GB2167443A - Fabricating structures from aluminium sheet - Google Patents

Description

1

SPECIFICATION

Amethodof fabricating structuresfrom aluminium 65 sheetand.structures comprising aluminium compo nents This invention relatesto a method of fabricating structuresfrom aluminium sheet and structures 70 comprising aluminium components.

Theterm "aluminium" as used here andthroughout the specification is intended to include aluminium alloys.

It iswell known for aluminium structuresto be fabricated by bonding components together after having pre-treated the surfaces of the components.

One such pre-treatment is DC phosphoric acid anodis ing as used in the aircraft industry, particularly by Boeing (British Patent 1,555,940), and this form of pre-treatment is considered to be one of the best available for long-term durability in structural applica tions. This durability isthoughtto depend on the structure of the oxide layer produced by DC phosphor ic acid anodising underthe Boeing conditions de scribed and many papers have been written on this subject (eg J D Venables etal, Appl. Surface Science 3, 1979,88-98). However, the Boeing process requires an anodising time of 5-60 minutes in a phosphoric acid electrolyte at a temperature of 10-300C. In practice an anodising time of 20-30 minutes is usually used, and clearly this is only suitable for batch treatment of components rather than as a continuous treatment for aluminium coil. Although oxide layerthicknesses are not reported in the patent examples, in practice a minimum thickness of 300-400 rim appears necessary to achieve the desired properties.

Oxide layers produced by the Boeing process have excellent properties as adhesive substrates, to the extent thatthey constitute a standard to which the rest 100 of the industry aspires.

It has also been proposed for aluminium structures to be "well-bonded", that is bonded with adhesive but also spot-welded.

According to a first aspect of this invention there is provided a method of fabricating structuresfrom aluminium sheet which comprises the steps of subjecting the aluminium sheetto an anodising process in an acidic electrolytefor a period of 2 minutes or lessto form an anodic oxide layerthereon: producing components of a desired shapefrom the aluminium sheet; and securing two components together by adhesive and by localised mechanical fastening means.

According to a second aspect of this invention there is provided a structure comprising aluminium compo- nents which are secured together by adhesive and by localised mechanical fastening means and in which the components have an anodic oxide layerformed thereon by being subjected to an anodising process in an acidic electrolytefara periGdof 2 minutesor less.

Preferredfeaturesoif this invenfLonwitt be apparent frorn the subsidiary claims of the specification.

The invention enables structures to be fabricated GB 2 167 443 A 1 from aluminium componentswhich have undergone a rapid pre-treatment butwhich nevertheless gives riseto bonds of strength and durability comparable to those achieved using the Boeing process.

Preferred forms of the invention also have the advantage thatthe anodising process can be carried out on aluminium sheet in coil form and can thus be effected continuously. Preferred forms of the invention also have the advantage of being spot-weldable.

Factors affecting features of this invention and their influence on preferred forms of the invention will now be discussed merely by way of illustration.

Equipmentfor continuous anodising of aluminium strip is well known, and is described for example in "Automation in Anodising" by W E Cooke (Aluminium Association, Aluminium Finishing Symposium, Chicago, March 1973). Suitable equipment includes an elongated bath with inlet and outlet ports for electrolyte and with opposed end faces having seals if necessary through which the continuous aluminium strip passes, the arrangement being such that the electrolyte preferablyf lows countercurrent to the strip. Two or more electrodes are positioned adjacent or indeed surrounding the moving strip, the electrodes being spaced in the direction of travel of the strip. Current leakage through the electrolyte is low because the electrolyte has a much lower conductivity thanthemetal.

In a preferred form of the invention the aluminium sheet is in the form of a coil forthe advantage of rapid anodising and for convenience of storage and transport. In orderforthe anodising processto be carried out continuously, the tail of one coil may bejoined to the head of the nextsothatthe sheetcan be continuously passedthrough a bath of electrolyte. Existing aluminium treatment plants generally have a line speed of at least 50 m/min, and often of 150-250 m/min. Thus, in orderto avoid the need forvery long electrolyte baths, the anodising process should take place rapidly. A contact time of 15 seconds between the sheet and the electrolyte is the longest that is likely to be practicable on existing plant. Electrolyte contact times of 1 to 6 seconds, and preferably 2 to 3 seconds, are likelyto be convenient, and times as short as 0.5 seconds are possible. The electrolyte contacttime at any particular line speed may be regarded as a fixed feature of the plant, and one aboutwhich the other process variables are adjusted. On certain types of plant much slower lines can be used and hence longer anodising times.

