Aluminum: Experience in Application
What you should know about welding aluminum
What you should know about welding aluminum
 |
In recent years, the use of aluminum in manufacturing has become more prevalent because of its light weight and other attributes that make it an attractive alternative to steel. In fact, the aluminum welding market is expected to grow at a rate of 5.5 percent annually based primarily on the assumption that the automotive industry will continue to increase its use of aluminum.
But, those experienced in the welding of steel will find aluminum to be a different breed – the normal welding characteristics of steel don’t always apply to aluminum. For example, aluminum’s high thermal conductivity and low melting point can easily lead to burnthrough and warpage problems if proper procedures are not followed.
In this article, we will first take a look at various alloying elements and how they affect aluminum; then we will turn our attention to welding procedures and the parameters that will create the best quality weld. Lastly, we will examine some new technology breakthroughs that make welding aluminum a little easier.
Alloying Elements
To understand aluminum, you must first understand some basics about aluminum metallurgy. Aluminum can be alloyed with a number of different elements, both primary and secondary, to provide improved strength, corrosion resistance and/or general weldability.
The primary elements that alloy with aluminum are copper, silicon, manganese, magnesium and zinc. But, before we examine them in detail and what they bring to aluminum, it is important to note that these alloys fall into two classes: heat-treatable or nonheat-treatable.
Heat-Treatable vs. Nonheat-Treatable Alloys
Heat-treatable alloys are those that can be heated after welding to regain strength lost during the welding process. To heat-treat an alloy means heating it at a high temperature, putting the alloying elements into solid solution and then cooling it at a rate which will produce a supersaturated solution. The next step in the process is to maintain it at a lower temperature long enough to allow a controlled amount of precipitation of the alloying elements.
With the nonheat-treatable alloys it is possible to increase strength through cold working or strain hardening. To do this, a mechanical deformation must occur in the metal structure, resulting in increased resistance to strain, producing higher strength and lower ductility.
Further Distinctions
To further designate aluminum alloys, they can also be classified by a temper designation which are as follows: F = As fabricated, O = Annealed, H = Strain hardened; W = Solution heat-treated and T = Thermally treated, which can designated heat treatment, or cold working aging. For example an alloy may carry the designation of 2014 T6. This means that it is alloyed with copper (2XXX series) and the T6 refers to the fact that it is solution heat-treated and artificially aged.
For purposes of this article, we will discuss wrought alloys, which are those aluminum alloys that are rolled from ingot or extruded with customer specified shapes. But please note that alloys can also be divided into cast alloys. Cast alloys are those used to manufacture parts from molten alloys of aluminum poured into molds. Cast alloys are precipitation hardenable but never strain hardenable. The weldability of these alloys is affected by casting type – permanent mold, die cast, and sand – since the casting surface is critical to welding success. A three-digit number, plus one decimal i.e. 2xx.x designates the cast alloys. Weldable grades of aluminum castings are 319.0, 355.0, 356.0, 443.0, 444.0, 520.0, 535.0, 710.0 and 712.0.
Alloying Elements
Now, that you understand some of the terminology, let’s take a look at the different alloying elements:
Copper (which carries a wrought alloy designation of 2XXX series) provides high strength to aluminum. This series is heat-treatable and mainly used in aircraft engine parts, rivets and screw products. Most 2XXX series alloys are considered poor for arc welding because of their sensitivity to hot cracking. These alloys are generally welded with 4043 or 4145 series filler electrodes, which have low melting points to reduce the probability of hot cracking. Exceptions to this are alloys 2014, 2219 and 2519, which are easily welded with a 2319 filler wire.
Manganese (3XXX series) added to aluminum yields a nonheat-treatable series used for general-purpose fabrication and build-up. Moderate in strength, the 3XXX series is used for forming applications including utility and van trailer sheet. It is improved through strain hardening to provide good ductility and improved corrosion properties. Typically welded with 4043 or 5356 electrode, the 3XXX series is excellent for welding and not prone to hot cracking. Its moderate strengths do prevent this series from being used in structural applications.
Silicon (4XXX series) reduces the melting point of aluminum and improves fluidity. Its principle use is as filler metal. The 4XXX series has good weldability and is considered a nonheat-treatable alloy. Alloy 4047 is becoming the alloy of choice in the automotive industry, as it is very fluid and good for brazing and welding.
Magnesium (5XXX series), when added to aluminum, has excellent weldability with a minimal loss of strength and is basically not prone to hot cracking. In fact, the 5XXX series has the highest strength of the nonheat-treatable aluminum alloys. It is used for chemical storage tanks and pressure vessels at elevated temperatures as well as structural applications, railway cars, dump trucks and bridges because of its corrosion resistance. It looses ductility when welded with 4XXX series fillers due to formation of Mg2Si.
Silicon and Magnesium (6XXX series) combine to serve as alloying elements for this medium-strength, heat-treatable series. It is principally used in automotive, pipe, railings, structural and extruding applications. The 6XXX series is somewhat prone to hot cracking, but this problem can be overcome by the correct choice of joint and filler metal. This series can be welded with either 5XXX or 4XXX series without cracking – adequate dilution of the base alloys with selected filler alloy is essential. A 4043 electrode is the most common for use with this series.
