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This 6 page Class Notes was uploaded by Jada Daniel on Wednesday September 23, 2015. The Class Notes belongs to MEM437 at Drexel University taught by Staff in Fall. Since its upload, it has received 40 views. For similar materials see /class/212407/mem437-drexel-university in Mechanical Engineering at Drexel University.
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Date Created: 09/23/15
5129262759607073 8896 Gd69 51 Describe the characteristics of 1 an alloy 2 pearlite 3 austenite 4 martensite and 5 cementite a Alloy composed of two or more elements at least one element is a metal The alloy may be a solid solution or it may form intermetallic compounds b Pearlite a twophase aggregate consisting of alternating lamellae of ferrite and cementite the closer the pearlite spacing of lamellae the harder the steel c Austenite also called gamma iron it has a fcc crystal structure which allows for a greater solubility of carbon in the crystal lattice This structure also possesses a high ductility which increases the steel s formability d Martensite forms by quenching austenite It has a bct bodycentered tetragonal structure and the carbon atoms in interstitial positions impart high strength It is hard and very brittle e Cementite also known as ironcarbide FesC it is a hard and brittle intermetallic phase 52 What are the effects of mold materials on fluid flow and heat transfer The most important factor is the thermal conductivity ofthe mold material the higher the conductivity the higher the heat transfer and the greater the tendency for the fluid to solidify hence possibly impeding the free flow of the molten metal Also the higher the cooling rate ofthe surfaces ofthe casting in contact with the mold the smaller the grain size and hence the higher the strength The type of surfaces developed in the preparation of mold materials may also be different For example sandmold surfaces are likely be rougher than those of metal molds whose surfaces can be prepared to varying degrees of roughness including the directions of roughness lay 59 Explain why the constant C in Eq 59 depends on mold material metal properties and temperature The constant C takes into account various factors such as the thermal conductivity of the mold material and external temperature For example Zircon sand has a higher thermal conductivity than basic silica sand and as a result a casting in Zircon of equal volume and surface area will require less time to solidify than that cast in silica 526 Why is the investmentcasting process capable of producing fine surface detail on castings The surface detail ofthe casting depends on the quality ofthe pattern surface In investment casting for example the pattern is made of wax or a thermoplastic poured or injected into a metal die with good surface finish Consequently surface detail ofthe casting is very good and can be controlled Furthermore the coating on the pattern which then becomes the mold consists of very fine silica thus contributing to the fine surface detail of the cast product 527 Explain why a casting may have a slightly different shape than the pattern used to make the mold After solidification shrinkage continues until the casting cools to room temperature Also due to surface tension the solidifying metal will when surface tension is high enough not fully conform to sharp corners and other intricate surface features Thus the cast shape will generally be slightly different from that of the pattern used 559 A round casting is 03 m in diameter and 05 m in length Another casting of the same metal is elliptical in cross section with a majortominor axis ratio of 3 and has the same length and cross sectional area as the round casting Both pieces are cast under the same conditions What is the difference in the solidification times of the two castings For the same length and cross sectional area thus the same volume and the same casting conditions the same C value in Eq 511 on p 205 on p 205 should be applicable The surface area and volume of the round casting is 2m1 25 0613 mg Amuud 2 hound m2 00353 111 mmuv mamem we and b vespemwe y wheve a 3mm W41 I W72 01 2 312 r3 7 I m b n BEBE m 5 mm a n 25H m The su ace 3123 mm eprserbased pan 5 522 a basm geumehy exHuHhe avea Euuauun devwauuns A 7 2m 255112 I 7 1002 1112 4am The vu ume 5 51m u U353 m2 Accuvdmg m Eq 5 M an p ZDS WE thus have 2 2 Twund Telllp V 5am Dem Eq 5 7 nnmmv nn pZEH mm hwm 7 Asa m W n n neghgwb e Thus mm Eq 5 5 We have 70 Fig 1 170L77111L7 y 29 uv su vmu uvvh 2 0 7 h 71 7 U1 mgn 2y Swm av y j r 7 2g f P P v2 770Pi 1i 7112 W 29 O 2 7 mm Subsmmmg mesa vesuhs mm the cummuny equauun gwen by Eq 5 Ewe have Al L39l A2L 2 All291111n29712 A 2 M NH 111 Whlch ls he deslved vela lunshlp Theveluve al 4n q malleable casl Hun and hlghrmanganese sleel7 lEtu AmH su 121254 m l m1 melals can be ublamed mm m lechmcal lltevaluve mm lmemel Fm example hum Kalpakjlam m n n n a Shrinkage Metal 3 u Aluminum Mnllcnblo ml Iron 089 Higlnnnngnnnse steel 26 From the following formula Lnnn Loll shrinkage Vc nd that for nlnmlmnn LAl 121U13 12156 in For malleable cast iron Llron 210089 12107 in mlrl for hlglnnangnneso eel Lmel12102612312iu ynn EXpEE Ed vange as u mnsl ulhev melals 570 The blank forthe spool shovm in the accompanying gure is to be sand cast out ofA319 an aluminum casting alloy Make a sketch ofthe wooden pattern forthis part Include all necessary allowances for shrinkage and machining 050 in 1 The sketch for a typical greensand casting pattern for the spool is shown below A cr sssectional view is also provided to clearly indicate shrinkage and machining allowances as well as the draft angles The important elements ofthis pattern are as follows dimensions in inc es a Twopiece pattern b Locating pins will be needed in the pattern plate to ensure that these features align properly c Shrinkage allowance 532 in1t d Machining allowance 116 in e Draft 3 lt 458 in gt 13953 in39 t l 3 typical 573 The optimum shape ofa riser is spherical to ensure that it cools more slowly than the casting it feeds Spherically shaped risers however are dif cult to cast 1 Sketch the shape ofa blind riserthat is to mold but also has the 39 a t 39 ti quot solidi cation time ofthe riser in part a to that ofa riser shaped like a right circular cylinder Assume that the volume ofeach riser is the same and that for each the height is equal to the diameter see Example 52 A sketch ofa blind riser that is easy to cast is shown below consisting ofa cylindrical and a hemispherical portions Hemisphere Note that the height ofthe cylindrical portion is equal to its radius so that the total height ofthe riser is equal to its diameter The volume V ofthis riser is r 3 V 777211 4 L Letting V be unity we have 3 13 T The surface area of the riser is A 27rrh mg 47052 57TT392 Substituting for r We obtain A 521 There fore from Eq 511 on p 205 the solidi cation time t for the blind riser Will be 2 V t C A From Example 52 we know that the solidi cation time for a cylinder with a height equal to its diameter is 0033C Thus this blind riser will cool a little slower but not much so and is easier to cast 1 2 v 00370 521 588 139 39 39 39 39 39 cast products Review each one and offer design solutions to avoid them Hiser I Fracture I u if Gate Casting 5 lb Sink mark Cold tearng 0 d By the student Some examples are for a fracture is at stress raiser a better design would utilize a more gradual let radius b fracture at the gate indicates this runner section is too narrow and thus it solidi ed rst hence the gate should be larger 596 Three sets of designs for die casting are shown in the accompanying gure Note the changes made to original die design number 1 in each case and comment on the reasons 1 By the student There are many observations usually with the intent of minimizing changes in section thickness eliminating inclined surfaces to simplify mod construction and to orient 2 anges sothat they can be easily cast a Pam39ng line 1 Parting line 31 Parting lme b m 2 l l u c