Boatbuilding in Aluminium Alloy.pdf
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Boatbuilcling
in Aluminium Alloy
,
Ernest H Sims
I
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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