Гричин С.В., Ульянова О.В. - Английский язык для инженеров сварочного производства, страница 13

Anoxidizing flame has an excess of oxygen and a reducing flame has an excessof fuel (carbon). An oxidising flame is used for cutting and a reducing flameis used for annealing e.g. to soften steel sheet metal.An acetylene flame (as is characteristic of most fuel/oxygen flames)has two parts; the light blue to white colored inner cone and the blue coloredouter cone. The inner cone is where the acetylene and the oxygen combine.The tip of this inner cone is the hottest part of the flame.

The outer cone iswhere hydrogen and carbon monoxide from the breakdown of the acetyleneand partial combustion of the inner cone combine with the oxygen in thesurrounding air and burns.A neutral flame has a well defined inner cone. A reducing flame has afeathery inner cone. An oxidizing flame has a smaller inner cone that issharply defined and is pale blue. The welder observes this while adjusting thefuel and oxygen valves on the torch to get the correct balance for the job athand. There is also a difference in the noise the flame makes. Adjusting theflame is not a hard thing to do after a little experience and practice.The size of the flame can be adjusted to a limited extent by the valveson the torch and by the regulator settings, but in the main it depends on the71size of the orifice in the tip.

In fact, the tip should be chosen first according tothe job at hand, and then the regulators set accordingly.Speaking23Imagine you are explaining to an apprentice how to set up theequipment. Use the right column of the table in the previous exercise and thetips below to give the instructions.First fasten fuel and oxygen tanks securely to a wall, a post or a portable cartin an upright position.Then remove … .After that … .……Having attached ... .…………After this preparation ... .…Finally … .Revision24Decode the abbreviations.SMAW, MMA, GMAW, MIG, FCAW, SAW, GTAW, TIG, EGW25Label the picture of a portable welding outfit with the words below.header pipeline valvefiller plugreleaseregulatorescape pipeflash arrestor chambercheck valve and drain plugacetylene cylinderFig.

10Portable welding outfit72PART 5. MODERN DEVELOPMENTSLead-in1In small groups discuss the trends of modern research in welding listedbelow. Decide which of them are of primary importance. Think of some othertrends. Report to the class.ØØØØØØnew welding methodsautomation of weldingcomputer controlenergy saving technologiesenvironmentally friendly technologiessafety improvementsReading 12You will read the text Friction Stir Welding (FSW).

Before you readdiscuss the following questions in the group.1. Is the method of friction stir welding a conventional one?2. What makes it conventional/unconventional?3. What material does it best fit for?3Skim the text and make its brief summary.Friction Stir Welding (FSW)H.H.

BhadeshiaFriction stir welding, a process invented at TWI, Cambridge in 1991,involves the joining of metals without fusion or filler materials. It is usedalready in routine, as well as critical applications, for the joining of structuralcomponents made of aluminium and its alloys. Indeed, it has beenconvincingly demonstrated that the process results in strong and ductilejoints, sometimes in systems which have proved difficult using conventionalwelding techniques.

The process is most suitable for components which areflat and long (plates and sheets) but can be adapted for pipes, hollow sectionsand positional welding. The welds are created by the combined action offrictional heating and mechanical deformation due to a rotating tool.

The73maximum temperature reached is of the order of 0.8 of the meltingtemperature.Fig. 11 Tool in operationFig. 12 HAZThe tool has a circular section except at the end where there is athreaded probe or more complicated flute; the junction between thecylindrical portion and the probe is known as the shoulder. The probepenetrates the work piece whereas the shoulder rubs with the top surface. Theheat is generated primarily by friction between a rotating-translating tool, theshoulder of which rubs against the work piece. There is a volumetriccontribution to heat generation from the adiabatic heating due to deformationnear the pin.

The welding parameters have to be adjusted so that the ratio offrictional to volumetric deformation--induced heating decreases as the workpiece becomes thicker. This is in order to ensure a sufficient heat input perunit length.The microstructure of a friction-stir weld depends in detail on the tooldesign, the rotation and translation speeds, the applied pressure and thecharacteristics of the material being joined. There are a number of zones. Theheat-affected zone (HAZ) is as in conventional welds. The central nuggetregion containing the onion-ring flow-pattern is the most severely deformedregion, although it frequently seems to dynamically recrystallise, so that thedetailed microstructure may consist of equiaxed grains. The layered (onionring) structure is a consequence of the way in which a threaded tool depositsmaterial from the front to the back of the weld.