During the anodising process a satisfactory balance must be achieved between anodic oxideformation on the aluminium sheet and dissolution of the oxide in the acidic electrolyte. Sufficent anodic oxide must be grown to give adequate structural strength to the oxide layer and to provide an adequate surface area to provide a good substrate for adhesive. Equally, dissolution of the anodic oxide layertakes place so thatthe surface area is effectively enlarged by attack of the pore wall structure. However, this attack must not be sufficientto cause breakdown and powdering of the oxide layer.

This print takes account of replacement documents submitted after the date of filing to enable the application to comply with the formal requirements of the Patents Rules 1982.

2 GB 2 167 443 A 2 Anodic oxide formation is essentially controlled by the anodising currentdensity used.Anodic oxide growth perunittime is substantially proportionalto anodising current density. With theshortcontact times available, the current density needs to be high to 70 achieve a sufficentlythick oxide layer. The current density is preferably at least 250 AIM2.

It is convenient to relate current density with electrolyte contacttime in orderto achieve a desired oxide thickness. This may be expressed by saying that 75 the total anodising input will usually be in the range 2xl 04to 12xl C)4, particularly3xl 04 to 6xl 04, C1m2.

Film attack is essentially controlled by the nature, concentration, and temperature of the electrolyte, with temperature being the most important factor. In considering the nature of this attack, it needs to be borne in mind that an anodic oxide film is created at the metalloxide interface, ie atthe innersurface of the oxide layer remote from the electrolyte. Chemical dissolution occurs atthe outer surface of the oxide layer, and it is thus the oldest remaining oxidethat is subject to attack.

The anodising electric current is preferrablyAC so thatthe aluminium sheet is alternately anodically polarised (during which time oxide growth predominates) and cathodically polarised (during which time chemical dissolution of the oxide predominates). Biased AC waveforms may be employed with advantageto achievethe desired balance between anodic growth of the oxide layerand chemical dissolution. TheACfrequency may be greateror (more likely) lessthan the standard 50 cls. Alternatively DC may be employed, either continuously or as a pulsed curreritto increase the extent of chemical dissolution (between the pulses) relative to growth of 100 the oxide layer.

Thevoltage is determined bythe value of current density at which one has chosen to operate. Hence it finds its own level according to the current density and temperature (it is quite markedly effected by temperature at constant current density). For example at the lower end of the temperature range, eg at350C, the voltage is about 40V for 600 amps/meter'. The voltage is reduced as the temperature goes up.

The temperature of the electrolyte is preferably at 110 least 25'C for short anodising times. If the electrolyte temperature istoo low, then no significant chemical dissolution of the oxide takes place during the (limited) electrolyte contact time and the surface area thereof is not increased. If the electrolyte temperature 115 is too high, then chemical dissolution may outpace oxide growth to the extent that a 11 oxide is redissolved as fast as it is formed. The preferred temperature range depends on the acids used in the electrolyte.

Generally, with an acid that attacks aluminium oxide 120 strongly a lower temperature is needed than with an acid that attacks the oxide less readily.

Electrolyte concentration has a much less marked effect on the rate of chemical dissolution of the oxide film than temperature. The dissolution rate increases 125 with electrolyte concentration and a concentration of at least 5% by weight of acid is found preferable in orderto achieve rapid anodising.

The oxide layerformed onthe aluminium sheet by the anodising is preferably relatively thin compared to 130 that produced in the Boeing process. If the components are to be spot- welded (described below in more detail) the thickness of the oxide layer is preferably keptto 50Orim or less otherwise the resistence of the layer may be too greatto enable satisfactory spotwelds to be easily formed. The thickness of the oxide layer is also preferably at least 15rim as belowthis level controlled chemical dissolution of the oxide is difficuitto achieve.

Theanodising process can be carried out in a numberof different electrolytes based on acids such as phosphoric acis and sulphuricacid orother acids in which porous aluminium oxide layers areformed, such as chrimic acid oroxalic acid.The electrolyte may also comprise a mixture of such acids.

A preferred electrolyte comprises from 5to 15% by weight of phosphoric acid. Phosphoric acid is capable of strongly attacking the anodic oxide layer so it is difficuitto achieve a balance between oxideformation and oxide dissolution during the anodising process particularly when short anodising times are needed to be compatible with existing process lines. With an anodising time of 15 seconds or less, the current density used should preferably be at least 250 A/M2 and maybe as high ascan be achievedwiththe equipment used, eg upto 3000AIM2 A preferred current density range is 300to 1500 AIM2.