But, those experienced in the welding of steel will find aluminum to be a different breed – the normal welding characteristics of steel don’t always apply to aluminum. For example, aluminum’s high thermal conductivity and low melting point can easily lead to burnthrough and warpage problems if proper procedures are not followed.
In this article, we will first take a look at various alloying elements and how they affect aluminum; then we will turn our attention to welding procedures and the parameters that will create the best quality weld. Lastly, we will examine some new technology breakthroughs that make welding aluminum a little easier.
Alloying Elements
To understand aluminum, you must first understand some basics about aluminum metallurgy. Aluminum can be alloyed with a number of different elements, both primary and secondary, to provide improved strength, corrosion resistance and/or general weldability.
The primary elements that alloy with aluminum are copper, silicon, manganese, magnesium and zinc. But, before we examine them in detail and what they bring to aluminum, it is important to note that these alloys fall into two classes: heat-treatable or nonheat-treatable.
Heat-Treatable vs. Nonheat-Treatable Alloys
Heat-treatable alloys are those that can be heated after welding to regain strength lost during the welding process. To heat-treat an alloy means heating it at a high temperature, putting the alloying elements into solid solution and then cooling it at a rate which will produce a supersaturated solution. The next step in the process is to maintain it at a lower temperature long enough to allow a controlled amount of precipitation of the alloying elements.
With the nonheat-treatable alloys it is possible to increase strength through cold working or strain hardening. To do this, a mechanical deformation must occur in the metal structure, resulting in increased resistance to strain, producing higher strength and lower ductility.
Further Distinctions
To further designate aluminum alloys, they can also be classified by a temper designation which are as follows: F = As fabricated, O = Annealed, H = Strain hardened; W = Solution heat-treated and T = Thermally treated, which can designated heat treatment, or cold working aging. For example an alloy may carry the designation of 2014 T6. This means that it is alloyed with copper (2XXX series) and the T6 refers to the fact that it is solution heat-treated and artificially aged.
For purposes of this article, we will discuss wrought alloys, which are those aluminum alloys that are rolled from ingot or extruded with customer specified shapes. But please note that alloys can also be divided into cast alloys. Cast alloys are those used to manufacture parts from molten alloys of aluminum poured into molds. Cast alloys are precipitation hardenable but never strain hardenable. The weldability of these alloys is affected by casting type – permanent mold, die cast, and sand – since the casting surface is critical to welding success. A three-digit number, plus one decimal i.e. 2xx.x designates the cast alloys. Weldable grades of aluminum castings are 319.0, 355.0, 356.0, 443.0, 444.0, 520.0, 535.0, 710.0 and 712.0.
Alloying Elements
Now, that you understand some of the terminology, let’s take a look at the different alloying elements:
Copper (which carries a wrought alloy designation of 2XXX series) provides high strength to aluminum. This series is heat-treatable and mainly used in aircraft engine parts, rivets and screw products. Most 2XXX series alloys are considered poor for arc welding because of their sensitivity to hot cracking. These alloys are generally welded with 4043 or 4145 series filler electrodes, which have low melting points to reduce the probability of hot cracking. Exceptions to this are alloys 2014, 2219 and 2519, which are easily welded with a 2319 filler wire.
Manganese (3XXX series) added to aluminum yields a nonheat-treatable series used for general-purpose fabrication and build-up. Moderate in strength, the 3XXX series is used for forming applications including utility and van trailer sheet. It is improved through strain hardening to provide good ductility and improved corrosion properties. Typically welded with 4043 or 5356 electrode, the 3XXX series is excellent for welding and not prone to hot cracking. Its moderate strengths do prevent this series from being used in structural applications.
Silicon (4XXX series) reduces the melting point of aluminum and improves fluidity. Its principle use is as filler metal. The 4XXX series has good weldability and is considered a nonheat-treatable alloy. Alloy 4047 is becoming the alloy of choice in the automotive industry, as it is very fluid and good for brazing and welding.
Magnesium (5XXX series), when added to aluminum, has excellent weldability with a minimal loss of strength and is basically not prone to hot cracking. In fact, the 5XXX series has the highest strength of the nonheat-treatable aluminum alloys. It is used for chemical storage tanks and pressure vessels at elevated temperatures as well as structural applications, railway cars, dump trucks and bridges because of its corrosion resistance. It looses ductility when welded with 4XXX series fillers due to formation of Mg2Si.
Silicon and Magnesium (6XXX series) combine to serve as alloying elements for this medium-strength, heat-treatable series. It is principally used in automotive, pipe, railings, structural and extruding applications. The 6XXX series is somewhat prone to hot cracking, but this problem can be overcome by the correct choice of joint and filler metal. This series can be welded with either 5XXX or 4XXX series without cracking – adequate dilution of the base alloys with selected filler alloy is essential. A 4043 electrode is the most common for use with this series.
عدل سابقا من قبل Argon في الأحد مارس 30, 2008 7:10 pm عدل 1 مرات
