Lesson Notes By Weeks and Term v3 - Junior Secondary 3
(b) Metals
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(b) Metals
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Subject: Basic Technology
Class: Junior Secondary 3
Term: 1st Term
Week: 2
Theme: Materials And Processing
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describe different ways of processing; explain the advantages and disadvantages of the different metal processing methods; identify metal alloys; explain the uses of common metal alloys.
casting. High forces are required, especially for cold working. Dies and tooling can be costly.
3. Joining: Description: Methods used to permanently or semi-permanently connect two or more metal pieces.
Sub-methods: Welding: A fabrication process that joins materials, usually metals, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to a strong joint. Nigerian
Example: Fabrication of metal gates, window frames, car chassis repair, construction of tanks and pipelines, structural steelwork. Common types include Arc Welding (SMAW, MIG/MAG) and Gas Welding.
Brazing/Soldering: Processes where two or more metal items are joined by melting and flowing a filler metal into the joint. The filler metal has a lower melting point than the base metals, which are not melted. Brazing uses filler metals with melting points above 450°C, while soldering uses filler metals below 450°
C. Nigerian
Example: Soldering in electronics (TVs, radios), plumbing (joining copper pipes), automotive radiator repair (brazing).
Mechanical Fastening: Using discrete components to hold metal parts together. This can be temporary or permanent. Nigerian
Example: Using bolts and nuts for structural connections, rivets for joining sheet metal (e.g., vehicle chassis, aircraft), screws for attaching panels.
Advantages: Welding creates strong, permanent joints. Brazing/Soldering allow joining dissimilar metals and are suitable for delicate components (soldering). Mechanical fastening allows for easy disassembly and repair. Relatively flexible for repairs and modifications.
Disadvantages: Welding can cause heat distortion, residual stresses, and requires skilled labour. Brazing/Soldering joints are generally weaker than welded joints and may be susceptible to corrosion. Mechanical fastening can add weight and may require drilling holes, potentially weakening the original material. Joints can loosen over time.
4. Machining: Description: A subtractive manufacturing process where material is removed from a workpiece by a cutting tool to achieve a desired shape, size, or surface finish.
Sub-methods: Turning: Used to produce cylindrical parts by rotating the workpiece against a cutting tool (e.g., on a lathe).
Milling: Uses a rotating multi-point cutting tool to remove material from a stationary workpiece (e.g., to create flat surfaces, slots, grooves).
Drilling: Creating circular holes using a rotating cutting tool (drill bit).
Grinding: Using an abrasive wheel to remove small amounts of material for precise dimensions and smooth finishes.
Advantages: Achieves high precision and accuracy. Produces excellent surface finishes. Capable of creating complex geometries and internal features. Can be used on a wide range of metals.
Disadvantages: Generates significant material waste (chips). Can be slow for large material removal. Requires skilled operators and precise machinery. Tooling costs can be high. Nigerian Context/
Examples: Fabrication workshops, automotive repair shops (e.g., reboring engine cylinders, machining brake drums), producing intricate machine parts, custom metal components.
5. Heat Treatment: Description: A group of industrial and metalworking processes used to alter the physical and sometimes chemical properties of a material. This involves heating and cooling operations, usually with metals, to obtain desired properties like hardness, strength, ductility, or toughness.
Common Processes: Annealing: Heating the metal to a specific temperature and then slowly cooling it. This softens the metal, improves ductility, relieves internal stresses, and refines grain structure.
Hardening: Heating the metal to a high temperature and then rapidly cooling it (quenching) in water, oil, or air. This increases hardness and strength but makes the metal more brittle.
Tempering: Reheating a hardened metal to a lower temperature and then cooling it, typically in air. This reduces brittleness and improves toughness, sacrificing some hardness.
Advantages: Allows tailoring of metal properties for specific applications. Can improve machinability, wear resistance, and shock absorption. Relieves internal stresses caused by previous processing.
