Title of Invention

A PROCESS FOR PREPARATION OF ALUMINIUM -ZINC MAGNESIUM-COPPER BASED ALLOYS"

Abstract

A process for the preparation of Al-Zn-Mg-Cu based alloys characterized by the steps of Melting a charge mixture at a temperature of 740-760°C of aluminum with 70% Al- 30% Cu, 95% Al-5% Zr and 90% Al-10% Mn as first, second and third master alloys, adding to the molten charge 50% Al- 50% Mg and 95% Al- 5% Cr as fourth and fifth master alloys together with elemental pure Zn one by one, subjecting the molten alloys to another step of heating at 760°C, subjecting the molten alloys to a steps of cooling as herein described, adding to the molten alloy 95% AI-5% Ti as sixth master alloy, degassing the molten melt to remove the dissolved gases like hydrogen and pouring it under argon atmosphere into a metallic mould characterised by the step of, subjecting the alloy to a step of homogenisation by heating at temperature range of 465+ 5°C for 30 to 40 hours, to form billets to eleminate dendiritic segregation in the cast microstructure, removing the oxidised layers formed on the surface of the said billets by scalping, subjecting the said billets to a step of extrusion processing at a initial billet temperature of 415-430°C to form extrusion rods, subjecting the extrusion rods to a steps of cooling preferably at the temperature range of 465 to 475°C for 2 to 5 hours, subjecting the rods to a step of solution treatment at the temperature of 465 to 475° for 2 to 5 hours followed by water quenching at room temperature, subjecting the extrusion rods to a step of stretching, subjecting the stretched material to a two stage of artificial ageing wherein the first stage of ageing is carried out at 95-105°C for 4-10 hours and the second stage ageing is carried out at 120-135°C for 10-20 hours.