As phosphoric acid attacks aluminium oxide so readily, it is difficu It to achieve sufficient oxide growth at high temperatures. It has not proved possible to generate an anodic oxide layer under AC conditions in a phosphoric acid electroltye at 90'C even with a eu rrent density of 1250 AIM2. When AC anodising is employed, the optimum electrolyte temperature is 1 ikely to be in the range 30 to 70'C. With DC anodisi rig, somewhat h ig her tem peratu res u p to 80'C may be useful.

With the optimum conditions described, anodising times as short as 0.5 seconds may be achieved.

Afurther advantage of using a phosphoric acid electrolyte isthatthe anodic oxide layerformed incorporates significant amounts of phosphate. Phosphate is known to be a hydration inhibitor with oxide surfaces, and as deterioration of the pre-treated surface often occursthrough hydration of the oxide, at least at its surface, the presence of a hydration inhibitor atthis point is beneficial.

Because the oxide is readily attacked bythe hot phosphoric acid electrolyte, rapid rinsing of the oxide layersurface is required after anodising, and this is readily achieved in a continuous coil process.

The result of the phosphoric acid anodising process is an aluminium sheet carrying a porous anodic oxide layerwhich contains phosphate ions, the pores of which are enlarged so thatthe effective surface area of the oxide layer is increased. The oxide layer is generally 15 to 200 rim thick. With an electrolyte contacttime of no more than 15 seconds, it is difficult to produce an oxide layer more than 200 rim thick.

Another preferred electrolyte comprises 10 to 30% by weight of sulphuric acid. Sulphuric acid attacks aluminium oxide less readily than phosphoric acid so the electrolyte is generally more concentrated and at a highertemperature than that used with phosphoric acid in orderto maintain a sufficient dissolution rate of 3 GB 2 167 443 A 3 theoxide. With a sulphuricacid electrolytethe anodising process is preferably effected at a temperatureof at leas1501C.The optimum electrolyte temperarange 7Tto 95'C.

With a. current density of at least 250 NM2 and an electrolytúscorlta,ct, tfr,neof between 0.5 and 15 seconds,the oxide layerformed generally has a thickness of 300 rim or legs..

Conditions similarto those described in UK Patent Specification No. 1235661 which discloses a method of anodising aluminium sheet in a sulphuric acid electrolyte in preparation forthe application of lacquer mayalso be suitable.

Afterthe anodising process, press lubricant which is selected to be compatible with the anodising process used and the adhesive subsequently applied, for instance Houghtodraw 7002 (Trademark) made by EdgarVaughan Limited, is applied onto the oxide layer. The aluminium sheet is then cut into discrete lengths or is recoiled for ease of storage. Alternatively, the aluminium sheet can be cut into discrete lengths before the press lubricant is applied. It is also possible to recoil the aluminium sheet afterthe anodising process forstorage and to apply the press lubricant after it has been uncoiled again, the sheet being cut into discrete lengths either before or afterthe application of the press lubricant. The press lubricant is preferably applied by machine, eg by spraying or roller coating, to ensure that a uniform coating is formed. As little press lubricant as is necessaryfor satisfactoryforming is used- usually less than 20 grammes/square metre, and preferably less than 5 grammes/square metre. Alight oil or separating agent such as dioctyl sebacate as used priorto coiling aluminium strip, or lacquer as used in the canning industry, are not suitable as press lubricants.

In some forming operations no press lubricant is required. Some structure can also be produced without anyforming operation.

Having produced discrete lengths of aluminium sheetcarrying the oxide layer and press lubricant by any of the routes described above, these are then formed into components of desired shapes. In the case of compoments for a motorvehicle body, this may involve pressing the sheet between dies and the punching of any holes required. An epoxide adhesive, for instance that produced by Permabond Inc under the Trade Mark ESP1 05, is then applied to the components which are assembled together in a jig.

The components are then secured together by localised mechanical fastening means, for instance spotwelds, while the adhesive is still fluid. The structure can thus be removed from the jig before the adhesive has cured. The adhesive is cured for 10-30 minutes at a temperature of 150'to 180'or such othertimes and temperatures as are suitable forthe particular adhesive used. Phenolic or acrylic adhesives can be used in place of theepoxi.de adhesive.

