Frame_materials.pdf

(150 KB) Pobierz
Frame Materials
Version 2.1
Written By Rickey M. Horwitz
The material contained in this section is protected by U.S.
copyright laws. Any unauthorized duplication or publication of the
material contained in this section is prohibited by law.
n the quest of building the ultimate HPV, the frame material becomes an
important issue. For building the Thunderbolt and Spitfire, I have narrowed
the materials down to four metals, Carbon steel, CroMo, Aluminum and
Titanium. Each of these materials has its strong and weak points, as I shall
attempt to point out in this chapter. I’ve also included a small essay on carbon
composites, as this material is quickly gaining acceptance in the bicycle
industry.
Throughout this chapter, I use the terms ‘Tensile Yield’ and ‘Tensile Ultimate’
to describe the strength attributes of the alloys mentioned. In laymen terms,
Tensile Yield (commonly referred to as Yield strength) is the applied pressure
value measured in PSI where the metal starts to bend or deform. Tensile
ultimate (or just plain Tensile Strength) is the applied pressure value measured
in PSI where the metal may expect to fail.
I
Carbon Steels
T
he most popular material for bicycle frames is carbon steel as it is
inexpensive and can be welded or bonded by a number of processes
making it ideal for mass production and for the home frame builder. Several
grades of carbon steel exist. The grades I am familiar with are 1010, 1018,
1020, and 1028. The last two digits represent the carbon content of the steel
as a 10th of a percent. Therefore, 1028 has .28% carbon, which is slightly less
than 4130 CroMo. The more percentage of carbon added, the harder and
stronger the steel. However, too much carbon causes the steel to become
brittle. Therefore, .4% carbon is about the maximum you shall see for frame
building.
For frame building, I would prefer to use 1028 if available. It offers a tensile
strength of 87,000 PSI and yield strength of 72,000 PSI, which is almost as
Page 1
hard as normalized Cro-Mo. However, plain old 2-inch muffler pipe might work
fine if the wall thickness is suitable.
Chromium Mollybendium (CroMo, 4130)
I
do not have extensive knowledge or a lot of experience frame building with
CroMo. I do understand that it is extremely tough and strong making it an
ideal choice for frame building. The strength to weight ratio of fully heat-
treated 4130 (most common) CroMo exceeds that of 6061-T6 aluminum.
However, the lower strength ‘normalized’ grade of 4130 CroMo is what is
widely used for bicycle frames, as it is less brittle. The best attribute of CroMo
is its inexpensive cost relative to other performance materials.
Because of the alloys used for 4130 CroMo, the material must be welded using
a TIG or MIG process, although it can also be brazed. The major alloys used for
making CroMo are Chrome and Mollybendium. Also known as ‘light alloy’ steel,
only 3% of alloy material is added to give this material its super strength. The
designation '4130' refers to the alloy and carbon content. The '41' represents
the alloy type and quantity and the '30' represents the carbon content as a
10th of a percent. Regardless of the alloy content, this metal TIG welds easily
and produces great results with little effort.
As with Aluminum, 4130 CroMo has different temper conditions. Likewise, the
temper levels affect the strength and characteristics of this alloy steel. As
mentioned, the WQ&T 4130 is extremely strong, perhaps to a fault. For
bicycle frame applications, the weaker normalized temper is used to maintain
Cro-Mo’s fine balance of longevity and strength (E.g., less prone to breaking).
In summary, tempering restores some of the ductility that may be lost after
the hardening heat treatment and quench. Alloy 4130 is tempered at between
750 F and 1050 F, depending upon the strength level desired. The lower the
tempering temperature the greater the strength. This normalized temper
upsets the strength to weight ratio in favor of aluminum.
Temper
Annealed
Normalized
WQ&T
Tensile strength Ultimate(PSI)
80,000
90,000
128,000
Yield Strength
(PSI)
50,000
70,000
113,000
For all-out performance, 4130 CroMo falls short to 6061-T6 aluminum. Many
bike aficionados claim that a steel frame will always outlive an aluminum frame
and is more forgiving.
Page 2
Aluminum (6061, 7005 and 7075)
A
luminum is quickly gaining ground as the material of choice for bicycles
such as Trek, Cannondale, Klien and other top brands. New technologies
have emerged recently that have lowered the cost of aluminum fabrication
making it a strong contender to CroMo. The proverbial 6061-T6-grade
aluminum is now the industry standard for quality aluminum frame
composition. This is in part because this type of aluminum can be easily
welded and is relatively strong. Lately, the bicycle industry has leaned towards
the 7005 series of aluminum for low to medium cost frames, as this type of
aluminum does not require a post heat treatment, hence lowing the cost
considerably.
Aluminum is extremely easy to work and machine making frame fabrication
painless. Depending on the diameter and wall thickness, aluminum tubing can
be easily formed using a standard conduit bender without use of a mandrel.
Since aluminum is relatively soft, it can be filed, drilled and sanded with ease.