Disadvantages: Can cause distortion or cracking if not performed correctly. Requires precise temperature control and cooling rates. Energy-intensive process. Nigerian Context/
Examples: Hardening and tempering of tools like cutlasses, hoes, axes, chisels to improve their cutting edge and durability. Annealing of wires to make them more pliable it, typically in air. This reduces brittleness and improves toughness, sacrificing some hardness.
Advantages: Allows tailoring of metal properties for specific applications. Can improve machinability, wear resistance, and shock absorption. Relieves internal stresses caused by previous processing.
Disadvantages: Can cause distortion or cracking if not performed correctly. Requires precise temperature control and cooling rates. Energy-intensive process. Nigerian Context/
Examples: Hardening and tempering of tools like cutlasses, hoes, axes, chisels to improve their cutting edge and durability. Annealing of wires to make them more pliable for bending.
B. Metal Alloys
1. Definition: An alloy is a mixture of two or more elements, at least one of which is a metal. The combination usually results in a material with properties superior to those of the individual pure metals.
2. Reasons for Alloying: Increased Strength and Hardness: Pure metals are often too soft for many applications.
Improved Corrosion Resistance: Adding specific elements can make an alloy more resistant to rust and degradation.
Enhanced Appearance: Alloys can be made with desirable colours or lusters.
Modified Melting Point: Alloys can have lower or higher melting points than their constituent metals.
Better Machinability or Castability: Some alloys are easier to work with.
3. Common Metal Alloys and their Uses: Steel: Composition: Primarily Iron (Fe) with a small percentage of Carbon (C) (typically 0.05% to 2.1%). Other elements like Manganese, Silicon, etc., are also present.
Properties: High strength, hardness (can be varied by carbon content and heat treatment), ductility.
Uses (Nigerian Context): Mild Steel: Most common, used for reinforcement bars (rebar) in concrete construction, gates, window frames, car bodies, roofing sheets, general structural work.
High Carbon Steel: Used for tools like cutlasses, hoes, chisels, hammers, springs, knives (due to its hardness and ability to hold a sharp edge).
Stainless Steel: Composition: Steel alloyed with a minimum of 10.5% Chromium (Cr), and often Nickel (Ni).
Properties: Excellent corrosion resistance (does not rust easily), good strength, hygienic, attractive appearance.
Uses (Nigerian Context): Cutlery (spoons, forks, knives), kitchen sinks, cooking utensils, surgical instruments, decorative architectural elements (e.g., railings, gates), food processing equipment.
Brass: Composition: Copper (Cu) and Zinc (Zn) as the primary constituents.
Properties: Golden appearance, good strength, excellent machinability, good electrical conductivity, good corrosion resistance (especially to water).
Uses (Nigerian Context): Plumbing fittings (taps, valves), musical instruments (trumpets, trombones), decorative items, door handles, electrical connectors, bullet casings, traditional sculptures and crafts.
Bronze: Composition: Copper (Cu) and Tin (Sn) as the primary constituents, sometimes with other elements like Aluminium, Manganese.
Properties: Stronger and harder than pure copper, good corrosion resistance, good wear resistance, excellent casting properties, distinctive reddish-brown colour.
Uses (Nigerian Context): Bearings, gears, bells, sculptures (e.g., the famous Benin Bronzes), marine fittings, propellers.
Duralumin: Composition: An alloy of Aluminium (Al) with Copper (Cu), Magnesium (Mg), and Manganese (Mn).
Properties: Light-weight, high strength-to-weight ratio, good workability.
Uses (Nigerian Context): Aircraft components (fuselage, wings), lightweight vehicle parts, ladders, scaffolding, high-strength structural components where weight is a concern.
Solder: Composition: Typically an alloy of Tin (Sn) and Lead (Pb) (though lead-free solders are now common, using tin with silver or copper).
Properties: Low melting point, good flow characteristics, good electrical conductivity.
Uses (Nigerian Context): Joining electrical components (circuit boards, wires), plumbing connections, repair of radiators.
Cast Iron: Composition: Iron (Fe) with a relatively high carbon content (2.1% - 4%), and Silicon (Si).
Properties: High compressive strength, excellent castability (pours well into moulds), good wear resistance, relatively brittle.
