Boatbuilding in Aluminium Alloy.pdf

Boatbuilcling

in Aluminium Alloy

Ernest H Sims

Aluminium and its Alloys

. . .

tiny particles from the basic bauxite. In the mid [')Ih century the method of isolation

chan~d, using sodium instead of potassium, enabling larger particles 1O b., fomled,

This in turn was improved upnn when m<;tallic aluminium ,,-as produced by

dissolvmg alumina;n a molten cryolite, obtaining "bout twenty-two pl',cent alumma,

then passing dee!ric currentS through the solution. As <1 result. the production costs

fell to a fraction of what they had b...en.

The method of ext"'''lion widdv ",cd rodav is known as the Electrulyti," Heduction

. .

Process. Fundamentally, this is hrought about by the breaking down of alumina ioto

aluminium and oxygen, Oxn;en combioing ",ith "arbon at the anode, is rdeased as

carbon dioxide gas. As cryolite melts at Hround (J~o°C and the alummium at around

6So°C, the pl"<>Cess is cootinuous, It h"s been e"timmed that it requin..s about 20,000

kilowatt hours of electricity to llrOO"cC a tOll "f Hluminium. and about f"ur IOns of

bauxite 10 make rn:o IOns of alumina. tl", whuk' pro<;ess prlxlucing ab"ut one ton of

metallic aluminium. During the process about J .500 pouods of carbon electtodes are

also consumed. It is e"ident from this that the production of aluminium is a highl)

complex and expensin operation. The most commercially pure aluminium has a Yet)

limited us<;, b<·ing rather soft and "'''ak. with a tensile strength of between 4 and (, tons

per squan: inch. Its "se lS normally limited to spinning. and deep draws of a complex

shape.

To produce a material ha"ing the corrosion resistance of aluminium, and at the

same time being commercially acce[ltahle hI' having higher strength t'HI"es, it is

ne""&lat)' 10 alloy with th" aluminium, vari,.", other metals, each introduced for a

\"ery specific purpose. This is what we know a, aluminium allo\'.

There are many deslgnated alloys of aluminium. Each one i, [lroduced III fulfil a

specific n ......d for a specitic industry. Wc ,hall conccrn ours<·lves only "'ith those alloys

that ha\'e a special iot"re't in a marine application.

'ne following are SOme of the metals used in ,'arying proponio,," in the alloying

proces", cO[lper, magnesium. silicon. iron, manganese, zinc. and chromium. The

percenm.,.-s uscd are small and qlry with th" physical requir"m"nts of the finished

product. For our pur[lo"", the gremest single dement is magnesium with between

four and five percent, For most ofthc others. it can ,'ary betw,,~n 0.1 and 1,0 pt."rc"nt.

The remainder. of course. is aluminium.

The disco,",>!")' of th., element aluminlUm wa~ made in the l'arlv part of the 1\llh

cemury. Al first, it was verv difficult and c.~ln::mch·~'()stk to estrac!. and isolate ,-u)'

In gennal, there are two types of wroul(ht alloy: (a) non-heat-treatable, and (b)

heat-treatable.

o 1\'1atenal in the annealed condition.

\ I i\1aterial in thc 'as manufactured' conditions-e.j:(., as rolled, as extruded

or drawn to size.

HI, Hz Strain hardened material suhjected to the application of cold work after

HJ, H4 annealing or to a combination of cold work and partial anncaling in order

H5, Hfi to secure thc specified "wd",ni"al prup"ni"s. Th" dC'Si ..",ali"", an: 1Il

H7, H.'! ascending order of t"nsik slr"ngth.

It is normal 10 use th" non-h"at-tr"atable alloys In the 0 or \'1 "omJition \\'h<'c<'

mll"h forming or weldmg IS to bc applied. As these allo,'s do not depend on heat

treatment to achien, their mechanical properties, the,' can be reheated without an\'

appreciable loss of strength... 1,150, they can be cold worked much more efficiently thml

the heat_treatable alloys,

\1 I

T Material \\'hi"h has bcen solution treated and requires no pre"'pllanon

lrcatmcnt

Solution heat-tr"atetl, "old worked and naturally aged,

Cooled from an ele"ated temperature shaping process and precIpitation-

treated.

\\'1' or TF Solution heat-treated and precipitation lre~ted,

T.H. Solution heal_trea,ed cold worked ,mtllhc!l pr"cipita,ion lrea,ed.

P Material which has be,,n precipitation lr"~ted onl\'.

Thu.• a single heM-treatment alloy in its strongest condition is indicated h,' ,he

,uffi.~ T, and a double-heat-treatml'nt alloy by lhl' suffix \V.I'.

As for non-heat-treatable alloys

T.D.

T.E.

He"t treallnent

Th.. r~ "r~ three type. of h~at treatment that are commonly applied [0 aluminium

alloys: (1) Annealing, (z) Solution treatment, (3) :\g~ing treatment,

(a) N"n_heat_lrealabl, "Iloys

The non-heat-treatable wrought alloys are indicated by the initiallener ')I'. These

allo)'s can be strengthened only by cold working and are softened b,' heating. -me

tempers or conditions in which thq' may be obtained range from soft or ann~alcd

temper to the fully work hard~n~d condition.

T~mpcr is indicated by the following symbols:

(h) f/fat-treatable a!l"n

The heat-treatable wrought alloys are mdicated hy the initiallener H, and condition is

described by the following suffix symhols:

0, .