Full Text

This invention relates to a process for preparation of Al-Zn-Mg-Cu based alloys. Al-Zn-Mg-Cu based AA7000 scries aluminium alloys are Uie highest strength aluminium alloy's which can be produced via inpot me In11urgica 1 route. These alloys therefore find ^'plications in light weight bridge, automobile, aircraft, armour material components etc. These alloys are provided ( «-i tb an anodic coating with the primary objective of protection against corrosion. In such cases, the coating thickness varies between 1 and ?,r)^im. When the principal function of the anodic coating Involves hard, wenr reRtstant * And abrasion reslstftnt surfaces, hard anodising process Is adopted. For engineering applications, -the hard nnodlc t cnntlnp of nt least Z5iim thickness are developed on the iJJoy. For certain applications where these alloys may be Used at high temperatures lor a short duration, formation of a continuous hard anodic oxide coatings of thickness beyond 70jim on these alloys may be critically required for such coatings to act as thermal barrier. The 'existing Al-Zn-Mg-Cu based AA7000 series commercial alloys, mainly comprise of Al , Zn (3 .8-8 .7 % ) , Mg(l .5-3.7%), Cu(0.4-2.6%), and Cr ( 0. 06-0. 35/^r+Ti as 0.25% maximum. The maximum permissible limit of the impurities Fe, Si and Mn which may be present in the commercial grade alloys are as high as 0.5% Fe , 0.4% Si and 0.3% Mn. These commercially available alloys have the ability to develop on them, a hard anodic coating of thickness (50 ^m) in one step. During the process of anodic coating, bath voltage rises rapidly and reaches 85V within 70 minutes due to which the process has to be interrupted at an intermediate stage and has to be carried out into 3-4 stages to get the anodic coating of thickness beyond 70/um. Even though, the desired coating thickness is achieved in multiple stages, yet it is of inferior quality to serve the required purpose. Al-Zn-Mg-Cu based alloy is mainly processed through ingot metallurgical route. In the process known in the art, the alloy compostion is first melted in an induction furnace and poured on to a suitable mould to obtain the cast ingot. The cast ingot is then subjected to thn homogenizing annealing at suitable temperature and is thereafter rubejrted to non-destructive testing to detect the casting defects. The sound ingots, for most of the uses, are subjected to hot deformation by direct extrusion. Thereafter, the material is solution heat treated, water quenched, stretched for stress relief purposes and subjected a to artificial aging at 120 C to obtain the desired strength through precipitation hardening. The components of interest, are then machined out from the longitudenal -3- sections of the fully heat treated materials and subjected to the hard anodising process. In the known process of hard anodising, the machined components are degreased in trichloroethylene followed by £> descaling at 55-65 C for 2-5 minutes in mixture of sulphuric acid (150 ml/litre) and chromium trioxide ( 60(7 / J i tre) . The components are then rinsed throughly in water. Uesmutting is subsequently carried out in a mixture of nitric acid (500mJ/1itre) and water (50ml/1itre ) for 2 minutes followed by thorough rinsing with water. The components are then subjected to hard anodising treatment. The components are made anode and lead is used as cathode. The electrolyte used for the hard anodising treatment, is a mixture of 22% sulphuric acid, 1.5% oxalic acid containing O.lwt% of aluminium sulphate and de-mineralised water (balance). The agitation in the bath is provided by air. For hard anodising the operating temperature of the r electrolyte is maintained between -8 to 0 C as compared to 20-30 C in the case of conventional anodising process. The direct-current power supply is used for the anodising process. A current' density varying in the range of 18-35 amp/sq ft is maintained constant during the process. The disadvantage with the above process is that bath voltage rises rapidly with anodising time and reaches to about 85V within only 70 minutes by which time a maximum anodic coating thickness 45~55yum is usually obtained. Due to this problem, anodising treatment has to be discontinued at the intermediate stage to avoid the increase in voltage beyond 85V so as to avoid burning of the coated components. M. The desired minimum- anodic coating thickness of * 70,um is obtained in two or* three stages where in each stage Lho but!, voltage is not allowed to exceed above 85V at each 01 the stage. The hard anodising treatment is stopped after 2-3 inturrupted stages after which anodic oxide coating thickness of 80-90 yum is obtained. Such an interrupted process for hard anodic oxide coating is unsuitable for commercial applications. Further disadvantage of the above known process is that in this process artificial aging to develop strength through precipitation hardening is carried out in one-step involving aging for 12-24h in the temperature range of 120- c • 135 C. Such alloys overage rapidJy giving rise to coarse distribution of precipitates even at aging temperature upto 135°C. The primary object of the present invention is to propose a process for preparation of . Al-2n-Mg-Cu based alloys with low levels of impurities of Fe, Si