4คุณสมบัติทางกลของชิ้นงานที่ขึ้นรูปด้วยผงโลหะ ที่มีความหนาแน่นต่างกัน เปรียบเทียบกับชิ้นงานที่ได้จากการรีดร้อนMaterialTensile strength, MPaTensile as Percent of Wrought Iron TensileElongation in 50 mm (2 in)Elongation as Percent of Wrought Iron ElongationWrought Iron, Hot Rolled331100 %30 %Powder Metal, 84 % density21465 %2 %6%Powder Metal, repressed, 95 % density28385 %25 %83 %
5BenefitsPM parts can be fabricated to final or near-net shape, thereby eliminating or reducing scrap metal, machining and assembly operationLower overall cost (less scrap lost)High melting point metals and composite materials can be producedPM is useful in making parts that have complex shapes or difficult to machinePermits a wide variety of alloy systemsProduces good surface finishes
6Benefits (Cont.)Provides materials which may be heat-treated for increased strength or increased wear resistanceProvides controlled porosity for self-lubrication or filtrationFacilitates manufacture of complex or unique shapes which would be impractical or impossible with other metalworking processesIs suited to moderate- to high-volume component production requirementsOffers long-term performance reliability in critical applicationsIs cost-effective
7DisadvantagesPorosity originates as the spaces between powder particles → low elongationExpensive powder
12Factors affected the QC of PM Powder characteristic: Apparent density (AD), the irregularity and porous texture decreases ADPowder preparation: most metal powder grains are coated by a thin oxide film but will be broken up during the pressing, stable oxide films e.g. SiO2 and Al2O3 cannot reduced during sintering leads to abrasive and tool wearType of compacting press, tool and dieType of sintering furnace, atmosphere, time and temperatureHeat treatment
135. Powder Manufacture PM Standards: Fine powder particles < 20 µm The finer the better preferably 2-10 µmShape of PowdersSponge-like for iron powder gives good green strengthSpheroidal particle gives high density and uniform distribution (see Fig. 2)
15Powder production: There are four main processes Solid-state reduction AtomizationElectrolysisChemical
161. Solid state reductionSolid state reduction is the most widely used for production of iron powder.Process:Selected ore is crushed and mixed with carbonContinuous furnace → Sponge ironFurther crushingSeparation of non-metallic materialSieving→ Irregular fine sponge-like powder
182. AtomizationAtomization: air, nitrogen, argon (for oxidisable metal) and water are commonly used. The particle shape is controlled by rate of solidification of metal droplets.Gas atomization gives spheroidalWater atomized gives a more irregular shape.Each powder produced by this method has the same chemical composition.
213. ElectrolysisBy choosing suitable conditions, such as electrolyte composition and concentration, temperature, and current density, many metals can be deposited in a spongy or powdery state.Further processing–washing, drying, reducing, annealing, and crushing–is often required, ultimately yielding high-purity and high-density powders.
22Copper is the primary metal produced by electrolysis but iron, chromium, and magnesium powders are also produced this way.Due to its associated high energy costs, electrolysis is generally limited to high-value powders such as high-conductivity copper powders.
244. Chemical process Use for production of a high purity, < 5 m. The powders produced can have a great variation in properties and yet have closely controlled particle size and shape.Oxide-reduced powders are often characterized as “spongy,” due to pores present within individual particles.
25Solution-precipitated powders can provide narrow particle size distributions and high purity. Thermal decomposition is most often used to process carbonyls. These powders, once milled and annealed, exceed 99.5 percent purity.
28Sinter-HIPSintered metals ~ 92% density is sufficient to ensure that open porosity at surface has been eliminated HIPed to full density.This process start from sintering then high pressure argon is introduced or vacuum sinter followed by HIPing in a separate apparatus for hard metal cutting tools.