the material matrix. AISI M3:2 high-speed steel made this way has better isotropy in toughness because its carbides are less oriented. Tool steels reach densities of 96-99.5% of their theoretical maximum. The refined structure eliminates rough eutectic carbide networks you'd find in conventional materials, which leads to better hot workability. This is a big deal as it means that steel billets can weigh more than 1 ton.

Aluminum-Lithium and Al-Si Alloys for Aerospace

The aerospace industry values Al-Li alloys for their exceptional weight-to-strength ratio. These alloys cut structural weight by 10-20% compared to traditional aluminum. Each 1% of lithium reduces density by 3% while making Young's modulus 6% higher. Modern aircraft like Airbus A330/350/380 and Boeing 747/777/787 now use third-generation Al-Li alloys such as 2195, 2196, and 2198. Spray-formed Al-Si alloys contain refined silicon particles of 3-5 μm, much smaller than the 100 μm particles in cast materials. These alloys resist wear better, expand less with heat, and are easier to cast.

Nickel-Based Superalloys and MMCs

The spray forming process offers a unique way to make nickel-based superalloys. It combines electroslag refining with spray forming to produce clean, uniform superalloys economically for critical fatigue-life applications. The deposit surface temperature plays a crucial role in controlling porosity for spray-formed IN718 and U720 nickel superalloys, with best results between 1240-1270°C. Spray forming creates two types of Metal Matrix Composites (MMCs): reactive spray forming with gas-liquid reactions and inert spray forming, where neutral particles mix into the metal spray.

Spray Metal vs. Cast Metal Microstructure

The performance differences between spray-formed and cast materials come from their distinct microstructures. Spray-formed materials have consistent equiaxed grains measuring 15-50 μm across the preform, while conventional casting creates irregular, often dendritic patterns. Spray-formed hypereutectic Al-Si alloys show fine, evenly spread silicon phases in an equiaxed aluminum matrix. Nickel superalloys made this way achieve ASTM 6 grain size with minimal isolated porosity, meeting aerospace standards for solution-annealed plus aged tensile properties. These materials last longer and resist fractures better than powder metallurgy alternatives because they don't have oxide contamination.

 

Applications in Advanced Manufacturing

The manufacturing sector has seen rapid growth in spray forming applications. This technology creates components with exceptional microstructure quality. The process also offers near-net shape production advantages.

Spray Formed Rings and Tubes for Turbines

Chemical industries use spray-formed tubes extensively. These tubes have found their most important role in aerospace propulsion systems. Sprayform Technologies International manufactures nickel superalloy rings up to 1,400 mm in diameter. The company operates as a joint venture between Pratt & Whitney and Howmet. Their aerospace sector production reaches 500 tons per year per shift. The nickel superalloy rings feature fine equiaxed grains (ASTM 5-8) without macroscopic segregation. These refined structures make hot workability better and allow more than 50% reduction in just two roll passes.

Flat Sheet and Strip Production via Linear Nozzles

Manufacturers create flat products using two main methods. They either scan (oscillate) the atomizer or use linear slit nozzles. These methods spread the spray cone over specific areas while depositing onto moving substrates. The sheets show refined grain structures that create metal strips with better corrosion resistance and mechanical properties. These materials work well in applications that need high strength-to-weight ratios and uniform microstructure.

Billet Production for Extrusion and Forging

Manufacturers create cylindrical billets by spraying off-center at angles onto rotating cylindrical substrates. Modern facilities can produce billets up to 0.5 meters in diameter and 2 meters in height. The process combines powder metallurgy principles with direct metal deposition to create preforms for later extrusion or forging. Automotive and aerospace industries use these billets after conventional deformation processing. The final components show better tensile strength and hardness than traditional wrought processing methods.

Protective Coatings and Cladding Applications

Spray forming technology serves as a great way to create protective surface treatments. The Process-defined Surface Deposition (PSD) technique adds metal coatings to engine components and high-temperature ceramic shields. Spray forming creates thick, dissimilar steel clad tubes with strong metallurgical bonds between layers in cladding applications. These interfaces can handle residual stresses and later thermo-mechanical processing. Large steel structures in offshore environments get protection from seawater corrosion through self-healing zinc coatings. This technology guards against corrosion for up to 25 years in harsh marine environments.

 

Advantages and Limitations Compared to Powder Metallurgy

Manufacturing technologies each have their strengths and limitations. Spray forming shows distinct advantages and drawbacks compared to 20-year-old methods. The process's economic success depends on balancing its production benefits against technical limitations.