The anodised aluminiurn coil or cut sheets can be storedfor upto 6 months in manytypical storage conditions without any significant deterioration in the oxide layer. The oxide layer is thus capable of providing a sound base for a strong and durable adhesive bond even if the sheet is stored for a considerable time between the anodising process and 130 the application of adhesive. In practice, it is essential thatthe anodised sheet is storage stable as there is often a delay of at least 48 hours (2 days) and usually more than 168 hours (7 days) between the anodising of the aluminium sheet atone site, eg atan aluminium mill, and the forming of the aluminium sheet into components of desired shape at another site, eg in a vehicle production line. The storage stability of the anodised aluminium sheet is, of course, enhanced if the press lubricant is applied before storage.

The aluminium sheet maybe degreased before the anodising process but one advantage of AC anodising is that it renders the surface of the sheet cyclically anodic and cathodicwith evolution of hydrogen atthe surface. this tends to separate any grease or other contamination from the surface of the sheet so the contamination is lifted off the surface. Air agitation can also be used to assist in the removal of contamination. As mentioned above, the electrolyte may also be passed through the bath in a direction opposite to that of advance of the aluminium sheet so that any contamination in the bath is swept away from the area of the bath where the sheet emerges from the electrolyte.

The press lubricant applied to the oxide layer may be oil, grease orwater based. The removal of an appropriately selected press lubricant remaining on the formed components priorto application of the adhesive is not necessary. Indeed, the complete removal of lubricant priorto application of the adhesive would be impracticable in a mass production line. The press lubricant may be pushed aside by the subsequently applied adhesive but may also become dispersed within the adhesive. The press lubricant should therefore be compatible with the anodised aluminium and with the adhesive so that is does not unduly affect bond durability and strength. The press lubricant should also be capable of ready removal priorto any painting operation even after being subjected to any elevated temperature at which adhesive has been cured.

The adhesive used in thejoints should be capable of retaining itsstrength undera widevariety of conditions such astemperature and humidity. The adhesive should wet the surface it is applied to but preferably be such that it does not sag or drip when applied to a vertical surface. Thixotropic materials maythus be preferred. The adhesive may be applied by any suitable method and may be applied to form a layerfrom about 0.1 to 3.0 mm thick in the final joint depending on joint geometry. The adhesive is preferably sufficentlyfluid to be squeezed out of the way at locations in thejointwhere pressure is applied by a spot- welding tool. It is also possible to use adhesive in a powdered or tape form. The adhesive is not usually applied overthe entire surface of the compoments although this maybe convenient when a powdered adhesive is used.

Resistence spot-welding is carried out through the adhesive whilst this is still in paste form using 5 mm truncated cone electrodes. An electrode pressure of 500 pounds (2.2 KN) is heldforatimeequalto 10 cycles of the electrical welding power prior to a 3 cycle weld at 23,000 amps, and is followed by a holding time of 10 cycles to allow tha molten slug of 4 GB 2 167 443 A 4 aluminium produced to solidify. Adjacent welds are spaced about6inchedto 1 foot(15to30crns) apart.

Ithas beenfoundthata structureformed inthe manner described above is strong enough to be load bearing and has durable bonds which substantially retain their strength with time. Itwill be appreciated that besides h ol ding the structure together when it is removed from thejig, the spot-welds or other localised mechanical fasteners also increase the strength of the joint between the bonded corn ponents and in particular increase the peel strength of thejoint.

Although it is possible for the spot-welds to be carried out at locations where there is no adhesive, it will generally befound desirable to spot-weld at a position where there is adhesive, the spot-welding being carried outthroughtthe adhesive before it has set.

As an alternativeto spot-welding, the components maybe rivetted together preferably using rivets which do not pierce both of the components so that the seal between the components is not broken. Otherforms of localised mechanical fasteners such as those which involve localised mechanical distortion of the components to secure them together, eg Tog-L-Lok (Trade Mark) of the BTM Corporation, may also be used.

The aluminium sheet may be an aluminium alloy such asthe 2000,3000,5000 or 6000 series of the Aluminium Association Incorporated Register.The optimum anodising conclitionswill generally differ for each alloy and tightercontrol of the conditions may be required with the 2000 Seriesthan with the othersto ensurethat a satisfactory oxide layeris produced. Itshould also be notedthat magnesium rich alloys of the 5000 Seriesform an oxide layer containing magnesium oxide (MgO) which is more soluble in acidic electrolytes so a lower tem peratu re may need to be used with such alloys.

Examples of conditions used in the anodising process will now be given merely byway of illustration.

