POWDER METALLURGY

1. POWDER METALLURGY Lecture 8 Petra Christian University

2. ATOMISATION TECHNIQUES • In this process, molten metal is separated into small droplets and frozen rapidly before the drops come into contact with each other or with a solid surface. • Typically, a thin stream of molten metal is disintegrated by subjecting it to the impact of high-energy jets of gas or liquid. Petra Christian University

3. Atomisation Technique • In principle, the technique is applicable to all metals that can be melted and is used commercially for the production of iron; copper; alloy steels; brass; bronze; low-melting-point metals such as aluminum, tin, lead, zinc, and cadmium; and, in selected instances, tungsten, titanium, rhenium, and other high-melting-point materials. • Production rate of commercial atomisation units are 400 kg/min. Petra Christian University

4. Atomisation Technique • Gas atomisation • Water Atomisation • Centrifugal Atomisation Petra Christian University

5. GAS ATOMISATION • The liquid metal stream is disintegrated by rapid gas expansion out of a nozzle. • Gas used are air, N2, He or Ar. • Application: nickel-base superalloys and many other highly alloyed materials. • There are horisontal and vertical gas atomiser. • The particle shape is spherical with a fairly wide distribution. • The product is homogeneous and has good packing properties. Petra Christian University

6. Horisontal Gas Atomisation • Low temperature atomisers. High velocity gas emerging from the nozzle creates a siphon, pulling molten metal into the gas expansion zone. A high gas velocity aids breakup of the metal. During flight through collection chamber, the droplets lose heat and solidify. Petra Christian University

7. VERTICAL GAS ATOMISATION • The melt is prepared by induction melting and is poured into the nozzle. The melt must be superheated over Tm. The gas jets can be formed by multiple nozzles arranged in a circle around the melt stream. Petra Christian University

8. The Melt Melt temperature Melt viscosity as it enters the nozzle. Alloy type, Metal feed rate Nozzle geometry The nozzle-to-melt distance The Gas Gas type Residual atmosphere Gas pressure Gas feed rate and velocity Gas temperature Operating Variables in the Gas Atomisation Process Petra Christian University

9. The Production Parameters for Ni-based Superalloy Powder Petra Christian University

10. The Formation of Metal Powder by Gas Atomisation Petra Christian University

11. Changes of the Droplet Shape • The droplet shape sequence, with distance from the nozzle, occurs as cylinder – cone – sheet – ligament – sphere. Depending on the amount of superheat and other variables, solidification can produce one of these shapes. Petra Christian University

12. The satellites present (Fig. 3.19 left) are due to turbulence and smooth particles (Fig. 3.19 right) formed by control of the flow near the nozzle. The elimination of satellites is importand for good packing and flow attributes. Petra Christian University

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14. WATER ATOMISATION • The most common technique for producing elemental and alloy powders from metals which melt < 1600oC. • High pressure water jets (single jet, multiple jets or an annular ring) are directed against the melt stream, forcing disintegration and rapid solidification. Petra Christian University

15. WATER ATOMISATION • The process is similar to gas atomisation, except for the rapid quenching and differing fluid properties. • Because of rapid cooling, the powder shape is irregular and rough, with some oxidation. Chemical segregation within an alloy particle tends to be quite limited. • Milling may be required after atomisation to eliminate particle bonding. Petra Christian University

16. Four Possible Particle Generation Mechanisms • Particles are generated by cratering, splashing, stripping, and bursting mechanism. • Bursting mechanism results in the smallest powders. • Typical mass flow rate: 5 kg water/kg metal powder. Petra Christian University

17. Process Control Variable • Water pressure • The nozzle-to-melt distance Petra Christian University

18. A 3.5% carbon steel is water atomised, oxidised, milled, decarburised, and annealed to form a sponge which is ground to form an iron powder. • Other materials produced by water atomisation include stainless steel, copper, brass, bronze, tool steel, cobalt, nickel alloys, precious metals, and low melting temperature metals (Pb, Sn, and Zn alloys). Petra Christian University

19. A Comparison Between Gas and Water Atomisation Petra Christian University

20. Water vs Gas Atomisation • The powder shape • The surface contamination Oxide coating can be removed by heating in hydrogen after atomisation. Petra Christian University

21. CENTRIFUGAL ATOMISATION • Reason for development of centrifugal atomisation is the desire to control particle size and the difficulties in fabricating powders from reactive metals. • Combination of a fusion process and a centrifugal force. The centrifugal force throws off the molten metal as a fine spray which solidifies into a powder. Petra Christian University

22. A consumable electrode (anode) made from the desired material and rotates at velocities up to 50,000 rpm. The electrode is melted at its end by either plasma arc or stationary tungsten electrode. Petra Christian University

23. Spherical Steel Powder Formed by Centrifugal Atomisation Petra Christian University

24. The Droplet Formation Events Petra Christian University

25. Some Examples of Centrifugal Atomisers Petra Christian University

26. Atomisation Limitations Petra Christian University

27. Forming Specific Metal Powders Petra Christian University

28. PRESS-AND-SINTER PROCESS • In this process, custom-blended metal powders are fed into a die, compacted into the desired shape, ejected from the die, and then sintered (solid-state diffused) at a temperature below the melting point of the base material in a controlled atmosphere furnace. • The compaction step requires the part to be removable from the die in the vertical direction with no cross movements of the tool members. The sintering step creates metallurgical bonds between the powder particles, imparting the necessary mechanical and physical properties to the part. • Conventional PM offers many advantages over the other consolidation methods. It offers the lowest manufacturing cost, including modest tooling costs. It also produces the closest tolerances in the finished parts. Since it is a net-shape processing technology, it yields parts requiring little or no secondary machining operations. Lastly, it makes available to designers and fabricators a wide variety of material systems from which to choose. • Parts produced through the press-and-sinter process are subject to certain limitations as well. Tooling and the maximum press tonnage capabilities impose size and shape constraints on parts that can be fabricated. Annual production quantities dictate how quickly the costs of tool set-ups and maintenance can be amortized. Finally, the presence of residual porosity in the parts will cause certain physical and mechanical properties to be lower than those of the wrought material. • Typical Press-and-Sinter Products—gears, sprockets, cams, ratchets, levers, clutch plates, pressure plates, housings, pole pieces, bearings, bushingsTypical Markets Using Press-and-Sinter Parts—automotive, appliances, power tools, hydraulics, lawn and garden, agriculture, off-road equipment, motors, firearms, recreational equipment, hardware, business equipment Petra Christian University

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