Near-Net Shape Efficiency and Reduced Waste

Spray forming's economic edge comes from cutting down process steps between the melt and the finished product. The process eliminates ingot casting, homogenization, and hot-rolling operations from conventional processing. This leads to:

• Much lower energy consumption
• Better manufacturing costs
• Production rates that reach 4100kg/(h · m) in certain cases

Notwithstanding that, we need to balance economic benefits against technical hurdles. The process uses nowhere near as many steps as powder metallurgy and conventional cast/forge methods. Yet the actual cost benefits depend on solving several key implementation challenges.

Porosity Control and Post-Processing Needs

Porosity remains the biggest problem in spray-formed materials. A typical spray-formed billet has 1-2% porosity and needs extra processing. Hot isostatic pressing (HIP) or thermo-mechanical processing can fix small pores (less than 30 μm). Some applications need complete porosity removal. Surface temperature plays a crucial role in porosity formation. Nickel superalloys show best results between 1240-1270°C. The spray angle also affects final density. Porosity drops to its lowest point when the average weighted impact angle stays under 25°.

Gas Consumption and Overspray Losses

High costs of inert gasses for atomization and material losses pose serious challenges to spray forming's economic viability. The process loses about 30% of material through overspray, splashing, and material bouncing off the semi-solid surface. Gas-metal ratios usually range from 1.5 to 5.5. Higher ratios mean lower yields. Many operators now use particle injector systems to reuse overspray powder to improve efficiency.

Comparison with Metal Injection Molding Yield

Metal injection molding (MIM) works great for complex shapes but costs more than traditional powder metallurgy. This happens because of gas atomization and binder compounding needs. Spray forming creates near-net shapes without removing binders. MIM achieves better final densities (over 95%) by using smaller powder sizes and higher sintering temperatures. Spray-formed materials usually need extra work to get rid of leftover porosity.

 

Conclusion

Spray forming represents a groundbreaking metallurgical breakthrough that reshapes the scene of complex alloy production. This process connects conventional casting, powder metallurgy, and metal injection molding through a unique blend of atomization and direct consolidation techniques. Manufacturers can now produce near-net-shape components with exceptional microstructural uniformity, minimal segregation, and densities that reach 96-99.5% of theoretical values.

The technology removes multiple intermediate processing steps. This streamlines processes and cuts overall production costs while creating specialized alloys that were hard to manufacture before. High-speed tool steels, aluminum-lithium aerospace components, and nickel superalloys show improved properties and performance characteristics with this technology.

Some challenges still exist. The biggest problems include residual porosity, gas consumption costs, and material losses through overspray. Companies adopt solutions like overspray recycling systems and optimized process parameters to tackle these issues.

The economic value of spray forming depends on weighing its technical advantages against implementation challenges. The original investment and operational costs might be higher than standard methods. However, knowing how to produce specialized materials with superior properties makes this worthwhile for many high-performance applications.

Spray forming will without doubt remain a vital technology that connects conventional metallurgy with advanced powder-based techniques. Manufacturers now have a powerful tool to create next-generation materials that meet tough performance requirements in aerospace, automotive, and industrial sectors. Research advances will help this technology become more efficient and cost-effective while expanding material capabilities. This cements spray forming's role in advanced manufacturing.

 

FAQs

Q1. What is spray forming and how does it differ from traditional manufacturing methods? Spray forming is an advanced manufacturing process that creates near-net-shape components by atomizing molten metal into droplets and depositing them onto a substrate. Unlike traditional casting or powder metallurgy, it combines rapid solidification and consolidation in a single step, resulting in materials with refined microstructures and minimal segregation.

Q2. What types of materials can be produced using spray forming? Spray forming is compatible with a wide range of alloys, including high-speed tool steels, aluminum-lithium alloys for aerospace applications, nickel-based superalloys, and metal matrix composites. It's particularly useful for materials that are difficult to manufacture through conventional methods.

Q3. What are the main advantages of spray forming? Key advantages include the ability to produce near-net-shape components with uniform microstructure, reduced processing steps compared to traditional methods, and the capability to create specialized alloys with enhanced properties. It also allows for the production of large billets and components with improved isotropy and performance characteristics.

Q4. Are there any limitations to spray forming technology? While spray forming offers many benefits, it does have some limitations. These include residual porosity in the final product (typically 1-2%), high costs associated with inert gas consumption for atomization, and material losses due to overspray. Additionally, some applications may require post-processing steps to achieve the desired final properties.

Q5. In which industries is spray forming technology most commonly used? Spray forming finds extensive applications in aerospace, automotive, and industrial sectors. It's particularly valuable in producing components for turbines, high-performance engine parts, and specialized tools. The technology is also used in creating protective coatings and claddings for various industrial applications.