EXAMPLE 1

Panelsof 5251 alloy were AC anodised in a 10% by weight phosphoric acid electrolyte at a temperature of 45'C and a current density of 600 A/M2 fora period of 10 seconds. The panels were rinsed immediately afterthe anodising process. The panels were then bonded in a perforated lap-shearjoint configuration using a toughened epoxy adhesive ESP105 (Trade Mark) produced by Permabond Inc. the initial bond strength was measured and the perforated joints were exposed to a neutral salt spray at43'Cfor periods of 2,4, and 8 weeks. Atthese intervals, samples were taken and the retention of initial bond strength monitored. As a control, material prepared as in British Patent specification 1555940 was also bonded and tested. This was 5251 alloy, DC anodised at 12V in 10% byweight phosphoric acid solution for

30 minutes.

Initial bondstrengths were identical; afterthe elapse of 8weeksthe retention of bond strength of the material prepared as described in Example 1 was 71.9% as compared to 70.1 % forthe DC prepared material. This demonstrates the potential performaz#0 of surfaces prepared by anodising using extremely short pre- treatment times. EXAMPLE2 In the second example, aluminium sheet 5251 is degreased using trichloroethylene vapour. The sheet is then subjected to alkaline cleaning using a 10% by volume aqueous solution of Oakite NST (Trade Mark) at 50'C. The sheet is immersed in this solution for a period of 5 minutes and then rinsed in running water fora period of 5 minutes. This treatment resulted in a water break free su rface.The surface is then deoxidised using a solution comprising 25911 of potassium dichromate, 50gli of sulphuric acid with small additions of fluoride, ammonium, aluminium, calcium and phosphate ions. A suitable solution is Deoxidiser No 1 (Trade Mark) produced by ICI pie. The sheet is immersed in this deoxidising solution for a period of 3 minutes and then rinsed in running waterfor a period of 10 minutes. This removes the pre-existing air- formed oxide layer. Next, the sheet is subjected to an AC anodising process fora period of 1 minute in a 10% by weight aqueous solution of orthophosphoric acid at 20'C with a current density of 80 A/M2. The sheet is final ly rinsed in ru nning waterfor a period of five minutes.

With an anodising time of 1 minutethis Example is applicableto a relatively slow moving line. EXAMPLE3 Example 3 is similarto Example 2 butthe sheetwas subjected to an AC anodising process fora period of 10 seconds in a 10% byweight aqueous solution of sulphuric aw)M at 900C with a current density of 1200 A1M2.

In order to assess the durability of the adhesive loo bonds formed in Examples 2 and 3, test strips were pre-treated and then bonded together using Permabond ESP1 05. Afirst set (D) of test strips was subjected to an AC anodising pre-treatmentin phosphoric acid as described in Example 2 for 1 minute and a second set (E) to a similar pre-treatment for2 minutes. Athird set (F) was subjected to an AC anodising pre-treatment in sulphuric acid as described in Example 3 for 10 seconds.A set (C) of control strips were also tested. The control strips were vapour degreased and alkaline cleaned as described in Example 2 and then deoxidised in a solution comprising sodium dichromate and sulphuric acid in accordance with the Boeing 5555 specification. This involved a DC anodising process in a 12% by weight orthophosphoric acid electrolyte at a temperature of 20to 25'C and at 10 voltsfora period of 20 minutes.

Test stripsfor each of the sets C, D, E, and 17were bonded together bythe adhesive ESP1 05 (Trade Mark) as described above. The lap shearstrength of these un-perforated bonds was tested afterthetest strips had been exposed to salt spray at430Cfor 2,4, 8,14,27 and 48 weeks. The results obtained are illustrated bythe accompanying graph which shows the fail u re strength of the u n-perfo rated joints with respeetto time for each of the sets. The strength retention after 48 weeks is also shown in Table 1.

GB 2 167 443 A 5 TME 1 PHS-TH INITIAL FAILURE STRENGTH FAILURE STRESS ASTER RETENTION S CMP&Y, ' 48 WEEKS: (NP'al 1 % 211L - o,,5 14.6 0.7 69.2 D 20.9 0.4 12.5 0.7 59.8 E 21.0 o.6 13.2 0.9 62.9 F 21.0 0.5 15.2 1.3 72.4 Strength retention after48 weeks Exposure to Salt Spray As will be seen, the strips pre-treated in sulphuric acid compare very favourably with those pre-treated bythe BAC 5555 process. The strips pre-treated in phosphoric acid also show strength retention after 48 weeks only slightly lowerthan that achieved with the Boeing 5555 process. This should be contrasted to strips which have not been pre-treated at all and which would lose all strength within a few weeks. The great advantage of the AC anodising process as compared to the Boeing 5555 process is that itforms an oxide layer which gives good strength retention and which can be spot welded through. It would not befeasibieto spot-weld through the oxide layer produced bythe Boeing 5555 pre-treatment.

