Aluminium Welding: Creating a Spark in Shipbuilding

October 13, 2012 8:45 am

With the increasing demand to create larger and faster ships, particularly for military service and the development of new, improved, high-performance aluminium base materials; aluminium welding has acquired an interesting and important place in ship building industry
Steel has an extensive account of providing superior mechanical properties to the ship building industry, but with one major disadvantage: weight. Increasing demands for size have forced ship designers to search for alternative materials which will reduce the weight of the ship without compromising strength. When properly designed, aluminium typically reduces the weight of small structures made of low-carbon steel by over 50 per cent. Weight issues have become increasingly important as advanced technology allows us to build larger ships. Since 1910, the maximum weight of ships has more than doubled, increasing from 46,000 tons to 109,000 tons. Increasing demands for size have forced ship designers to search for alternative materials to reduce the weight of the ship without compromising strength. Dramatic technological advances have allowed aluminium to meet or exceed the minimum strength requirements for normal strength steels currently used in the shipbuilding industry. Another advantage of aluminium is its resistance to corrosion, which is superior to steel – it corrodes over 100 times slower than conventional structural carbon steel used to build ships.
This report elaborates the weight reduction, strength, corrosion resistance, and cost of replacing conventional structural steel with lighter-weight aluminium alloys in the shipbuilding industry.   Advantages • Structural design of a ship should seek to minimise weight. This will reduce cost and minimise the loss of cargo dead-weight due to structure. • Weight reduction not also increases fuel efficiency. As a ship gets larger it becomes increasingly difficult to design for fuel efficiency without sacrificing other aspects. In addition, larger ships require larger power plants, which require more fuel. The larger engines and massive quantities of fuel add weight to the already bulky ships. Storage of the fuel also becomes a question. Weight issues have become increasingly important as advanced technology allows us to build larger and larger ships. • Aluminium has higher corrosion resistance over steel; this results in increased ship life. Weight Impressive technological advances in strength have allowed aluminium to emerge as a possible replacement for ocean-going ships. With a density of 2.70 g/cm3, aluminium is roughly one-third the weight of steel (r = 7.83 g/cm3). Table 1 highlights the strength to weight ratio of different aluminium alloy and carbon steel.
Table 1: Strength to weight ratio of different aluminium alloy and carbon steel Corrosion
ASTM material code
Material
type
 
Typical
Ultimate Tensile Strength, ksi
 
Density,
g/cm3
 
 
Strength-to-
Weight Ratio
 
7075-T6
Aluminium
 
 
83
2.80
 
 
822
 
2024-T361
 
Aluminium
 
72
 
2.80
 
713
 
5056-H18
 
Aluminium
 
63
 
2.66
656
 
 
6061-T6
Aluminium
 
 
45
2.71
 
 
459
 
3004-H38
 
Aluminium
 
41
 
2.71
 
418
 
Fibreglass
 
Fibre
 
19
1.43
 
 
367
 
6063-T5
 
Aluminium
 
27
 
2.74
 
273
 
1020 Carbon Steel
 
Steel
 
60
7.86
 
 
211Aluminium, as indicated by its position in the electromotive force series, is a thermodynamically reactive metal. Among structural metals, only beryllium and magnesium are more reactive. However, aluminium has excellent corrosion resistance due to an extremely adherent oxide film that forms on the surface whenever it is exposed to air or water. This oxide film is highly protective and because it is more thermodynamically inactive, prevents aluminium from corroding further. When exposed to extremely corrosive materials, such as salt water, the oxide film may break down and further corrosion or pitting may occur but at a much lower rate than carbon steel (Table 2). In contrast, steel’s oxide layer, rust, does not provide a highly protective layer, and as a result, steel continues to corrode. Corrosion behaviour of various aluminium and steel alloys in seawater is shown in Table 2.
Table 2: Corrosion behaviour of various aluminium and steel alloys in seawater
Aluminium Alloy
Corrosion Rate, µm/yr
% Change in Tensile Strength
Steel Alloy
Corrosion Rate, µm/yr
5083-O
0.9
0.0
Structural Carbon Steel (depending on chemistry & temperature)
 
 
 