The cost of aluminum is higher than most frame building materials. Aluminum
is derived in two ways: either from ore or from recycled scrap. Due to its low
melting point of 660°C (steel: 1535 °C), processing or recycling of aluminum is
relatively easy. On the other hand, extraction from ore is another story. It
requires that bauxite be turned into an aluminum oxide, and then converted
into aluminum using electrolysis. It takes four tons of bauxite to produce one
ton of aluminum. The complete process consumes almost 75 KW/hours of
energy to produce a single pound of aluminum.
Home builders may find aluminum fabrication challenging as it requires a TIG
or MIG process and is more difficult to weld than CroMo or carbon steel using
the same process. Additionally, the 6061-T6 series aluminum losses half its
total strength when welded. Consequently, a post heat treatment process is
required to regain its full T6 strength. The 7005 series aluminum retains
enough of its original strength (it much stronger than 6061 to begin with) that
a post heat treat is not always required.
Page 3
The 6061 series of aluminum is alloyed with magnesium & silicon. However,
these are only the majority of alloying elements, as other elements are
included such as copper, zinc, manganese and titanium. A total of 4% of
alloying elements comprise the 6061 specification. The 6061 series is divided
into grades or designations that refer to the post temper process. The most
common grades are T0, T4, and T6. The grades, processes and strengths are
depicted below:
6061 Series Aluminum
Temper
T0 or O
T4
T6
Process
fully annealed aluminum
Solution heat treated and naturally aged
Solution heat treated and artificially aged
Tensile
Yield(PSI)
Ultimate.(PSI)
11,600
8,000
35,000
21,000
45,000
40,000
The 7005 series aluminum is alloyed primarily with zinc, making it even
stronger than the proverbial 6061. As mentioned, this aluminum does not
always require a solution heat treatment, as it loses less of its overall strength
when annealed. An artificial aging treatment is all that is required making this
aluminum desirable for frame builders who don’t have heat-treating resources.
On the downside, some metallurgist suggest that any of the 7000 series
aluminums (although a stronger aluminum) do not stand up to fatigue as well
as the 6061 series aluminum. Perhaps this is why no one has produced a
heat-treated 7005 aluminum frame, as it may lead to premature failure.
Therefore, it is predictable that a 7005 aluminum frame may not hold up as
long as a heat-treated 6061 frame.
7005 Series Aluminum
Temper
T0 or O
T6
Process
fully annealed aluminum
heat treated and artificially aged
Tensile Ult(PSI)
28,300
50,800
Yield(PSI)
11,600
42,100
Just for conversation, I'll also mention the 7075 series aluminum. As with
7005, this type of aluminum is heavily alloyed with zinc making it incredibly
tough and strong. However, it has so much zinc, it cannot be easily welded,
making it's use limited to machined components or glue bonded assemblies.
In closing 6061-T6 aluminum is .29 the weight of normalized 4130 CroMo steel
and is .4 times as strong. Hence, aluminum’s strength to weight ratio is
higher.
Page 4
Titanium (3Al-2.5V)
3
Al-2.5V titanium is an exotic alloy metal that has properties roughly that of
315 Stainless steel. However, this alloy is almost half the weight and is as
strong as 4130 CroMo. As with aluminum, titanium metal is not found as a
free element. However, it is the ninth most abundant mineral in the earth's
crust. It is usually present in igneous rocks and in the sediments derived from
them. It is found in the minerals rutile (TiO2), ilmenite (FeTiO3), and sphene,
and is present in titanates and in many iron ores. The energy and process to
produce titanium makes this metal very expensive.
The 3AL-2.5V series of titanium alloy is 40 to 60% stronger than unalloyed
titanium. The main alloys in this series include aluminum and vanadium.
Back in the mid ‘70s, Teledyne manufactured the first commercial titanium
bicycle frame called the ‘Titan”. Although a technical marvel (extremely light
frame in its day), the Titan was plagued by reliability problems and production
was short lived. This negative publicity gave titanium the reputation as an
exotic, over-priced, and unreliable material for bicycles. The root cause of the
Titan’s demise was the type of titanium used. Back in the mid ‘70s high
performance titanium alloys were not commercially available. It would be
another 15 years before the 3Al-2.5V series of titanium alloy would become
readily available to the consumer market.
With the advent of the 3Al-2.5V series of titanium alloy, many bicycle
manufacturers have produced frames of unequalled performance. The 3AL-
2.5V series of titanium alloy is 40 to 60% stronger than unalloyed titanium.
The main alloys in this series include aluminum and vanadium. There are
other titanium alloys much stronger than the 3Al-2.5V series, but these alloys
are difficult to weld and machine.
3AL-2.5V Titanium
Process
Alpha annealed
ST 925 deg. C, Aged at 480 deg. C
Tensile Ult(PSI)
90,000
132,500
Yield(PSI)
72,500
120,000
For manufacturing, titanium presents a few challenges. First, the metal is
extremely tough and doesn’t machine as easily as 4130 Cro-Mo or aluminum.
Many machinists have compared its qualities to that of nickel or hardened
stainless steel.
Secondly, when molten titanium is subjected to oxygen
(>1130 c), it produces a titanium dioxide making the structure extremely
brittle. TIG welding titanium requires intense consideration, as any heated
zones of the weld (both upper and lower sides) must be in a total inert
Page 5

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin