Chapter 6 Powder Metallurgy (PM) 1302 310 Engineering Metallurgy Dr.Sukangkana Lee

Introduction โลหะผงวิทยา เป็นกระบวนการขึ้นรูปชิ้นงานจากผงโลหะ โดยที่ผงโลหะจะถูกอัดในแบบเป็นรูปร่างตามแบบ และผงโลหะจะเชื่อมเข้าด้วยกันโดยการอบในบรรยากาศที่ควบคุม จะทำให้ได้ชิ้นงานที่มีขนาดที่แน่นอน, ต้องการการตัดแต่งน้อย, มีผิวที่สวยงาม และสามารถผลิตในปริมาณที่มากได้

โลหะผงวิทยาถูกนำมาขึ้นรูปโลหะเพื่อผลิตชิ้นงานในอุตสาหกรรมต่าง ๆ ที่มีรูปร่างซับซ้อน หรือวัสดุที่ขึ้นรูปได้ยาก เช่น bearings, structural components and cutting tools กว่า 60 ปี สามารถผลิตได้ทั้งวัสดุที่เป็นโลหะ และ วัสดุผสม ชิ้นงานที่ผลิตได้จะมีความหนาแน่นสูง สามารถผลิตชิ้นงานที่มีรูพรุนได้

คุณสมบัติทางกลของชิ้นงานที่ขึ้นรูปด้วยผงโลหะ ที่มีความหนาแน่นต่างกัน เปรียบเทียบกับชิ้นงานที่ได้จากการรีดร้อน Material Tensile strength, MPa Tensile as Percent of Wrought Iron Tensile Elongation in 50 mm   (2 in) Elongation as Percent of Wrought Iron Elongation Wrought Iron, Hot Rolled 331 100 % 30 % Powder Metal, 84 % density 214 65 % 2 % 6% Powder Metal, repressed, 95 % density 283 85 % 25 % 83 %

Benefits PM parts can be fabricated to final or near-net shape, thereby eliminating or reducing scrap metal, machining and assembly operation Lower overall cost (less scrap lost) High melting point metals and composite materials can be produced PM is useful in making parts that have complex shapes or difficult to machine Permits a wide variety of alloy systems Produces good surface finishes

Benefits (Cont.) Provides materials which may be heat-treated for increased strength or increased wear resistance Provides controlled porosity for self-lubrication or filtration Facilitates manufacture of complex or unique shapes which would be impractical or impossible with other metalworking processes Is suited to moderate- to high-volume component production requirements Offers long-term performance reliability in critical applications Is cost-effective

Disadvantages Porosity originates as the spaces between powder particles → low elongation Expensive powder

Products and applications

Raw Materials Mixing Forming Sintering Optional Operations

Factors affected the QC of PM Powder characteristic: Apparent density (AD), the irregularity and porous texture decreases AD Powder 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 wear Type of compacting press, tool and die Type of sintering furnace, atmosphere, time and temperature Heat treatment

5. Powder Manufacture PM Standards: Fine powder particles < 20 µm The finer the better preferably 2-10 µm Shape of Powders Sponge-like for iron powder gives good green strength Spheroidal particle gives high density and uniform distribution (see Fig. 2)

Powder production: There are four main processes Solid-state reduction Atomization Electrolysis Chemical

1. Solid state reduction Solid state reduction is the most widely used for production of iron powder. Process: Selected ore is crushed and mixed with carbon Continuous furnace → Sponge iron Further crushing Separation of non-metallic material Sieving→ Irregular fine sponge-like powder

Milling

2. Atomization Atomization: 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 spheroidal Water atomized gives a more irregular shape. Each powder produced by this method has the same chemical composition.

GAS ATOMIZATION Fine powder Collection chamber

3. Electrolysis By 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.

Copper 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.

Electrolytic

4. 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.

Solution-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.

Sponge iron-reduced ore

Hot isostatic pressing Isostatic compaction Vacuum melting and Cold forging Hot isostatic pressing Plus hot forging Direct hot isostatic pressing

Sinter-HIP Sintered 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.

Metal injection Moulding (MIM) Powder + Binder Blending Granulation Injection Moulding Green body Debinding Brown body Sintering 99% Density Finished part