Weldability of materials .. Copper and copper alloys

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Weldability of materials .. Copper and copper alloys

مُساهمة من طرف Admin في الجمعة مارس 07, 2008 3:13 pm

Weldability of materials


Copper and copper alloys


Copper and copper alloys are chosen because of their corrosion resistance and electrical and thermal conductivity.
The various types of copper alloys are identified and guidance is given on processes and techniques which can be used in fabricating copper alloy components without impairing their corrosion or mechanical properties or introducing defects into the weld.

Material types


The alloys are grouped according to the principal alloying elements. Although there are UK standards (BS 2780-2875) for the alloy designations, the alloys are more commonly known by the generic type:









  • C Pure copper
  • CH Copper with small alloy additions
  • CZ Brasses such as copper-zinc (Cu-Zn)
  • NS Nickel silvers such as copper-zinc-nickel (Cu-Zn-Ni)
  • PB Bronzes such as copper-tin (Cu-Sn) (phosphor bronze alloys also contain phosphorus)
  • G Gunmetals such as copper-tin-zinc (Cu-Sn-Zn) (some alloys may contain lead)
  • CA Aluminium bronze such as copper-aluminium (Cu-Al) (most alloys also contain iron and many nickel)
  • CN Cupro-nickels such as copper-nickel (Cu-Ni)


A number of popular alloys are listed in the Table together with the recommended filler metal (compositions of TIG and MIG filler wires are given in BS2901 Part 3).










Typical alloys and recommended filler for inert gas welding
Alloy group Typical alloys Recommended filler
CoppersTough pitchC7, C8
Phosphorus deoxidisedC7, C8
BrassesLow zinc, up to 30% ZnC9, C13
High zinc, 40% ZnNot Recommended
Nickel Silvers20% Zn / 15% NiC9, C13
45% Zn / 10% NiNot Recommended
Silicon Bronze3% SiC9
Phosphor Bronze4.5% to 6% Sn / 0.4% PC10
Aluminium Bronze< 7.8% AlC12, C12 Fe
> 7.8% AlC13, C20
6% Al / 2% SiC23
GunmetalLow leadC10, C9, C13
LeadedNot Recommended
Cupro - Nickel10%NiC16, C18
30% NiC18




In terms of weldability, alloys have quite different welding characteristics. Copper, because of its high thermal conductivity, needs substantial preheat to counteract the very high heat sink. However, some of the alloys which have a thermal conductivity similar to low carbon steel, such as cupro-nickel alloys, can normally be fusion welded without a preheat.

Copper


Copper is normally supplied in the form of









  • oxygen bearing, tough pitch copper
  • phosphorus deoxidised copper
  • oxygen-free copper


Tough pitch copper contains stringers of copper oxide (<0.1% oxygen as Cu 2 O) which does not impair the mechanical properties of wrought material and has high electrical conductivity. Oxygen-free and phosphorus deoxidised copper are more easily welded.
TIG and MIG are the preferred welding processes but oxyacetylene and MMA welding can be used in the repair of tough pitch copper components. To counteract the high thermal conductivity, helium and nitrogen-based gases, which have higher arc voltages, can be used as an alternative to argon.

Avoiding weld imperfections



In fusion welding tough pitch copper, high oxygen content leads to embrittlement in the heat affected zone (HAZ) and weld metal porosity. Phosphorus deoxidised copper is more weldable but residual oxygen can result in porosity in autogenous welds especially in the presence of hydrogen. Porosity is best avoided by using appropriate filler wire containing deoxidants (Al, Mn, Si, P and Ti).
Thin section material can be welded without preheat. However, over 5mm thickness all grades need preheat to produce a fluid weld pool and avoid fusion defects . Thick section components may need a preheat temperature as high as 600 deg.C.

Copper with small alloying additions


Low alloying additions of sulphur or tellurium can made to improve machining. However, these grades are normally considered to be unweldable.
Small additions of chromium, zirconium or beryllium will produce precipitation hardened alloys which, on heat treatment, have superior mechanical properties. Chromium and beryllium copper may suffer from HAZ cracking unless heat treated before welding. When welding beryllium copper, care should be taken to avoid inhaling the welding fumes.