Graph 30- 25- C, Ef 20- 5_ 0- 1 1 1 1 11 1 F c E D 400 EXPOSURE (days) Comparison of durability of pretreatments C to F in salt-spray @ 4loC Example 4

In Example 4, aluminium sheetof 5251 alloyO.7to 2.0 mm thick is AC anodised in an electrolyte comprising 15% byweight sulphuric acid at a temperature of 80'C. The charge input atthe surface of the sheet is arranged to be 12000 conlombs1M2 which is achieved using a current density of 1200A/M2 forafLmeGf 10 seconds. This anodising process forms a porous oxide layer about 0. 15 microns thick on the surface ofthe aluminium sheet.

In orderto assess bond durability of joints formed onsheetanodised inthiswayandto makecomparisonswith otherprocessesa number of tests were Q carried out.

In the firsttest, strips cut from 5351 aluminium sheet anodised in the manner described in Example 4 were bondedtogether and exposed to neutral salt spray for a range of times and the shearstrength of the bonds then measured by the perforated lap shear method. Various thicknesses of oxide layer were used and the adhesive ESP1 05 mentioned above as well as that produced by the 3M Company u nderthe Trade Mark EC2214 were used. SimIlartest strips were prepared from aluminium sheet which had undergone a conventional Boeing phosphoric acid anodising pre-treatment process (PAA) for30 minutes and these were tested in the same mannerfor comparison. The results of these tests are shown in Table 2.

As will be seen, the AC anodising pre-treatment gives results similar to those using DC phosphoric acid anodising according to the Boeing process.

6 GB 2 167 443 A 6 7AEME 2 SALT SPPAY TESTING Pre-Tteatment Adhesive Initial 2 Vkeks Salt Spray Testing Time Bond 8 Vkeks 20 hXTks Strength Bond % Strength 1 Bond % Strength Bond Strength M/2 S th Retention Strength Retention Strength Retention Z7 M/.2 M/m2 PAPL 30 min 2214 21.4 17.2 81.4 12.7 59.3 13.4 62.6 PAA 30 in ESP 105 21.4 111.8 87.9 17.1 79.9 15.3 71.5 Hot Ar 0.05 u),' 2214 19.7 14.1 71.7 12.9 65.4 11.0 5S.S Hot AC 0.05 u)= ESP 105 21.3 18.9 88.6 17.2 80.6 15.2 71.3 a Hot AC 0.15 u) 2214 21.2 17.1 80.6 13.4 63.0 12.7 60.1 r Hot AC 0.15 u) o" ESP 105 21.9 17.278.6 17.7 80.7 14.4 65.9 Ebt AC 0.3 u) o 2214 21.3 16.0 75.4 13.3 62.7 13.9 65.4 I M.

Ebt Ar 0.3 u) ESP 105 20.8 18.8 90.7 16.3 78.6 14.4 69.6 Teststrips which had been pre-treated bythe anodising process described above and then stored in various conditions before being bonded together were also tested. Three different storage conditions were used:

OD- Office Conditions (dry and relatively warm) 20 CW- Exposure in a deep shelter (cold and relatively humid) HW- Humidity Cabinet (hot and relatively humid) The results of these tests are shown in Table 3. Table 3 showsthatthe AC anodised pre-treated surface withstands storage in reasonable conditions for at least6 months without affecting subsequent bond strength but rapidly deteriorates underhotwet 17k9LE conditions. This is similarto results found using conventional phosphoric acid anodising (not illustrated).

Finally, tests were carried out with bonded test strips being stressed and exposed to a humid atmosphere. These tests were also carried out on strips which had undergone pre-treatment bythe Boeing phosphoric acid anodising process (PAA). The results of thesetests are shown in Table 4. The resu Its for strips with 0.05 micronsthick oxide layer produced by AC anodising and those with a 0.15 and 0.3 microns thick layer are similar to those with the Boeing phosphoric acid anodising.

