ME 310 Manufacturing Processes Final
Explain the reason for drafts in molds.
Ability to remove the pattern from the sand-mold without damaging the mold.
Why is there density variation in the compacting of powders? How is it reduced?
The density depends on the compacting pressure
What is the function of a core?
A core is used to form hollow regions of a casting
How is weldablity defined?
A material ability to be welded
Complex cast components can be readily made using lost foam casting that cannot be readily made using other sand casting methods. Clearly explain the fundamental reason why this additional shape-making flexibility is possible for lost foam casting.
Able to make any geometry shape, it will make exactly what the foam shape is
List 2 distinct advantages of metal casting as a shape-making method compared to other shape-making methods such as forging, extrusion, etc.
Able to make large, complex shapes and at low cost
List 2 distinctive advantages of adhesive bonding and 2 distinct disadvantages compared to other joining methods.
Advantage: fast and automated; could join two unrelated materials that couldn't be joined otherwise Disadvantage: not very strong and tough; not at high temperatures
List 1 advantage and 1 limitation of the green sand casting process compared to high pressure die casting process.
Advantage: less cost Disadvantage: thin walls not possible
List the advantages and disadvantages in using a lubricant in forging operations.
Advantages: Improving metal flow, act as a parting agent Disadvantages: Act as a thermal barrier
What is the difference between machining allowance and dimensional tolerance?
Allowance is planned deviation from normal dimension Tolerance is limit of acceptable unintended deviation from normal dimension
Give three applications where adhesive-bonding is the best joining method.
Automotive brake-lining assemblies, laminate windshield glass, appliances, helicopter blades, honeycomb structures, and aircraft bodies and control surfaces
Why are mechanical joining methods such as bolting and riveting typically more expensive and typically more ineffective than arc welding processes for most applications?
Because they require more detailed molds to make threads better and stronger to fasten They are ineffective because they can get loose and untighten the two pieces it is joining Bolting and riveting are more expensive because they have to be made first and then applied to hold pieces together
What are the similarities and differences between consumable and nonconsumable electrodes?
Both establish arc, but non-consumable electrodes do not melt Inert gases are used for shielding
What is the basic difference between additive manufacturing and rapid prototyping?
Can manufacture a prototype but you cannot prototype a product
Why is casting an important manufacturing process?
Casting is the ideal process to creating large and complex shapes
Describe the methods used in metal powder compaction.
Cold compaction-pressing, isostatic pressing, rolling, extrusion, injection molding Hot compaction- isostatic pressing
What is the difference between cold, warm. and hot forging?
Cold forging involves either impression die forging or true closed die forging with lubricant and circular dies at or near room temperature Warm forging of steel runs from above room temperature to below the recrystallization temperature, or from about 800 to 1,800 degrees Fahrenheit Hot forging is the plastic deformation of metal at a temperature and strain rate such that recrystallization occurs simultaneously with deformation
What causes burrs? How can they be reduced or eliminated?
Dull tool edges contribute to large burr formation Reduce by having good working tools
Why are draft angles required in forging dies?
Easier removal of forged part
Describe the differences between expendable and permanent molds
Expendable molds are molds that have to be broken up to remove the cast (have to make a new mold for every casting) Permanent molds are designed to be reused and able to withstand multiple operations and created to be easily removed
How does extrusion differ from rolling and forging?
Extrusion is pushing billet through a die to get the near-net-shape
Explain the difference between extrusion and drawing.
Extrusion is pushing while drawing is pulling billet through a die
What is flash? What is its function?
Flash is extra material on the side of the die Its function is to make sure the die is filled properly
Describe the methods by which sheet glass is made.
Float Method- molten glass is fed into a long bath and floats over a bath of molten tin Drawing Method- passing molten glass through a pair of rolls Rolling Method- molten glass is squeezed between powdered rollers forming sheets
How is fluidity defined? Why is it important?
Fluidity is defined by its capability of molten metal to fill mold cavities It is important because it affects the castability to make parts
Are fluxes necessary in brazing? If so, why?
Flux is essential because it prevents oxidation and removes oxide films
What differences, if any, would you expect in the properties of casting made by permanent mold vs. sand casting?
Grain sizes (small in permanent casting), microstructure (fine in permanent casting), segregation of alloying elements (less in permanent casting), gas and inclusion entrapment in permanent casting
What are the major types of sand molds? What are their characteristics?