and Mn. More particularly, the object is that the Fe, Si and Mn impurities in the Al-Zn-Mg-Cu based alloys are restricted below the threshold limits disclosed in the invention. Another object of the present invention is to proposv: a process for preparation of . . Al-Zn-Mg-Cu based alloys which can be hard anodised to anodic oxide coating thickness beyond 70yum, in one single operational anodising step without any interruptions during the anodising process. These and other objects and advantages wil] be more clearJy understood from the detailed description that follows/ figures and specific example which is intended to be typical of/ rather than in anyway limiting on the scope. Still another object of the present invention is to propose a process for preparation of improved AI-Zn-Mg-Cu alloys wherein artificial aging to obtain peak aged properties is carried out in two stages, 4-10 hours at 95-105°C during first stage followed by second stage aging at 120-135°C for 10-20 hours, which gives rise to a fine and uniform distribution of the strengthening phase precipitates in the matrix, and greatly minimizes the formation of coarse precipitates at the grain boundry. Yet a further object of the present invention is to propose a process for preparation of AI-Zn-Mg-Cu alloys wherein heating to the homogenisation temperature is carried out between at 465j: 5°C for 30-40 hours which helps to obtain a fine and uniform distribution of precipitate involving recrystallsation inhibiting elements such as Zirconium and chromium, which helps to effectively control the grain structure of the alloy in the wrought microstructure, and this in turn has a decisive influence in obtaining a uniform precipitate morphology in the finally heat treated microstructure. A further object of the present invention is to propose a process for preparation of AI-Zn-Mg-Cu based alloy capable of being hard anodised to develop a superior quality continuous hard anodic coating without any discontinuities/disruptions in the local regions of the hard anodic coating. A still further object of the present invention is to propose a process for preparation of an commercially more suitable AI-Zn-Mg-Cu based alloys which are amenable for hard anodic oxide coating thickness beyond 70nm in an uninterrupted single stage thereby enhancing commercial suitability of the anodic oxide coating process for these alloys. : 7 : According to the present invention there is provided a process for the preparation of Al-Zn-Mg-Cu based alloys characterized by the steps of: a) Melting a charge mixture at a temperature of 740-760°C of 7.624 wt of aluminum with 0.466 wt of 70% AI- 30% Cu, 0.24 wt of 95% Al-5% Zr and 0.050 wt of 90% Al-10% Mn as first, second and third master alloys, bj adding to the molten charge 0.5 wt of 50% AI- 50% Mg and 0.360 wt of 95% AI- 5% Cr as fourth and fifth master alloys together with 0.560 wt elemental pure Zn ore by one, c} subjecting the molten alloys to another step of heating at 760°C, d) subjecting the molten alloys to a steps of cooling as herein described, e) adding to the molten alloy 95% Al-5% Ti as sixth master alloy, f) degassing the molten melt to remove the dissolved gases like hydrogen and pouring it under argon atmosphere into a metallic mould, g) subjecting the alloy to a step of homogenisation by heating at temperature range of 465+_5°C for 30 to 40 hours, to form billets to eleminate dendiritic segregation in the cast microstructure, h) removing the oxidised layer? formed on the surface of the said billets by scalping, • 8 : i) subjecting the said billets to a step of extrusion processing at a initial billet temperature of 415-430°C to form extrusion rods, j) subjecting the extrusion rods to a steps of cooling preferably at the temperature range of 465 to 475°C for 2 to 5 hours, k) subjecting the rods to a step of conventional solution treatment wherein the rodes are degreased in trichloro ethylene followed by descaling in the mixture of sulphuric acid & chromium trioxide at the temperature of 465 to 475° for 2 to 5 hours followed by water quenching at room temperature, 1) subjecting the extrusion rods to a step of stretching, m) subjecting the stretched material to a two stage of artificial ageing wherein the first stage of ageing is carried out at 95-105°C for 4-10 hours and the second stage ageing is carried out at 120-135°C for 10-20 hours. Further in accordance with the present invention a charge mixture of 99.8% and above purity primary aluminium and a first, second and third master alloys namely, 70% AI-30% Cu, 95% AI-5% Zr, 90% AI-10% Mn are prepared. The said charge mixture is melted in an induction furnance by heating at a temperature of 740-760°C. A fourth and fifth master alloys namely, 50% AI-50% Mg, 95% AI-5% Cr alongwith elemental pure Zn in the ingot form are added to the molten charge in the sequential order. The molten alloy is then subjected to a step of heating to a temperature of 760°C for about 10 minutes. The molten alloy : 9 : is then subjected to a step of cooling at a temperature of about 740°C and to which a sixth master alloy namely 95% Al-5% Ti is added for grain refinement. The molten melt is degassed to remove the dissolved gases like hydrogen and it is then poured into a metallic mould under