Superplastic Forming of Multipass Friction Stir Processed Aluminum-Magnesium Alloy

  • PDF / 2,079,446 Bytes
  • 10 Pages / 593.972 x 792 pts Page_size
  • 79 Downloads / 226 Views

DOWNLOAD

REPORT


UCTION

FRICTION stir processing (FSP) is a new grain refinement technique, wherein a simple rotating tool with a small pin at the bottom of cylindrical shoulder refines microstructure by stirring action of the pin.[1,2] Pin length is made to plunge into work piece until shoulder touches top surface, generating heat due to friction between tool shoulder, and material surface. Plastic deformation of the material by stirring action results in grain refinement in a limited area around pin.[3,4] Microstructures of different grain sizes can be produced by altering rotation rate and traverse speed, i.e., heat input to the material.[5] Microstructure developed during FSP exhibits inhomogeneity in thickness direction like fine grains in nugget zone and coarse elongated grains in TMAZ/HAZ.[6,7] Several studies related to FSP have shown good superplastic properties induced in various materials by single and multipass processing.[8–10] Single pass FSP of an Al-Mg alloy resulted in grain refinement in the range of 3.5 to 8.5 lm and showed superplastic behavior.[5] Mishra et al.[11] showed superplasticity in 7075 Al alloy after two-step multipass FSP. However, ductility was reported to be higher in the nugget region of the second pass compared to that in the transition zone and nugget zone of first pass. Variation in superplastic properties of material from different passes of multipass FSP was reported despite having similar microstructure.[12] S. PRADEEP, Ph.D. Student, and VIVEK PANCHOLI, Associate Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247 667, Uttarakhand, India. Contact e-mail: [email protected] Manuscript submitted September 18, 2013. Article published online September 26, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

Recently in 28 pass FSPed 5086 Al-Mg alloy, homogeneity of the nugget area was demonstrated by showing uniform deformation behavior in samples extracted from different locations.[13] Su et al.[14] in a 7075 Al alloy showed sub-micrometer grains with uniform size in all the passes after bulk multipass processing. Superplastic forming was also attempted after multipass FSP in 7000 series Al alloys.[15,16] Superplastic forming is a process in which gas pressure is applied on one side of the blank held between dies, producing near net complicated shapes.[17] However, thickness variation in the formed component is a limiting factor. In a free flow hemispherical bulging, the thickness and effective strain vary from edge to the apex of the bulge. At the edge of the bulge, strain is lower, whereas thickness is more and reverse is true at the apex. Thickness variation in the formed bulge is a strong function of strain rate sensitivity (m) of a material.[18] High m value promotes bulging with lower thickness variation. Variable pressure forming was proposed as one of the approaches to maintain high m value at the apex during bulging.[19] Inhomogeneous microstructure in the blank was also reported to reduce thickness variation in the bulge

Data Loading...