Green Sand- the sand is mixed with clay and water and it is inexpensive Cold Box- organic and inorganic binders are blended in sands to bond grains chemically for greater strength, the molds are more dimensionaly accurate but are more expensive than green sand
Describe the drawbacks to having a riser that is too large or too small.
Having risers that are too large means longer solidification times and increase in scrap metal Too small of risers won't completely compensate for the liquid to solid shrinkage
What types of steel have poor weldability? What is the specific weld zone problem that is the cause of this poor weldability? Describe the special welding practices that are commonly used to weld steels that have poor weldability?
High alloy carbon steel The heat affect zone To weld steel with poor weldability is have high heat input and/or preheat the speciem, martensite, and slow cooling rates
Why are steels more difficult to cast than cast irons?
High temperatures (1650C) is required to melt steels and the high reactivity of steels with oxygen during melting and pouring of metal
Explain reasons for hot tearing in casting.
Hot tearing occurs because the casting cannot shrink freely during cooling
Describe 2 separate reasons why more than 1 riser might be needed to successfully eliminate shrinkage from a casting.
If the casting is too large, parts will solidify before it can reach the end, leaving a hole in the middle of the cast, and shrinkage will be past the tolerance level
List important considerations when selecting pattern materials for sand molds.
Important considerations of determing casting pattern material are size & shape of casting, dimensional accuracy & quantity of casting, and molding process
Why is the surface finish of a rolled product better in cold rolling than in hot rolling?
In cold rolling, there are no oxides on the surface, the grain sizes are larger which mean a smoother surface finish
Why is the intermediate shape of a part important in forging operation?
Intermediate shape is in strong connection with the final shape of the product
Why is the investment-casting process capable of producing fine surface detail on castings?
Investment casting uses wax and plastics to create their patterns that are then treated with plaster. the molten metal is then poured in to the mold. because wax is very smooth both the liquid and solid form the mold has a very smooth surface finish
If you need only a few castings for the same design, which three processes would be the most expensive per piece cast?
Investment, Permanent mold, and Die
Why is ultrasonic testing (UT) commonly used for weld non-destructive evaluation?
It is a non-destructive way to evaluate internally the part, it is also simplier and faster than other ways of evaluation
Why is shielding metal-arc welding a commonly used process?
It is portable and flexible, all welding positions, low equipment cost
What is jiggering? What shapes does it produce?
Jiggering is a motion in which the clay bat is formed by means of templates or rollers It produces axisymmetric parts
Explain what is meant by solid-state welding.
Joining takes place without fusion at the interface of the two parts to be welded
Describe what occurs to metal powders during sintering.
Mechanical bond> metallurgical bond Particles become closer together reducing porosity
Why do most metals shrink when they are cast?
Metals shrink because contraction of molten metal as it cools prior to its solidification, contraction of metal during the phase change from liquid to solid, and contraction of the solidified metal as its temperature drops to ambient temperature
What are the advantages of integrated snap fasteners?
Non-permanent , semi-permanent, and permanent Design flexibility Low to high cost
Explain the difference between open-die and impression-die forging?
Open-die forging is a metal work piece placed between two flat dies and reduced height by compression Close-die forging is a work piece that takes the shape of the die cavity while being forged between two shaped dies
How are the individual wax patterns attached on a "tree" in investment casting?
Patterns are attached to a wax sprue by means of a heated metal tool
What are the forms of raw materials for processing plastics into products?
Pellets, granules, powders or liquid, paste
Name the type of materials typically used for permanent-mold casting processes.
Permanent casting mold use very high strength materials like cast iron, bronze, graphite
Describe briefly the production steps involved in making P/M parts.
Powder production > Blending > Compaction > Sintering > Finishing Operations
What are the advantages of the lost-foam casting process?
Process is relative simple (no parting lines, cores, or riser systems) Inexpensive flask are used for the process The casting requires minimal finishing and cleaning operations
What types of finishing operations are used on ceramics? On glass? Why?
Products are mostly net-shape because any finishing operations could cause them to break since they are so brittle
Explain the difference between punching and blanketing.
Punching is having the center of a piece of sheet metal removed to have shape desired Blanking is removing the outer most edges to leave the inner sheet metal
What are the differences between the solidification of pure metals and the solidification of metal alloys?
Pure Metals- constant solidification temperature and composition remains unchanged Metal Alloys- has a range of freezing temperatures and composition continuously changes
What is the difference between solidification of pure metals and metal alloys?