Uses (Nigerian Context): Manhole covers, engine blocks, heavy machinery bases, pipes, stove grates, agricultural machinery parts. --- This section provides a detailed explanation of metal processing methods and metal alloys, including their advantages, disadvantages, and uses, with specific examples relevant to Nigerian contexts. A. Metal Processing Methods Metal processing refers to the various techniques used to convert raw metals (often from ingots or sheets) into finished products or components, altering their shape, size, and sometimes their internal structure and properties.
1. Casting: Description: This method involves melting metal and pouring it into a mould cavity, which is a hollow form created to the desired shape. The molten metal then cools and solidifies within the mould, taking its shape. Once solidified, the casting is removed from the mould.
Process: Pattern Making: A replica of the final part (the pattern) is made, usually from wood, plastic, or metal.
Mould Making: The pattern is used to create a mould cavity in sand, plaster, or metal.
Melting: The metal is heated to its molten state in a furnace.
Pouring: The molten metal is carefully poured into the mould cavity.
Cooling and Solidification: The metal cools and solidifies within the mould.
Fettling: The solidified casting is removed from the mould, and excess material (like risers and gates) is trimmed off. Further cleaning and finishing may be done.
Advantages: Allows for the production of complex and intricate shapes that are difficult or impossible to make with other methods. Can produce very large and heavy components. Suitable for brittle metals that cannot be easily shaped by deformation. Relatively low cost for mass production once the mould is made.
Disadvantages: High energy consumption due to melting. Potential for defects like porosity, shrinkage, or cracks if cooling is not controlled. Surface finish may be rough, requiring subsequent machining. Mould making can be complex and expensive for one-off items. Nigerian Context/
Examples: Production of cast iron manhole covers, engine blocks, stove parts, aluminium cooking pots (often done locally using sand casting), traditional bronze sculptures (e.g., Benin Bronzes).
2. Forming (Plastic Deformation): Description: These methods involve applying mechanical forces to deform solid metal blanks into desired shapes without changing their volume significantly. The metal is shaped while it is below its melting point, often at elevated temperatures (hot working) or at room temperature (cold working).
Sub-methods: Forging: Shaping metal by localized compressive forces, typically using hammers (manual or power) or presses. It can be hot or cold. Nigerian
Example: Local blacksmiths making hoes, cutlasses, axes, and other farm implements. Industrial forging produces crankshafts, connecting rods, spanners.
Rolling: Passing metal between two rotating rolls to reduce its thickness, increase its length, or create specific cross-sectional shapes (e.g., sheets, plates, I-beams). Nigerian
Example: Production of steel reinforcement bars (rebar), roofing sheets, aluminium sheets for various uses.
Extrusion: Pushing a metal billet through a die opening to create a uniform cross-section. Like squeezing toothpaste from a tube. Nigerian
Example: Aluminium window frames, door frames, pipes, electrical conduit.
Drawing: Pulling metal through a die opening to reduce its cross-section and increase its length. Primarily used for producing wires and rods. Nigerian
Example: Production of electrical wires (copper, aluminium), barbed wire, nails.
Advantages: Improves the mechanical properties of the metal (e.g., strength, toughness) by refining the grain structure. Minimizes material waste compared to machining. Good surface finish can be achieved, especially with cold working. High production rates for certain products.
Disadvantages: Requires heavy and expensive machinery (e.g., rolling mills, forging presses). Limited to simpler shapes compared to casting. High forces are required, especially for cold working. Dies and tooling can be costly.
3. Joining: Description: Methods used to permanently or semi-permanently connect two or more metal pieces.
Sub-methods: Welding: A fabrication process that joins materials, usually metals, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to a strong joint. Nigerian
Example:* Fabrication of metal gates, window frames, car chassis repair, construction of tanks and pipelines, structural Preparation: Teacher: Gather samples of different processed metals (e.g., welded joint, forged tool, machined part, cast aluminium pot), and samples of common alloys (e.g., steel spoon, brass key, bronze souvenir, stainless steel fork). Prepare charts or diagrams illustrating processing methods and alloy compositions. If possible, prepare short video clips demonstrating casting, welding, or forging.