Brasses (copper-zinc alloys) and nickel silvers


When considering weldability, brasses can be conveniently separated into two groups, low zinc (up to 20% Zn) and high zinc (30 to 40% Zn). Nickel silvers contain 20 to 45% zinc and nickel to improve strength. The main problem in fusion welding these alloys is the volatilisation of the zinc which results in white fumes of zinc oxide and weld metal porosity. Only low zinc brasses are normally considered suitable for fusion welding using the TIG and MIG processes.

Avoiding weld imperfections


To minimise porosity, a zinc-free filler wire should be used, either silicon bronze (C9) or an aluminium bronze (C13). High welding speeds will reduce pore coarseness.
TIG and MIG processes are used with argon or an argon-helium mixture but not nitrogen. A preheat is normally used for low zinc (<20% Zn) to avoid fusion defects because of the high thermal conductivity,. Although preheat is not needed in higher zinc content alloys, slow cooling reduces cracking risk. Post weld heat treatment also helps reduce the risk of stress corrosion cracking in areas where there is high restraint.

Bronzes (tin bronze, phosphor bronze, silicon bronze and gun metal)


Tin bronzes can contain between 1% and 10% tin. Phosphor bronze contains up to 0.4% phosphorus. Gunmetal is essentially a tin bronze with up to 5% zinc and may additionally have up to 5% lead. Silicon bronze contains typically 3% silicon and 1% manganese and is probably the easiest of the bronzes to weld.

Avoiding weld imperfections


These are generally considered to be weldable, apart from phosphor bronze and leaded gun metal, and a matching filler composition is normally employed. Autogenous welding of phosphor bronzes is not recommended due to porosity, but the risk can be reduced by using a filler wire with a higher level of deoxidants. Gun metal is not considered weldable due to hot cracking in the weld metal and HAZ.

Aluminium bronze


There are essentially two types of aluminium bronzes; single phase alloys containing between 5 and 10% aluminium, with a small amount of iron or nickel, and more complex, two phase alloys containing up to 12% aluminium and about 5% of iron with specific alloys also containing nickel and manganese and silicon. Gas shielded welding processes are preferred for welding this group of alloys. In TIG welding, the presence of a tenacious, refractory oxide film requires AC (argon), or DC with a helium shielding gas. Because of its low thermal conductivity, a preheat is not normally required except when welding thick section components.

Avoiding weld imperfections


Rigorous cleaning of the material surface is essential, both before and after each run, to avoid porosity. Single phase alloys can be susceptible to weld metal cracking and HAZ cracking can occur under highly restrained conditions. It is often necessary to use matching filler metals to maintain corrosion resistance but a non-matching, two phase, filler will reduce the cracking risk. Two phase alloys are more easily welded. For both types, preheat and interpass temperatures should be restricted to prevent cracking.

Cupro-nickels


Cupro-nickel alloys contain between 5 and 30% nickel with specific alloys having additions of iron and manganese; 90/10 and 70/30 (Cu/Ni) alloys are commonly welded grades. These alloys are single phase and generally considered to be readily weldable using inert gas processes and, to a lesser extent, MMA. A matching filler is normally used but 70/30 (C18) is often regarded as a 'universal' filler for these alloys. As the thermal conductivity of cupro-nickel alloys is similar to low carbon steels, preheating is not required.

Avoiding weld imperfections


As the alloys do not contain deoxidants, autogenous welding is not recommended because of porosity. Filler metal compositions contain typically 0.2 to 0.5% titanium, to prevent weld metal porosity. Argon shielding gas is normally used for both TIG and MIG but in TIG welding, an argon-H2 mixture, with appropriate filler, improves weld pool fluidity and produces a cleaner weld bead. Gas backing (usually argon) is recommended, especially in pipe welding, to produce an oxide-free underbead.

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رد: Weldability of materials .. Copper and copper alloys

مُساهمة من طرف ENG_3ALIM في الثلاثاء مارس 25, 2008 11:17 pm

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