EFF= OF S M PRE-MA AM BMIM =4 Adhesive Storage salt t MS Storage Time Bond Strength 2 ks 1 4 s 8 YLeeks 1 12 Weeks Pre-treatment iticns in after Storage Bond Bond Months M1.2 Str th Strength Strength Strew" MN1r2 MgI,,2 MhTI.2 M1,,2 Hot AC OD 1 21.5 15.6 14.1 14.8 14.2 (0.15 U) 2 21.4 15.7 14.6 14.7 14.3 in sulphuric acid 4 20.6 14.0 13.9 14.7 - 6 22.3 15.7 14.6 15.3 14.9 (Initial Bond C 1 21.7 15.6 14.0 14.8 14.1 Strength 19.9) 2 21.1 15.0 13.5 14.2 14.0 4 20.2 15.7 13.2 14.7 - 6 21.7 15.4 14.5 14.9 14.9 2214 Adhesive EW 1 7.8 4.9 3 8 4 6 4 3 2 - 65 4.6 4:9 5:1 4:8 4 8:2 43 2. 8 4.1 6 1.5 2:4 1 S 2.4 2.5 TABLE 4

BLDúúPq TEST3W (AEED, ESP 105) Time to Initial Sustained Ehilure Strength Stress (Days) Pre-treatment (M1M2) level, 1.t 2nd, 3rd PAA/30 20.1t 0.9 5 68 74 78 Hot AC O.OSU 5 78 96 135 Hot AC} o, 0.1su r 19.8 2.1 5 52 63 78 Hot AC 0.3u 20.4 t 1.5 5 55 CL Tests have also been carried out on strips which have been subjected to AC phosphoric anodising using a variety of conditions to determine whether they could be spot-welded through satisfactorily. An electrolyte comprising 10% by weight of phosphoric acid was used with a range of current densities. Each of the strips was anodised for a period of 2 minutes.

The temperature of the electrolytewas 2WC but in the firsttestwith a current density of 4000Alm2 the temperature rose to 40'C. This testwas therefore repeated with the electrolyte maintained ata temperature of 1 WC. ltwill be appreciated that as thistest was conducted to test the weldability of anodised strips, the conditions used have been selected to maximise the thickness of the oxide, layer formed and 1 7 GB 2 167 443 A 7 do notnecessarily represent preferred conditions for producingstrong durable joints. The conditions used an_d results achieved are summarized in Table 5. As wil[beseen,all the test strips could be satisfactorily 5 spot-welded together.

A strip anodised in accordance with Example 3 was also tested and could be spot-welded satisfactorily.

The anodising process described above are believed to remove the airformed oxide layerfrom the aluminium sheet and replace this by anew anodic oxide layer.

"le 5 Spot-Nelding Strips Anodised in 10% by 1Abight phosphoric Acid Current nensity Anodising Tim Teuperature Spot-welded? (A/.2) (Minutes) (cc) (Yes/kb) so 2 20 Yes 250 2 20 Yes 500 2 20 Yes 4000 2 20 440 Yes 4000 2 10 Yes The new anodic oxide layer comprises a nonporous barrier layer portion and a porous structure above this barrier layerwhich portion may have a total thickness of at least30nm. Different conditions in the anodising process produce differences in the structure and proportions of these two components. The porous nature of the new oxide layer may provide a keyto which the subsequently applied adhesive can be securely bonded. An increase in the surface area of the oxide layerthus tends to improve the bond which can be made to the subsequently applied adhesive. The porous structure formed bythe anodic process is attacked by the acidic electrolyte so the initial pore structure is enlarged. This again increases the effective surface area of the oxide layer and permits better penetration of the subsequently applied adhesive into the pores. It has been found that structures formed in the manner described above can be strong enough to form the structural, load bearing parts of a motorvehicle body. Accelerated test also indicate the such structures are capable of retaining adequate strength underthe wide variety of conditions that a motorvehicle generally encoun- ters fora time at least equal to the useful service life of the vehicle. The anodising processes described may be carried out much more quickly than many of the pre-treatments used in the prior art. The anodised sheet can be cut and formed without causing substantial damage to the porous oxide layer, even when the forming of the sheet involves pressing it between dies, so the oxide layer is still abieto provide abase for strong and durable adhesive bonds. As the anodising process may also be effected before the

Claims (37)

sheet is cut into discrete lengths it can also be carried out continuously and can be careful ly controlled. CLAIMS

1. A method of fabricating structures from aluminium sheetwhich comprises the steps of: subjecting the aluminium sheet to an anodising process in an acidic electrolyte fora period of 2-minutes or less to form an anodic oxide iayerthereon; producing components of a desired shapefrom the aluminium sheet; and securing two components together by adhesive and by localised mechanical fastening means.

2. A method as claimed in claim 1 in whichthe anodising process is effected by means of AC.

3. A method asclaimed in claim 1 or2 in which the anodising process is effected ata temperature of at least 2WC.

4. A method as claimed in any preceding claim in which the anodising process is effected with a current density of at least 25ONrn2.