argon atmosphere. The alloy is subjected to a step of homogenising. The step of homogenising is carried out in a temperature range of 465i 5°C for 30-40 hours. The ingot is then heated to homogenisation V temperature at the heating rate of 25-35°C per hour. The homogenisation annealing eleminates dendiritic segregation in the cast microstructure. The surface of the billet is scalped to remove the oxidised layers from the surfaces and then the billets is subjected to the non destructive testing to detect casting defects. The said billets are then subjected to extrusion processing using 10:1 round bar extrusion ratio at initial billet temperature of 415-4300C, preferably at 420°C, container temperature of 370-390° preferably at 380°C and ram speed of 0.8 to 3 mm/sec preferably Imm/sec. The mechanical processing of the billet can also be carried out by any other deformation route such as rolling. The extrusion rods are then cooled and subjected to solution treatment wherein the components are degreased in trichloro ethylene followed by descaling in the mixture of Sulphuric Acid 65 chromium trioxide at the temperature of 465 to 475° for 2 to 5 hours followed by water quenching at room temperature range of 465 to 475°C for 2 to 5 hours followed by water quenching at room temperature. The extrusion as obtained by the above step are stretched to obtain 1.5 to 3.0% permanent set for stress relief purposes. The stretched material is subjected to two-stage artificial aging at 95-105°C for 4-10 hours in the first stage followed by a second stage aging in the temperature range of 120-135°C for 10-20 hours. The components from the longitudinal sections of the round bar extrusions are then machined out. By the present Invention the 1 itnl tat ionsof the prior art can be largely overcome by a process which rnrtrlclp the Jevel of. impurities in the AI -7.n ~Mf» - n The source of impurities like Fe and Si in commercial 7ft00 series Al alloys is the primary alliminjum whirh IP ur-rr1 a^ n part of i h e charge for I lie alloy male MIR. For Mie conmercial production of 7000 scricp Al alloys, hip. h purity 'tt i UITI (AJ) is not used. MI addition, for l-rlfrr mix inn 'lie alloy charges ditriiifr meltinp fen obtain; MR rieotifi mixture of alloy mo J t , a combination of primary All, various master alloys and suitable Al alloy scraps are used. These alloy scraps are the source of Mn impurity in the Al Zn Mg-Cu- based alloys. The beneficial effects of restricting of impurities below the above mentioned limits is that this does not allow the formation of an Iron (Fe) — rich. Al -I7 'e- -S i -Mn-Cu particles based consituent phase. This phase survives the hard anodising treatment, locally inhibit the nucleation and growth of the anodic oxide film in local regions and inhibit the formation of continuous coating . Besides constituent particles get partly coated with a thin oxide film of high electrical resistance. The latter gives rise to added resistance to the electrical path during anodic coating. This together with electrical resistance provided by the barrier layer gives rise to observed high bath voltage within a shorter period. Restricting the level of impurities of Mn and Si in the AJ-Zn-Mg-Cu based alloys, inhibits the formation of Al Fe-Si-Mn-Cu .based inter-metallic particles which are responsible for the observed rapid increase in bath voltage with anodising time. With restricted level of Mn and Si, less coarse Fe-rich Al Fe Cu particles base constituent phase is formed which dissolves only the restricted amounts of Mn and Si in it. This phase is not found to cause the bath voltage to rise rapidly with time during the hard anodising process. Restricting the level of Iron (Fe) reduces the volume fraction of Fe-rich Al-Fe-Cu based constituent particle. Combined reduction of Mn , Si and Fe in the alloy forms Al Fe Cu based particles of reduced volume fraction, size and number density. The consequence is that the Al-Fe-Cu particles of smaller size readily get either dissolved or/and oxidised during the anodising process, thus not giving rise to any discontinuity in the anodic oxide film. Thus restricting the level of Fe- impurity in Al-Zn-Mg-Cu alloy aids the process of formation of a continuous hard anodic oxide coating on these alloys. The impurities in Al-Zn-Mg-Cu based alloys are kept below the above-mentioned threshold limits by taking the charge mixture of 99.8% and above purity primary aluminium and master alloys like Al-Cu, Al-Zr etc made out of 99.8% aluminium. In the known process, the molten metal is poured in air into the moliCHx whereas in the process of the present invention, the molten metal is poured under argon atmosphere into the mould. This reduces the oxidation of the melt, thus reducing the formation of dross in the mould. Further in the known processes, the alloy ingot is heated to the homogenisation temperature at an arbitrary rate or at a high rate whereas in the process of the present invention heating is carried at constant rate of 25-35° C per hour which helps' to obtain a fine and uniform distribution of precipitate involving recrystal1isation inhibiting elements such as Zirconium and chromium. This in turnxcontrols the wrought grain structure, and finally the precipitate morphology in the fully