 
120
5086-O
0.9
-2.7
105
5454-H34
1.0
-0.7
85
5456-H321
1.6
-1.1
70
5456-O
0.4
-0.4
 Aluminium can be formed through either casting or wrought processes. The designation “wrought” indicates that the alloys are available primarily in the form of worked products, such as sheet, foil, plate, extrusions, tube, forgings, rod, bar, and wire. The working operations and thermal treatments transform the cast ingot structure into a wrought structure. The structure influences the strength, corrosion resistance, and other properties of an aluminium alloy. This study deals only with wrought aluminium alloys because they possess superior strength and corrosion resistance properties to cast aluminium alloys. Disadvantages Two disadvantages of aluminium are:• Aluminium alloys cannot meet the maximum yield strengths required in certain Ship building applications – only high-strength, low-alloy steels meet these strength requirements. • Aluminium, at about ` 102.37 per Kg, costs roughly five times more than steel, at about ` 22.9 per kg. Developments in High-Strength Aluminium Alloys for Marine Applications In recent years, progress has been achieved by aluminium producers in the development of improved aluminium alloys specifically targeted at the shipbuilding industry. In 1995 the aluminium manufacturer Pechiney of France registered the aluminium Alloy 5383 and promoted this material to the shipbuilding industry as having improvements over 5083 alloy. These improvements provided potential for significant weight savings in the design of aluminium vessels and included a minimum of 15 per cent increase in the postweld yield strength, improvements in corrosion properties, and a 10 per cent increase in fatigue strength. These developments, coupled with formability, bending, cutting, and weldability characteristics at least equal to that of 5083, made the 5383 alloy very attractive to designers and manufacturers who were pushing the limits to produce bigger and faster aluminium ships.   In 1999, the aluminium manufacturer Corus Aluminium, Germany, came out with the aluminium base Alloy 5059 (Alustar). This alloy was also developed as an advanced material for the shipbuilding industry, providing significant improvements in strength over the traditional 5083 alloy. The 5059 alloy is promoted by Corus as providing improvements in minimum mechanical properties over Alloy 5083. These improvements are referenced as being a 26 per cent increase in yield strength before welding and a 28 per cent increase in yield strength (with respect to Alloy 5083) after welding.
Early testing on the 5059 (Alustar) base alloy indicated that problems could be encountered relating to the weld metal not being capable of obtaining the minimum tensile strength of the base material in the heat-affected zone. One method used to improve the weld tensile strength was to increase the amount of alloying elements drawn from the plate material into the weld. This was assisted by the use of helium additions to the shielding gas during TIG welding, which produces a broader penetration profile that incorporates more of the base material. The use of 5556 filler metal rather than the 5183 filler metal can also help increase the strength of the deposited weld material.
Obviously these high-performance vessels require high-quality welding. The training of welders, development of appropriate welding procedures, and implementation of suitable testing techniques are essential in producing such a high- performance product.
The Future With the increasing demand to create larger and faster ships, particularly for military service and the development of new, improved, high-performance aluminium base materials, it is apparent that aluminium welding has acquired an interesting and important place in ship building industry.
The most popular welding process for Aluminium is TIG. MIG process also is picking up due to increased productivity angle. But in MIG process, the wire feeding is a critical aspect. The preferred method for feeding soft aluminium wire long distances is the push-pull method. Specially designed drive rolls are needed. Drive-roll tension has to be set in such a way to deliver an even wire-feed rate. Excessive tension will deform the wire and cause rough and erratic feeding; too-little tension results in uneven feeding. Both conditions can lead to an unstable arc and weld porosity. 
Table 3: Ador Welding’s aluminium welding consumables
  Process
AWS classification
AWL brand
Application
GTAW
ER5183
Tigfil 5183
For total aluminium ships
GTAW
ER5356
Tigfil 5356
For total aluminium ships
GTAW
ER5556
Tigfil 5556
For total aluminium high speed shipsTigfil 5183 5183 is an aluminium filler that has improved strengths on alloys such as 5086 compared to 5356 that may not meet the needed tensile. Commonly used for welding of marine components, drilling rigs, cryogenics, railroad cars, storage tanks – base metals of 5083, 5086, 5456, to each other or to 5052, 5652 and 5056.
Tigfil 5356 Magnesium Aluminium Alloy filler metal that is used to weld Aluminium Alloys 5050, 5052, 5083, 5356, 5454, and 5456. The post-anodising colour tint is white making it a good choice for anodising applications. The saltwater corrosion resistance is very good, making it an ideal choice for many marine applications. Average tensile strength of weld is 38,000 psi.  Tigfil 5556 5556 is an aluminium filler that has good ductility and improved crack resistance due to the content of manganese, magnesium and zinc. Commonly used for welding of base materials 5154, 5254, 5454 and 5456.
Conclusion The feasibility of replacing steel with aluminium in the shipbuilding industry depends primarily on the application and cost constraints. Demands for greater ship size have forced designers to search for alternative materials to reduce ship weight while maintaining strength. Aluminium alloys meet or exceed the minimum yield strength requirements for normal strength steels and have superior corrosion resistance (steel corroded at a rate of 120 micrometer per year, while in a similar study, aluminium corroded at a rate of only 1 micrometer per year).  However, because of higher costs, aluminium may not be always economical. For high-strength applications, ship builders sacrifice corrosion resistance and weight reduction in favour of the greater strength provided by HSLA steels. When normal strength materials are adequate, ship builders are going in for using aluminium alloys to reduce ship weight and improve corrosion resistance.
Courtesy: Ador Welding Ltd. For more details, contact at cmo@adorians.com or visit www.adorwelding.comn