BMM3643_Lect 34 Joining

Slide 1

BMM3643 Manufacturing ProcessLecturer: Dr. Siti Rabiatull Aisha IdrisEmail: [email protected]: 09-4246349 1112.0 JOINING PROCESSES (continue)2Page 3Lesson Objectives:

After todays lecture, students are expected to:Analyze the quality and testing of welded joint, weld design and process selection

3BMM 3643Page 4IntroductionWeld quality depends on geometry, presence of cracks, residual stresses , inclusions and oxide filmsMicro structure and grain size depends on the temperatures BMM 3643Page 5Solidification begins with formation of columnar grains which is similar to castingWeld metal has a cast structure because it has cooled slowly, it has grain structureResults depends on alloys, composition and thermal cycling to which the joint is subjected.Pre-heating is important for metals having high thermal conductivityWithout preheating, the heat produced dissipates rapidly through the rest of the parts being joined.Solidification of Weld metalBMM 3643Page 6Weld Joint StructureCharacteristics of a typical fusion-weld zone in oxyfuel-gas and arc welding.

Grain structure in (a) deep weld and (b) shallow weld. Note that the grains in the solidified weld metal are perpendicular to their interface with the base metal (see also Fig. 10.3). (c) Weld bead on a cold-rolled nickel strip produced by a laser beam. (d) Microhardness (HV) profile across a weld bead.BMM 3643Page 7Weld QualityWelding discontinuities can be caused by inadequate or careless application The major discontinuities that affect weld quality are:porosity, slag inclusions, incomplete fusion & penetration, weld profile, cracks, lamellar tears, surface damage and residual stresses BMM 3643Page 81. PorosityCaused by gaseous released during melting of the weld area but trapped during solidification, chemical reactions or contaminantsThey are in form of spheres or elongated pocketsPorosity can be reduced by Proper selection of electrodesImproved welding techniquesProper cleaning and prevention of contaminantsReduced welding speeds Weld Quality (cont.)BMM 3643Page 92. Slag InclusionsCompounds such as oxides ,fluxes, and electrode-coating materials that are trapped in the weld ZonePrevention can be done by following practices :Cleaning the weld bed surface before the next layer is depositedProviding enough shielding gasRedesigning the jointWeld Quality (cont.)BMM 3643Page 103. Incomplete FusionProduces lack of weld beads Practices for better weld :Raising the temperature of the base metalCleaning the weld area, prior to the weldingChanging the joint design and type of electrodeProviding enough shielding gasWeld Quality (cont.)BMM 3643Page 11Discontinuities and Defects in Fusion WeldsExamples of various discontinuities in fusion welds.

Examples of various defects in fusion welds.BMM 3643Page 124. Lack of Penetration Incomplete penetration occurs when the depth of the welded joint is insufficientPenetration can be improved by the following practices :Increasing the heat InputReducing the travel speed during the weldingChanging the joint designEnsuring the surfaces to be joined fit properlyWeld Quality (cont.)BMM 3643Page 135. Weld ProfileUnderfilling results when the joint is not filled with the proper amount of weld metal.Undercutting results from the melting away of the base metal and consequent generation of a groove in the shape of a sharp recess or notch.Overlap is a surface discontinuity usually caused by poor welding practice and by the selection of improper material.Weld Quality (cont.)BMM 3643Page 146. Cracks Cracks occur in various directions and various locationsFactors causing cracks:Temperature gradients that cause thermal stresses in the weld zone Variations in the composition of the weld zone.Embrittlement of grain boundariesInability if the weld metal to contract during coolingWeld Quality (cont.)BMM 3643Page 15Cracks in Welded JointsTypes of cracks developed in welded joints. The cracks are caused by thermal stresses, similar to the development of hot tears in castings.

BMM 3643Page 16Cracks are classified as Hot or Cold.Hot cracks Occur at elevated temperaturesCold cracks Occur after solidificationBasic crack prevention measures :Change the joint design ,to minimize stresses from the shrinkage during coolingChange the parameters, procedures, the sequence of welding processPreheat the components to be weldedAvoid rapid cooling of the welded componentsWeld Quality (cont.)BMM 3643Page 177. Lamellar tearsOccurred due to the shrinkage of the restrained components in the structure during cooling.Can be avoided by providing for shrinkage of the members & changing the joint designWeld Quality (cont.)BMM 3643Page 188. Surface Damage These discontinuities may adversely affect the properties of welded structure, particularly for notch sensitive metals.Weld Quality (cont.)BMM 3643Page 199. Residual StressesBecause of localized heating & cooling during welding, expansion and contraction of the weld area can cause the following defects:Distortion, Warping and buckling of welded partsStress corrosion crackingFurther distortion if a portion of the welded structure is subsequently removedReduced fatigue lifeWeld Quality (cont.)BMM 3643Page 20Residual Stresses and DistortionResidual stresses developed in a straight butt joint. Note that the residual stresses in (b) must be internally balanced.

Distortion of a welded structure.BMM 3643Page 21Stress relieving of weld Preheating reduces problems caused by preheating the base metal or the parts to be weldedHeating can be done electrically,in furnace,for thin surfaces radiant lamp or hot air blastSome other methods of stress relieving : hammering or surface rolling Weld Quality (cont.)BMM 3643Page 22Testing of Welded JointsQuality of the welding joint is established by welded jointEach technique has capabilities ,limitations and sensitivity reliability and requirement for special equipment and operator skill.BMM 3643Page 23Destructive TechniquesTension TestLongitudinal & transverse tension tests are performedStress strain curves are obtainedTension-Shear TestSpecifically prepared to simulate actual welded joints and procedures.Specimen subjected to tension and shear strength of the weld metal

BMM 3643Page 24Weld Testing(a) Specimen for longitudinal tension-shear testing; specimen for transfer tension-shear testing; (b) wraparound bend test method; (c) three-point bending of welded specimens (see also Fig. 2.11).

BMM 3643Page 25Destructive Techniques (cont.)Bend testDetermines ductility and strength of welded joints.The welded specimen is bend around a fixture The specimens are tested in three-point transverse bending These tests help to determine the relative ductility and strength of the welded joints

BMM 3643Page 26Other destructive testingFracture Toughness TestCorrosion & creep testsTesting of spot welds-Tension-Shear-Twist-Cross-tension-Peel

BMM 3643Page 27Non-Destructive testingOften weld structures need to be tested Non-DestructivelyNon-Destructive testing are :VisualRadiographicMagnetic-particleLiquid-penetrantUltrasonicBMM 3643Page 28Weld design & Process SelectionConsiderations:Configuration of the components or structure to be welded, and their thickness and sizeMethods used to manufacture the componentsService requirements, Type of loading and stresses generatedLocation, accessibility and ease of weldingEffects of distortion and discolorationAppearanceCostsBMM 3643Page 29Weld DesignSome design guidelines for welds. Source: After J.G. Bralla.

BMM 3643Page 30Weld DesignsExamples of weld designs used

BMM 3643Page 31Reminder25/10/2007 (Thursday) 8.00 pmTest 2 and Lab QuizChapters covered: anything after test 1 up to week 11 lectures.Already uploaded into E-learningBMM 3643Page 32Important NoteDue to high demands from you, there will be no class on Wednesday.BMM 3643Page 33FinallyI would like to wish you happy holiday and Selamat Hari Raya Aidilfitri.I would like to seek your forgiveness for anything wrongs that Ive doneSee you again later insyaallahAny questions?? 34

35