Pure Metals- constant solidification temperature and composition remains unchanged Metal Alloys- has a range of freezing temperatures and composition continuously changes
Outline the steps involved in processing ceramics and glasses
Raw material > Forming/Shaping > Drying > Firing/Sintering > Finishing
How are seamless tubes produced?
Rotary piercing and rolling, or extruding
Briefly describe the specific problems associated with 'solid-to-solid shrinkage' and 'liquid-to-solid shrinkage' in metal castings and solutions used by metal casters to solve each of these problems.
S>S: oversize pattern L>S: risers
What is shrinkage allowance? Machining allowance?
Shrinkage allowance is how much to allow so cracking can be avoided during solidification process Machining allowances are included in pattern dimensions and increase with size and section thickness of the casting
Estimate the solidification time.
Solidification time is inversely proportional to the surface area squared
A GMAW weld is made on a plain carbon steel and stainless steel (lower thermal conductivity) using the same welding conditions. Which material will have a larger fusion zone? Explain.
Stainless steel because low thermal diffusion means larger fusion zone
What is superheat? How does superheat influence the fluidity of a metal or alloy?
Superheat is the liquid under pressure above its boiling point (doesn't boil) It delays solidification
What is superheat? Why is it important?
Superheat is the liquid under pressure above its boiling point (doesn't boil) It delays solidification
What is springback? What is negative springback?
The bending angle is the beginning angle to which the operator overbends so that the metal springs back to the desired bent angle Occurs in V-die bending, caused by the nature of the deformation occurring within the sheet metal just when the punch completes the bending operation at the end of the stroke
Give several reasons why forgings are typically not cost effective for low production volume parts.
The dies are expensive Machining will need to be done Tooling is expensive
Although the cost of a laser power supply for welding and cutting is quite high compared to arc welding power supply, give specific reasons why the use of lasers for welding or cutting can be cost justified.
The distribution of the weld is decreased with low heat input and a single pass
Describe the features of forming-limit diagrams
The failure zone is above the line of the strains (major/minor) which mean it won't preform well at those deformations
Describe the principle of resistance-welding processes.
The heat required for welding is produced by means of electrical resistance across the two components to be joined
Clearly explain the reason(s) for the basic shape of the 'relative cost per piece' vs. 'number of pieces' curves below. Also explain the relative positions of the curves for sand casting and permanent mold casting of connecting rods.
The more product that is produce, the cheaper it will become to make the next one since the setup cost will be decrease will each sequential part At some point, when there are more than 100,000 products being produced, the cost of sand casting compared to permanent casting will be more expensive
Using appropriate sketches, show how a typical coffee mug should be oriented in a mold to be successfully cast using conventional green sand casting methods.
The mug needs to be lying on its side with the parting line going through the lenght (center of the handle) A core is needed to keep the hollow area hollow Drafts need to be used where the part is perpendicular to the parting line
Explain why some joints may have to be preheated prior to welding.
The need for preheating increases with these factors: thickness of parts being welded, lower heat input, higher speed of welding
How can you tell whether a certain part is forged or cast? Explain the features that you would investigate.
The parting line of a casted part would be small while a forged part would either have a large parting line or none (depending if it was hammered off)
How does sheet-metal forming differ from rolling, forging, and extrusion?
There is no change in the cross section in sheet-metal forming compared to rolling, forging, or extrusion
Fundamentally explain why hardness readings on a test specimen should not be taken too close together or too close to the edge of a test specimen.
They can't be taken to close together because of plastic deformation
For what types of welding applications is resistance spot welding (RSW) most suited?
Thin sheet welding Steel
Why are protective atmospheres necessary in sintering? What would be the effects on the properties of P/M parts if such atmosphere were not used?
To control the carburization and decarburization of iron and iron-based compacts, and to prevent oxidation of the powders
What is the purpose of flux? Why is it not needed in gas tungsten-arc welding?
To prevent oxidation, nitrates, and other inclusions and, with the resulting slag, to protect the molten-weld pool
Why are the metal parts in three-dimensional printing often infiltrated by another metal?
To provide good heat-transfer capabilities as well as wear resistance
What is mill scale? Why does the presence of mill scale affect the selection of an appropriate welding process for steel?
Very thin layer of brownish black material that forms when steel is hot rolled
Why does die casting produce the smallest cast parts?
Wall thicknesses less than those attainable by other casting methods are possible and the velocity of metal in the runners is higher than other processes; small parts can be cast before the runner solidifies.
