Valery Marinov, Manufacturing Technology

Transcript

  CHAPTER CONTENTS 1 METAL CASTING 1 Selection of castings of various materials, shapes, and sizes1.1 Overview of Casting Technology  Denitions Casting nomenclature Heating and pouring  Solidication and cooling 1.2 Casting Processes Expandable mold casting  Permanent mold casting 1.3 Casting quality  Casting nomenclature  The gure in the right shows the nomencla-ture of mold and castings in sand casting. 1.1 OVERVIEW OF CASTING TECHNOLOGY   Advantages Complex shapesNet-shape ability Very large partsVariety of metalsMass productionDisadvantagesPoor accuracy Poor surfaceInternal defectsMechanical propertiesEnvironmental impact Casting nomenclature Defnitions  Casting is a process in which molten metal ows into a mold where it solidies in the shape of the mold cavity. The part produced is also called casting  . Mold preparation Metal heating Pouring ProcessingCooling Casting technology involves the next steps:   Metal Casting  2 Valery Marinov, Manufacturing Technology  The pouring cup, downsprue, runners, etc., are known as the mold gating system , which serves to deliver the molten metal to all sections of the mold cavity. Gating system in sand casting  Heating and pouring  Heating  The total heat required is estimated as the sum of     Heat to raise the temperature to the melting point    Heat of fusion    Heat to raise the molten metal temperature to the temperature of pouring  Pouring  Major factors affecting the pouring action    Pouring temperature    Pouring rate    TurbulenceSome important equations in pouring: Pouring cupDownsprueGates to castingRunner Runner wellRunner extensionDownsprue well Velocity of the liquid metal at the base of the sprue: Gravitational acceleration constant  Sprue height   ghv  2  Volumetric ow rate: Casting’s cross-sectional area Q = vA Mold lling time:  MFT = V/Q  Mold cavity volume Fluidity  Fluidity is a measure of the capability of a metal to ow into and to ll the mold before freezing. It denes to the great extend the quality of casting.Factors affecting uidity:    Pouring temperature    Metal composition    Heat transfer to the surroundings    Viscosity of the liquid metal Standard testing method  Pouring cupDownsprueSpiral moldLimit of flowbefore freezing In the foundry practice, test for uidity is carried out for each ladle just before pouring the molten metal into the mold   Metal Casting  3 Valery Marinov, Manufacturing Technology  Solidifcation and cooling  Solidication of metals Pure metals   solidify at a constant temperature equal to their freezing pointMost alloys freeze over a temperature range Chvorinov’s rule  TST—total solidication timeC m —mold constantV—volume of the casting  A—surface area of the casting n—constant, usually n=2 nm AVCTST       Solidication time Example: Calculate the total solidication time for a 10/100/200-mm steel plate if C m  = 0.2 min/mm 2 Solution:   Metal Casting  4 Valery Marinov, Manufacturing Technology  ShrinkageDirectional solidication    By a proper design of the casting     By external and internal chillsSeveral riser designs are used in prac-tice as shown in the gure. The riser must remain molten until after the casting solidies.  The Chvorinov’s Rule is used to cal-culate the riser’s dimensions. Riser design Open BlindTopSide CastingMold Riser  CastingMold Riser  CastingMold Riser  CastingMold Riser  Possible types and positions for risers in sand casting  Riser design