heatLveated microstructure in an effective manner, further new feature of the process is that in the known / processes, artificial aging to develop strength through precipation hardening is generally carried out in one step involving aging for 12-24 hours in temperature range of 120-13b° C. Such alloys overage rapidly giving rise to a coarse distribution of precipitates even at aging temperature below 130°C.' In the present invention aging is carried out in two stages, in the first stage for 4-10 hours at 95-105*C followed by aging at 120-130"C for 10-20 hours during second stage. This heat treatment practice greatly encourages t>he formation of fine and uniforni precipitates in the matrix and minimizes the formation of coatee grain boundary precipitates. The invention will be more fully understood from th discussion of the following representative example of alloy form studied the evaluation of the present in-'-on1 ion. However/ such an ex amp.IP ir n«f inipnrl For a 10 kg melt/ a mixture of 7.624 kgs of 99. H* purity primary aluminium (i.e. 99.8% al and the balance being 0.11% Fe and 0.09 wt% Si impurities), 0.466kg of Al-30% Cu, 0.24kg of Al-5% Zr master alloys and 0.050 kg of Al-10% Mn master alloy is charged into the induction furnance. The charge is heated to 740°C. When the charge has melted/ 0.50 kg of Al-50% Mg master alloy, 0.360 kg of Al-5% Cr alloy and 0.560 kg of pure Zn in the ingot form at* added in the above sequence. The charge is superheated to and whole matelal is held at this temperature for 10 minutes. The temperature is then reduced to 740 C and 0.200 kg of Al-5% Ti master alloy is added for grain refinement purposes. After 5 minutes, 25g of FOSEC made Degasser-190 pellets were added for degassing purposes. The molten metal is then poured jjnder argon atmosphere into a metallic mould of sui table size. When the melt had solidified, the ingot was cleared of the portions having casting defects. A cylindrical billet of 85mm diameter and 120mm height was fabricated out of the cast ingot. The billet was subjected to the homogenising annealing at 465 C for 35 hours and air cooled. The billet was scalped and was subjected to the extrusion processing. Extrusion was carried out using 10:1 round bar extrusion ratio and at an initial billet temperature of 420 C , container temperature of 380 C and ram speed of Imm/sec. The extrusion is air cooled. It !*»»? then subjected to 6 solution treatment at 470 C for 2 hours followed by water quenching at room temperature. The quenched extruda'tes were stretched to obtain 1.8% permanent set for stress relief purposes. The extursion was then subjected to artificial aging at 100° C for 8 hours followed by ntijicial aging at 120" C for 15 hours. from the fully heat treated extrusion rod, component samples of 90x20x5 mm were machined out form the longitudinal direction of the round bar extrusions and are subjected to the hard anodising treatment. For the purpose of evaluation of the alloy, a sample components of commercial alloy AA7075 with weight composition of Al -5 .6Zn-2 . 5Mg-l . 4Cu-0 .12. Cr containing 0.10 Ti & 0.12Zr and impurity level of 0.20Si, 0.18Mn^( referred in the following description as AlloyZ) was evaluated with the sample component of alloy (referred in the following description as alloyl) prepared by the process of the present invention with impurity level of 0.05Si, 0.04Mn and O.lZFe. The components of alloyl and alloy2 were subjected to known hard anodising process using a current density of 18 amp/SQjft and a total maximum anodising time of 130 min was used in order to obtain the desired coating thickness of 80±10 yum. After anodising, the components were thoroughly rinsed with water. The anodic coating thickness was measured using iso-scope based on eddy current principle for non-conducting oxide coatings. The cross-sections of the an od ic coatings together with the chemistry and morphology of the constituent particles present in the heat-treated materials as well as within the anodic coatings were characterised using a combination of scanning electron microscopy (SEM) and electron probe micro analyser (EPMA). A quantitative x-ray WDS system attached to EPMA was used to analyse the chemistry of the constituent particles. The chemical analysis was carried out on polished but unetched samples. The scanning electron micrographs were taken in the back scattered electron imaging mode. Fig l(a) & (b) show tht, scanning electron micrographs of the longitudenal sections cf the fully heat treated extrudates of alloyl and alloy? respectively. The micrographs typically show the presenc- of "stringers" i.e. rows of particles (produced as a result -.f the break-up of the constituent particles during working) along the extrusion direction. Fi 11 electron micrograph of the transverse section of extrudates of aJloy2. In this direction, the stringers are viewed end-on and they appear as non-uniformly distributed small particles across the extrudate section. Comparison of Figl (a) and (b) shows that number density of Fe-rich particles in ailoyZ is considerably higher. This is consistent with the presence of higher amounts of impurities in alloy??. Fig. 2 shows the elemental x-ray maps of the costituent particles present in alloyl. This together with