Page 14 - Jan-Feb 2018
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                                      general, increases with total solute     ranges of (1.4-2.2) wt % and (1.2-2.0)
                                      content in the alloy, the element        wt %, respectively. Like AA7085 and
                                      copper has been identified to greatly    all other high strength 7xxx series
                                      accelerate the precipitation reactions,  alloys, zirconium is present in this
                                      thus greatly influencing the quench      alloy in trace amounts.
                                      sensitivity of this alloy. Taking into
                                      account these aspects, ALCOA (USA)          Also, in DMR7xxx alloy, high purity
                                      in 2002 came up with an aluminium        primary aluminium (i.e., 99.9 wt %
                                      alloy AA7085 having the composition      plus) are utilised for the preparation
                                      (wt %) of aluminium-(7-8) zinc-          of this alloy in order to compensate
                                      (1.2-1.8)   magnesium-(1.3-2.0)          the loss in ductility, fracture toughness
                                      copper-(0.08-0.15) zirconium. It         and other mechanical properties
                                      is noteworthy that this alloy has        because of the increase in the zinc
                                      substantially lower copper and           content.
Al-Cu-Mg-Zn Phase diagram at 460 0C magnesium compared to the other
and 6 wt % Zn                         existing high strength 7xxx series       Further, in the actual DMR7xxx
                                      aluminium alloys. An obvious             alloy composition, the copper and
property required by the designers consequence of this is the attainment       magnesium contents are so chosen
for thick sections of these materials of low strength values in the alloy in   that the alloy lies in the α-aluminium
is low quench sensitivity. Reduced the desired tempers. These strength         phase field of the aluminium-copper-
quench sensitivity greatly improves values obviously decrease further in       magnesium-zinc phase diagram at
the through-thickness properties. LT and ST directions.                        460 °C, for the purpose of
Although, the quench sensitivity, in                                           homogenisation. This would not
                                                  With      this               allow  the  abneodicpresSent(Ala2Ct uMthge)
                                                  background,                  phase   to
                                                  DMRL has come                solution treatment temperature of
                                                  up with an alloy             460 °C. Accordingly, the DMRL
                                                  DMR7xxx having               patented DMR7xxx alloy, containing a
                                                  higher strength              suitable amount of zinc, would possess
                                                  than the existing            a combination of high strength, high
                                                  alloy AA7085                 SCC resistance together with the
                                                  [Indian Patent No.           reduced level of quench insensitivity.
                                                  268056 (2015)].
                                                  The related Patent              As-cast round ingots of DMR7xxx
                                                  Cooperation Treaty           (580 mm diameter) have been
                                                  (PCT) application            successfully produced at HAAL,
                                                  got the approval             Aurangabad. The resultant sound
                                                  of the European              billets following homogenisation,
                                                  patent authority,            scalping and ultrasonic testing have
                                                  got the grant of a           been extruded at OFAJ, Nagpur to
                                                  patent from China            produce extruded sections of suitable
                                                  [CN 101835915                thicknesses, i.e., t of 20 mm through
                                                  B (2013)], and               100 mm, and forging stocks of
                                                  awaiting  grant              diameter 240 mm.
                                                  of patents from
                                                  Germany, France                 The extruded sections/flats have
                                                  and South Korea. In          been heat treated to T7651 temper.
                                                  DMR7xxx alloy, the           Desired through-thickness properties
                                                  zinc content varies          have been obtained in 100 mm thick
                                                  between 8 wt % to            extruded sections at the laboratory
                                                  10 wt %, while               scale. These materials are currently
(a) DMR7xxx ingots of 580 mm diameter, (b)-(d) extruded the copper and         being heat treated at OFAJ to realise
sections of thicknesses 20 mm, 70 mm and 100 mm,  magnesium                    the desired mechanical properties at
respectively, and (e) 240 mm diameter extrusions  contents are in              the industrial scale.
as forging stock.                                 the composition

14 January-February 2018                                    Aluminium Alloy Technologies for Advanced Defence Systems
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