Materials processing and joining
Learn about casting defects and precision casting, strain and force calculations for rolling, extrusion and deep drawing, springback, the carbon equivalent and heat input in welding, cold cracking and sensitization, solid-state joining, and sintering and HIP in powder metallurgy with multiple-choice questions.
Materials processing and joining: key points
Metal parts are made by casting, in which metal is melted and poured into a mold; plastic working, in which the solid is deformed; welding and joining, which connect members; and powder metallurgy, in which powder is compacted and sintered. In casting, risers and the order of solidification are designed to prevent shrinkage cavities from solidification shrinkage and gas porosity. In plastic working such as rolling, forging, extrusion and deep drawing, dimensions and the forces needed are estimated using constant volume and true strain, and hot working can break up the cast structure and refine it.
In welding, a heat-affected zone forms around the melted region; in steel, hardening by rapid cooling and hydrogen-induced cold cracking are problems, and in stainless steel, sensitization by carbide precipitation. Cooling is managed through the carbon equivalent, heat input and preheating, and friction stir welding, which does not melt the material, is also used. In powder metallurgy, green compacts are sintered below the melting point, and dies are designed to allow for the dimensional shrinkage. It is essential for high-melting materials and cemented carbides, and remaining pores can be eliminated by hot isostatic pressing.
All Materials processing and joining questions (15)
- What is the main cause of a "shrinkage cavity," a void that tends to form inside a casting, especially at the center of a thick section that solidifies last?
- Which casting method makes a pattern of the same shape as the product from wax, coats it with several layers of refractory slurry and hardens them, then heats it to melt out the wax and pours melt into the resulting mold?
- A plate 20 mm thick is cold rolled in several passes to 5.0 mm, assuming the width does not change. Which value is closest to the magnitude of the true strain in the thickness direction, ln(h₀/h)?
- When carbon steel parts of the same shape are made by casting and by hot forging, the forged part generally has better toughness and fatigue strength. Which reason is correct?
- An aluminum alloy billet 100 mm in diameter is hot extruded into a round bar 25 mm in diameter. Using the extrusion ratio R = A₀/A (the ratio of cross-sectional areas), take the ideal extrusion pressure, ignoring friction and redundant deformation, as p = Y ln R with a flow stress Y = 50 MPa. Which value is closest to p?
- Ignoring changes in thickness and the radius at the bottom corner, a cylindrical cup with a bottom, 50 mm in diameter and 40 mm high, is made from a circular blank in a single deep-drawing operation. Assuming the surface area does not change, which value is closest to the diameter of the blank?
- When a sheet is bent and removed from the die, its bend angle partly recovers by elastic recovery (springback). Which is likely to show larger springback? Assume the same sheet thickness and bend radius.
- The carbon equivalent used as a guide to the weldability of steel is Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 (mass %). Which value is closest to Ceq for a steel with 0.20% C, 1.20% Mn, 0.30% Cr, 0.10% Mo, 0% V, 0.30% Ni and 0.15% Cu?
- Arc welding is carried out at a voltage of 25 V, a current of 200 A and a welding speed of 300 mm/min. With an arc thermal efficiency of 0.80, which value is closest to the heat input per mm of weld, Q = ηVI/v?
- In welding high-strength steel, preheating is used to prevent cold cracking (hydrogen cracking), which appears in the heat-affected zone and elsewhere within minutes to days after welding. Which statement correctly describes the effect of preheating?
- When austenitic stainless steels such as SUS304 are welded, intergranular corrosion can occur in the heat-affected zone a short distance from the weld line (weld decay). Which combination of cause and countermeasure is correct?
- Which statement correctly describes the features of friction stir welding (FSW)?
- A green compact at 70% relative density is sintered to 95% relative density. Assuming isotropic shrinkage and no change in mass, which value is closest to the linear shrinkage (fractional decrease in length) due to sintering?
- To make WC–Co cemented carbide, WC powder is mixed with Co powder, compacted and sintered. Which statement correctly describes this sintering?
- Hot isostatic pressing (HIP) is used to eliminate closed pores remaining inside powder-metallurgy parts and castings. Which describes the method correctly?