the quantitative x-ray WDS analysis carried out on the as-polished samples of alloyl on EPMA revealed that the particles appearing bright are Fe-rich Al-Fe-Cu-Mn based phase having 22% Fe, 5.8% Cu and 0.3% Mn and those appearing dark grey are Mg^Si particles. Similar analysis carried out on alloy 2 also revealed the presence of Mga.Si and a Fe-rich phase, however this Fe-rich phase was found to be complex one having 22% Fe, 5.2% Si, 4.1% Mn and 3.6% Cu and 0.85% Cr(Fig 3). Fig 4(a) and (b) show the bath-voltage time curves for alloyl & alloy2. Fig £f {a) shows that the voltage increases steadily with time for 120 min with the terminating voltage of 83V. The thickness of the oxide coating was found to be in the rang of 85-90/im whilst, in the case of alloy2, 83V was reached within only 70 min and the anodising treatment had to be stopped to avoid the increase in voltage beyond 85V, thus avoiding the buring of Mte components. The average thickness of the anodic coating under this condition was found to be 50+ 5/tm. The desired thickness of 85-90;jm coating for alloyZ could be obtained in two further steps, with the bath voltage not being allowed to exceed 85V, at each step. Fig. S'\ a) & (b) show the cross-sections of anodic oxide films developed on alloyl (after 120 min) and alloyZ (after 130 min) respectively. The thickness of anodic ' oat ing in case is about 90um. However in case of alloyl, anodic coating is continuous whilst in case of alloy2, continuity of the anodic coating is greatly disruppted. Fig.6(a) shows the cross section of the anodic oxide film developed on alloyZ after ther hard anodising treatment was inerrupted after 70 min. This together with elemental x-ray maps shown in fig 6(b) show that the remains of the Al-Fe-Si-Mn-Cu based particles are always present in the regions of discontinuity of the anodic film. No trace of Mg j. Si particle could be found at those sites implying that they dissolve during the anodising treatment. The increase in bath-voltage at a much a much faster rate for alioy2 is understood to be primarily due to the increasing number of Ai-Fe-Si-Mn-Cu based particles being encountered with time during anodising treatment Fig 6 thus, provides for the first time, the direct evidence showing the inhibition of the formation of the anodic film by a constituent particle. The hexagonal shaped Fe-rich particle arrowed in Fig 6(a) has stopped the formation of the anodic film locally and has caused the anodic film to grow bypassing it. Such features are not nb.qervT:] in the case of alloyl. The presente of an iron(Fe) • rn rich AI-Ve-Si-Mn-Cu based constituent particle hag thus been directly proved to be deterimental to the quality of the bird anodic coating on conrnercial all 0*7 AA7075. I Claim: 1) A process for the preparation of Al-Zn-Mg-Cu based alloys characterized by the steps of: (a) Melting a charge mixture at a temperature of 740-760°C of 7.624 wt of aluminum with 0.466 wt of 70% AI- 30% Cu, 0.24 wt of 95% Al-5% Zr and 0.050 wt of 90% Al-10% Mn as first, second and third master alloys, (b) adding to the molten charge 0.5 wt of 50% Al- 50% Mg and 0.360 wt of 95% Al- 5% Cr as fourth and fifth master alloys together with 0.560 wt elemental pure Zn one by one, (c) subjecting the molten alloys to another step of heating at 760°C, (d) subjecting the molten alloys to a steps of cooling as herein described, (e) adding to the molten alloy 95% AI-5% Ti as sixth master alloy, (f) degassing the molten melt to remove the dissolved gases like hydrogen and pouring it under argon atmosphere into a metallic mould, (g) subjecting the alloy to a step of homogenisation by heating at temperature range of 465+ 5°C for 30 to 40 hours, to form billets to eleminate dendiritic segregation in the cast microstructure, (h) removing the oxidised layers formed on the surface of the said billets by scalping, (i) subjecting the said billets to a step of extrusion processing at a initial billet temperature of 415-430°C to form extrusion rods, 0) subjecting the extrusion rods to a steps of cooling preferably at the temperature range of 465 to 475°C for 2 to 5 hours, (k) subjecting the rods to a step of conventional solution treatment wherein the rodes are degreased in trichloro ethylene followed by descaling in the mixture of sulphuric acid & chromium trioxide at the temperature of 465 to 475° for 2 to 5 hours followed by water quenching at room temperature, (1) subjecting the extrusion rods to a step of stretching, (m) subjecting the stretched material to a two stage of artificial ageing wherein the first stage of ageing is carried out at 95-105°C for 4-10 hours and the second stage ageing is carried out at 120-135°C for 10-20 hours. (2) A process for the preparation of AI-Zn-Mg-Cu based alloys substantially as herein described and illustrated.

Documents:

1378-del-1996-abstract.pdf

1378-del-1996-claims.pdf

1378-DEL-1996-Correspondence-Others-(19-08-2010).pdf

1378-del-1996-correspondence-others.pdf

1378-del-1996-correspondence-po.pdf

1378-del-1996-description (complete).pdf

1378-del-1996-drawings.pdf

1378-del-1996-form-1.pdf

1378-del-1996-form-13.pdf

1378-DEL-1996-Form-15-(19-08-2010).pdf

1378-del-1996-form-19.pdf

1378-del-1996-form-2.pdf

1378-del-1996-form-26.pdf

1378-del-1996-form-3.pdf