5. A method as claimed in any preceding claim in which the aluminium sheet is subjected to the anodising process fora period of 15 seconds or less.

6. A method as claimed in any preceding claim in which the aluminium sheet is subjected to the anodising process fora period of at least 0.5 seconds.

7. A method as claimed in any preceding claim in which the electrolyte comprises at least 5% by weight of acid.

8. A method as claimed in any precedimg claim in which press lubricant is applied to the aluminium sheet afterthe anodising process and before the components are produced therefrom.

9. Amethod asclaimed in claim 8 in which the aluminium sheet is initially coiled, is uncoiled before the anodising process and is re-coiled after the application of press lubricant.

10. A method asclaimed in claim gin which the re-coiled aluminium sheet is uncoiled and cut into discrete lengths before the components are cut therefrom.

11. Amethod as claimed inclaim8 inwhich the aluminium sheet is initially coiled, is uncoiled before the anodising process and is cut into discrete lengths afterthe anodising process, the press lubricant being applied either before or afterthe aluminium sheet is cut.

12. A method asclaimed in claim 11 in which the aluminium sheet is recoiled afterthe anodising process and then uncoiled again before it is cut and before the application of press lubricant.

13. Amethodasclaimed in claim 10, 11 or12 in which the discrete lengths of aluminium sheets are formed in a press to produce the components.

14. A method as claimed in claim 14 in which the aluminium sheet, whether in coil form or indiscrete lengths, is stored for at least48 hours (2 days) after the application of press lubricant and before components are formed therefrom.

15. A method as claimed in claim 14 in which the aluminium sheet, whether in coil form or indiscrete lengths, is stored for at least 168 hours (7 days) after the application of press lubricant and before components are formed therefrom.

16. A method as claimed in any preceding claim in which the localised mechanical fastening means comprise spot-welds.

17. A method as claimed in any preceding claim in which the localised mechanical fastening means comprise rivets.

18. A method as claimed in any preceding claim in which the electrolyte comprises phosphoric acid.

19. A method as claimed in claim 18 in which the 8 anodising process is effected at a temperature of 80T or less.

20. A method as claimed in claims 3,4,5,6,7 and 19 in which the anodising process is effected ata temperature in the range 30'Cto 70T and with a current density of 3000 NM2 orless and in which the electrolyte comproses 15% by weight of acid or less.

21. Amethod asclaimed in anyof claims 1 to 17 in which the electrolyte comprises sulphuric acid.

lo

22. Amethod as claimed in claims 3 and 21 in which the anodising process is effected at a temperature of at least 50T.

23. A method as claimed in claims 4,5,6,7 and 22 in which the anodising process is effected ata temperature of WC or less and in which the electrolyte comprisesfrom 10 to 30% byweight of acid.

24. A structure comprising aluminium components which are secured together by adhesive and by localised mechanical fastening means and in which the components have an anodica oxide layerformed thereon by being subjected to an anodising process in an acidic electrolytefor a period of 2 minutes or less.

25. A structure as claimed in claim 24 in which the anodic oxide layer has a thickness of 50Onm or less.

26. A structure as claimed in claim 24or25 in which the anodic oxide layer has a thickness of at least 15nm.

27. A structure as claimed in any of claims 24to 26 in which the adhesive is an epoxide adhesive.

28. A structure as claimed in any of claims 24to 27 in which the localised mechanical fastening means - comprise spot-welds.

29. A structure as claimed in any of claims 24to 28 in which the localised mechanical fastening means comprise rivets.

30. A structure as claimed in anyone of claims 24 to 29 in which the electrolyte comprises phosphoric acid.

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31. A structure as claimed in claims 25,26 and 30 in which the anodic oxide (ayerhas a thickness of 20Onm or less.

32. A structure as claimed in any of clams 24to 29 in which the electrolyte comprises sulphuric acid.

33.A structure as claimed in claims 25,26 and 32 in which the anodic oxide layer has a thickness of 30Onmorless.

34. A structure as claimed In any of claims 24to 33 which forms part of a motorvehicle body.

35. Astructure as claimed in claim 34which forms part of a structural, load bearing part of a motor vehicle body.

36. A method substantially as hereinbefore de- scribed with reference to the examples.

37. A structure as hereinbefore described with referenceto the examples.

Printed in the United Kingdom for Her Majesty's Stationery Office, 8818935, 5186 18996. Published at the Patent Office, 25 Southampton Buildings, London WCPA lAY, from_which-copies may be obtained.

GB 2 167 443 A 8