Students: Bring common metal items from home (with permission) if requested, for identification.
Lesson Introduction (10 minutes): Teacher: Begin by displaying various metal items (e.g., a steel spoon, a brass key, an aluminium pot, a piece of welded metal). Ask students to identify them and discuss their uses. Prompt them to think about how these items got their specific shapes and properties.
Teacher: Introduce the topic: "Metals are transformed from their raw state into useful products through various processing methods, and mixing metals together creates alloys with enhanced properties." Teacher: State the learning objectives in learner-friendly language. Content Delivery and Activities (35 minutes): Exploring Metal Processing (Objective 1 & 2): Teacher Activity: Explain each metal processing method (Casting, Forming, Joining, Machining, Heat Treatment) one by one, using clear definitions, diagrams (if available), and real-life examples from Nigeria.
Teacher Activity: For each method, clearly explain at least two advantages and two disadvantages.
Student Activity: Students will observe the displayed samples and discuss how each might have been processed. They will take notes on the descriptions, advantages, and disadvantages of each method.
Teacher Activity: Facilitate a brief Q&A session after explaining each method to check for understanding. For example, "What method do you think was used to make this aluminum pot, and what is one advantage of that method?" Identifying and Understanding Metal Alloys (Objective 3 & 4): Teacher Activity: Define an alloy and explain the reasons for creating alloys (e.g., improved strength, corrosion resistance).
Teacher Activity: Introduce common metal alloys (Steel, Stainless Steel, Brass, Bronze, Duralumin, Solder, Cast Iron). For each alloy, state its main constituent metals and describe at least two common uses, emphasizing Nigerian examples. Display physical samples of each alloy if available.
Student Activity: Students will identify the presented alloy samples. In pairs or small groups, they will brainstorm where they have seen these alloys used in their homes or communities. They will note down the compositions and uses of each alloy.
Teacher Activity: Lead a class discussion on student findings, correcting misconceptions and reinforcing correct information. "Where in Nigeria do we see extensive use of steel?" (Construction, vehicle parts). "What local craft uses bronze?" (Benin artwork).
Wrap-up and Review (5 minutes): Teacher: Briefly summarize the key points covered: different metal processing methods, their pros and cons, and common metal alloys with their uses.
Teacher: Ask quick recall questions to check immediate understanding (e.g., "Name one advantage of forging over casting," "What are the main components of brass?"). ---
Local Fabrication and Artisanship: Application: Observe metal workshops in local markets (e.g., Aba, Nnewi, Ladipo). Students can see blacksmiths forging tools (hoes, cutlasses), welders fabricating gates and window frames, and local foundries making aluminium pots or cast iron parts.
Integration: The lesson directly relates to the skills and materials used by these local artisans, demonstrating how theoretical knowledge translates into practical trades and entrepreneurship within the Nigerian economy. Students can identify processing methods and alloys being used in their immediate community.
Infrastructure and Construction: Application: Visit a construction site or observe buildings, bridges, and roads being built. Identify the use of steel reinforcement bars (rebar), steel roofing sheets, aluminium window frames, and welded structural components.
Integration: This connects the study of metals to the development of national infrastructure. Students can understand why specific metals (e.g., steel for strength, duralumin for lightweight structures) and processing methods (e.g., rolling for rebar, extrusion for frames, welding for joining) are chosen for different parts of buildings and infrastructure projects.
Automotive and Mechanical Industries: Application: Examine vehicles (cars, motorcycles, trucks). Identify components made from various alloys (e.g., steel chassis, cast iron engine blocks, aluminium alloy wheels, brass fittings in the engine). Observe car repair workshops where welding and machining are commonly performed.
Integration: This topic provides insights into the materials science behind vehicles and machinery. Students learn why specific alloys are chosen for engine parts (e.g., cast iron for durability and heat resistance, duralumin for lightweight pistons) and how repair techniques like welding are essential for maintenance and fabrication in the Nigerian automotive sector. ---