It seems that cylindricalsheets of material are extruded during each rotation of the tool, which on aweld cross-section give the characteristic onion-rings.74The thermomechanically-affected zone lies between the HAZ andnugget; the grains of the original microstructure are retained in this region,but in a deformed state. The top surface of the weld has a differentmicrostructure, a consequence of the shearing induced by the rotating toolshoulder.The MachineThis is a picture of a friction stir welding (FSWshows a typical) machine. This one is at the Joiningand Welding Research Institute (JWRI) of OsakaUniversity, Japan.The ToolFig.

13 Friction-stirwelding machineBelow you can see an illustration of sometypes of tools. Each tool has a shoulder whoserotation against the substrate generates most of theheat required for welding. The pin on the tool isplunged into the substrate and helps stir the metal inthe solid state.Fig. 14 The toolsThe Fixture and WeldThe two halves to be joined must be rigidly fixed before the weldingoperation (first picture below). The pin, which is an integral part of the tool,is plunged into the metal to help stir it up; the shoulder of the tool generatesmuch of the heat. As the weld is completed, the tool is withdrawn leavingbehind a hole. The weld is designed so that such regions can be discardedfrom the component.

The presence of a hole may not be appropriate whenwelding pipes or storage vessels. The hole can be avoided by designing thetool such that only the pin can be retracted automatically and gently into theshoulder, leaving behind an integral weld.75Fig. 15 The fixtureFSW of SteelSteel can be friction stir welded but the essential problem is that toolmaterials wear rapidly. Indeed, the wear debris from the tool can frequentlybe found inside the weld.

The process would therefore be used in specialcircumstances where other welding methods are inadequate. Thesecircumstances have yet to be clarified. There are so many good methods bywhich steel can be welded. The example below is the FSW of 316L stainlesssteel. Notice that the sample becomes red-hot during welding.Fig 16 Obtaining a weldSince the tool gets red hot, it is necessary to protect it against theenvironment using a shielding gas. A possible use of FSW in the welding ofsteels is in the context of stainless steels.

Austenitic stainless steels can easilybe welded using conventional arc welding and other processes. However,FSW can offer lower distortion, lower shrinkage and porosity. Moreimportant is the avoidance of fumes containing hexavalent chromium whichis carcinogenic. In addition, chemical segregation effects associated withwelding processes involving solidification are avoided. Such segregation can76lead to a degradation of corrosion resistance since electrochemical cells areset up between solute-rich and poor domains.Friction Stir Welding of Cast Aluminium AlloyThe most popular aluminium casting-alloy contains about 8 wt% ofsilicon.

It therefore solidifies to primary aluminium-rich dendrites and aeutectic mixture of aluminium solid-solution and almost pure silicon. Thelatter occurs as coarse silicon particles which tend to be brittle. The cast alloyusually has some porosity. Friction stir welding has the advantage that itbreaks up the coarse silicon particles and heals any pores by the mechanicalprocessing, as illustrated below.Fig. 17A section through a friction stir weld made in an Al-Si casting alloy.There are pores indicated in the base metal (BM).

HAZ represents theheat affected zone, TMAZ – the thermomechanically affected zone, and SN –the stir nugget. The photographs in this section have kindly been provided byProfessor H. Fujii of JWRI, Japan.77Fig. 18 Optical micrographs of regions (a), (b) and (c) of the stir nugget.The location of these regions is identified in macroscopic section presentedabove.Optical micrographs showing the microstructure in (a) the base metal;(b) heat-affected zone; (c) the thermo mechanically affected zone, whereconsiderable refinement of the silicon has occurred.Tensile strength Proofstress Elongation (%)Fracture local(MPa)(MPa)Joint 150851.6BMWeld 179875.3TMAZSN2519614.4SNThe refinement of silicon and elimination of porosity leads to bettermechanical properties in the weld than in the base plates.Vocabularyductilethreadedpinshoulderdebrisvolumetricshrinkagesoluteequiaxedгибкий, ковкий, поддающийся обработкес резьбой, нарезнойцапфабуртик, поясокосколки, обломки, обрезки, ломобъемныйусадочная деформациярастворенное вещество, растворравноосный4Read the Russian text below and correlate it with the text Friction StirWelding (FSW) in the previous activity.