[lOATIWILDI,\(; IN "LL'~IIS'L\1 Al.l.OY

(I,AnMa!in/:

A IXri<;>d of half 10 four hours sl InTlp"'TaIUr"S belwecn JSo and 3~oC is lufficit..m to

,;Qfl~"1' aluminium allnys for cold ",ork'ng or 10 ",lie"e lOlemal sln;ss. 'In" lOOb""r lin~

and hil/her lemperalUr" a", ad,·issbJr.. for I"" rn<:'d'um and high S1J"CT1j:1lh aJ~"5 when

maximum IIOftOC$S is requ,red, 'Ilw r~tc of c....li.'2 from ."" an""",linl:: Icmperalure

hal a comidt,rable inAucoce on me, final ru.rdflO'Sll.. Cfl"lir'l/l: in sl,1l air is ,.;ttisfacoory for

mo5l PU!"JXl""'i. but for the- loWflil po5Sibl" hardnl"$$ lhe material shoukJ be ~'(lOled In

Ihe fumacr al a controlled I1lle of I-."S:S tnan 2Q"C per hour u"lil ,h" I",",penllu'" is

below 2oo"C,

(:) SQ/,,/;(m /rro,m,,",

Solution treatnwm ccrllsisls of healing for ""ri.. ds of fmm half lu twemy-fcrur hours al

pr""cribed lempO'r1llUreS belween .. 60 and 5-+5°C, 111e malcrial is Ih"n quench"d, '['ll"

mcchanical prope"i". of mOSI alloy" ar" impro""d w ,'ar)'illj:l degr"". b)' SOllllion

IrCUlmCnt. The 'en,ile slren,"th and proof <II'''"" arc considerahlv incr"a",d o"cr Ihe

prOIX";"S in the annealcd conditiun but Ih" c1olll(ation is usuall~' redllccd, In

comparison to the 'as wroul(ht' condition, th" imprO""m"m aft"r .olulion treatment

appli". 10 all three prupcrric~, The tem!>"ralure of Ihe WaleI' of Ihe 'lo~'nd, has a

conaiderabl" e!fecr on Ih<· m~"Chanical propeni"" of the dell'""" of internal 51"""', Th"

cold waler q...,neh gi,~ maximum """"hanle-.Il prope"i~, hUllhe ad,-.lOla b ", is off~1

br the imroduclIon of high im<'rruill stress, Tl"l' boilinll wal"r quench kll"<"'ll leu

residual S1rcn,ll"1h and rrducea Ih~ risk of dismnlOn but tho:.. lttrmltlh and hanloes~ lin.:

luwu.

IJ) Agnng Irw/,..""t

'I'he impl'O'o~m In stnmglh fulluwin.'l soIU1ion tro:lllmo!1lt CIIn be funher incn... sed

br llgeinll" or p~pillltion, Some alloys hc~n 10 harden rapIdly at room l"mpcrlllUre

immediatdy aft"rqu"nchinll' Allhoullh Ihe proo:n,low. down aftera few h""rs, Ihe

maximum impro..",",,,nl in titrcnglh and hardn"'Sli is IlpprO<leh"d aft"r Ii"e da\'s. For

man)' alloys llj(C;nll 31 room tcmpcr~lu"", does not produo: Ihe n..S1 pruprrlOcs and a

treallllcnl at temperaturu be"""",\ 110 and 21See is IICC"ssar)',

'n,,, heal-trealable alloys Ih"n are Ih... mosl difficult to fom" Bul. aimosl hoeClIUll<: of

thin, Ihey ma~' be con.idered mOsl suitable fur Jur,!!it udinals, string"", "Ie" illasmul:h

as, becaose ofthe;r re.islane<: fO shaping, Ihe)" m,,)" pmd",.., a fa; rer lin", pro"ided Ihe

shape iSIlOl! complex or "x~"<,s.;,'e. E"en h"re, hOl"e""',;1 's possible to pro"ide local

heallu aid ..,,'cre fom'ing, ,J. b'OOd method iSlo u:;e lK"<,lylcncoxl"R"" wilh a lar~, soft

ria",,; falher than th" hard o"Yllen AalTU'_ Care musl be ,;xerci;;.,d nOllo COIK1:ntrate

thc heat in one Spol. hUI spread OWl' a fairly la~e arca, It is JXISl;ihle. also, lhal ,he

slrcnj1;!h pro!>"ni"" would be aff"",cd.

The malerial considi.'red by many aUlhoritio:s '" I., the mosl suiTable for boal

comuucrion ""Ienor u:sr, is for pial" to British Standard Spe<.-ificalion I ~ 77 ="PS and

ALl'''ll;o..;Jl;''1 A'\U ns ALLOYS

for extrusioos B.S. '47b '<E8. '111lS is a magneSIum allo)" with the follo\\';ng

eharacterist;cs:

Strength: ?\Iedium (tensile about ,8 tons per sq. in.)

Dunility: ['Jig-h

Formability: Vcrv good

Corrosion resistance: El<ccllcOl

Wcldabilitv: Vcrv good

For superstructures a stronger sheet material could be used with ad",mtao:e. such as

B,S. 1470 1\S6 Hz "r H3. dCpt:nding on the amotJOl of forming. This has a hIgher

proof stress, and would produce a stiffer panel ;n the thinner sheet ga\.gcs,

It should be, and often is. the ",sponsihilil\' of the desigoer to stipulate the typ" and

grad... of alllminium alloy to b ... lls{'d, Occas;onally. the builder has to decide, either

because the designer ha, o"t so specified. or becaus« til<' maler;als or;ginally specified

arc not available, and a substitutt' must be accepted. In these circumstances. the

builder should prm'ide himself with a good "'urking knowledge of the characteristics

of the ,'anous alloys and the" conditions. The alumlOium maouf""lurns are al\\'a"s

"cry generous in their ad";n' ,,,,<1 literal un', ,,,,,I ad"antage should be taken of this.

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