Fig 5 shows a bicycle frame made from chromoly steel tubes. The frame tubes are joined using TIG welding. The finished frame is protected with a powder-coat...

(a) An alloy is a mixture of two or more metals, or a metal combined with a non-metal, that is produced to achieve improved or different properties compared to the individual elements. [1] For example, chromoly steel is an alloy of iron with small amounts of chromium and molybdenum added to increase strength and fatigue resistance beyond what plain carbon steel can achieve.

(b)(i) Two mechanical properties that make chromoly steel suitable for a bicycle frame: [2]

1. High tensile strength - chromoly allows thin-walled tubes to be used that still support the rider's weight and the dynamic forces of pedalling, cornering, and braking without permanently deforming. Thin walls mean a lighter frame. [1]
2. Good fatigue resistance - the frame is subjected to repeated stress cycles from road vibrations, pedalling loads, and impacts over thousands of kilometres. Chromoly's fatigue resistance means the frame can endure these cycles over a long service life without developing cracks. [1]

(b)(ii) Two advantages of titanium over steel for a bicycle frame: [2]

1. Titanium is significantly lighter than steel (approximately 45% less dense), reducing the overall weight of the frame, which is a critical performance factor in competitive cycling. [1]
2. Titanium has excellent corrosion resistance and does not rust. The frame does not need a protective coating (paint or powder coat), saving additional weight and eliminating the risk of corrosion under chips in the finish. [1]

(c)(i) TIG stands for Tungsten Inert Gas. [1]

(c)(ii) Description of TIG welding: [3]

1. A non-consumable tungsten electrode is held in the welding torch. An electric arc forms between the tungsten tip and the workpiece, generating intense, focused heat that melts the parent metal (chromoly steel) at the joint. Unlike MIG, the electrode does not melt and is not consumed. [1]
2. A separate filler rod of matching or compatible material is fed manually by the welder's other hand into the molten weld pool to build up the joint and fill any gap between the tube ends. [1]
3. An inert shielding gas (usually argon) flows through the torch nozzle and surrounds the tungsten electrode and weld pool. This gas shield prevents atmospheric oxygen and nitrogen from reacting with the molten metal, which would cause oxidation, porosity, and a weak, contaminated weld. The result is a clean, high-quality, precisely controlled weld bead. [1]

(d)(i) Two stages of preparation before welding the frame tubes: [2]

1. Clean and degrease the tube ends to remove oil, grease, fingerprints, and other surface contaminants. Any contamination in the weld zone would vaporise during welding and create gas pockets (porosity) that weaken the joint. [1]
2. File or machine the tube ends to the correct mitre angle so they fit tightly together with minimal gaps. A close-fitting joint ensures the weld penetrates evenly and the tubes maintain their designed alignment. [1]

(d)(ii) Work hardening occurs when a metal is deformed at room temperature (cold working), such as being bent, hammered, rolled, or repeatedly stressed. The deformation distorts the metal's internal crystal grain structure, forcing dislocations in the lattice to pile up and tangle. [1] This makes the metal harder and stronger but more brittle, reducing its ductility. If further cold working continues without relief, the metal may crack. In the area around a weld, thermal expansion and contraction during cooling can introduce internal stresses that have a similar effect. [1]

(e)(i) Normalising process and its purpose: [3]

1. The welded steel frame is heated in a furnace to above its upper critical temperature (approximately 900 degrees C for medium carbon steel) and held at this temperature until the heat has soaked through the entire frame evenly. [1]
2. The frame is then removed from the furnace and allowed to cool in still air (not quenched in water or oil, and not left in the furnace). [1]
3. The purpose is to relieve the internal stresses introduced by welding and to refine the grain structure. The reheating allows the distorted grains to recrystallise into a uniform, fine-grained microstructure, restoring consistent mechanical properties (strength, ductility, toughness) throughout the frame, including the heat-affected zones around each weld. [1]

(e)(ii) Difference between normalising and annealing: [2]

Both processes involve heating steel above its critical temperature, but they differ in cooling rate. Normalising cools the steel in still air, which produces a finer grain structure and a slightly harder, stronger material. [1] Annealing cools the steel very slowly inside the furnace (or buried in sand/vermiculite), producing a coarser grain structure that makes the metal softer and more ductile. Annealing is used when the metal needs to be as soft as possible for further machining or forming, while normalising produces a tougher, more refined structure suited to a finished component. [1]

(f)(i) Stages of the powder coating process: [3]

1. The steel frame is thoroughly cleaned, degreased, and surface-prepared. Any rust, mill scale, or old coatings are removed by shot blasting or chemical treatment to create a clean, bare metal surface. [1]
2. The frame is given an electrostatic charge (grounded), and dry thermoplastic or thermosetting powder is sprayed from a gun that imparts an opposite electrostatic charge to the powder particles. The charged particles are attracted to the grounded metal surface and adhere evenly, building up a uniform layer of powder. [1]
3. The coated frame is placed in a curing oven at approximately 180 to 200 degrees C. The powder particles melt, flow together into a continuous film, and chemically cure (cross-link) to form a hard, smooth, durable coating that is bonded to the metal surface. [1]

(f)(ii) Two advantages of powder coating compared to traditional wet paint: [2]

1. Powder coating produces no volatile organic compounds (VOCs) or harmful solvent emissions, making it significantly more environmentally friendly and safer for workers than solvent-based wet paints. [1]
2. The resulting coating is thicker and more durable than most wet paint finishes, providing better resistance to chipping, scratching, fading, and weathering. [1]

(g) The two main metals in duralumin are aluminium and copper. [1] Duralumin is significantly harder and stronger than pure aluminium because the copper atoms disrupt the aluminium crystal lattice, preventing dislocations from moving easily. This gives the handlebars greater rigidity and resistance to bending under the rider's grip and steering forces, while retaining much of aluminium's low density. [1]