Hi Tommyjoking!
I respectfully have to disagree with your presumption that most aluminum boat manufacturers use 4043 filler on 5XXX series aluminum because of costs savings... If they did we'd have alot of litigation going on involving the manufacturers and their customers with respect to the weld cracking issues alone notwithstanding all of the other issues that would arise as a result.
Here's a .pdf you'll find interesting:
http://www.alcotec.com/us/en/education/knowledge/techknowledge/aluminum-filler-alloy-selection.cfmHere's an excerpt:
"The high purity 1000 Series Alloys and Alloy 3003 are easy to weld with 1100 or 4043 filler alloys. High purity alloys can be welded with base alloy filler if desired. Alloys possessing from 1% to 2.5% magnesium, such as 3004 and "5052", are very sensitive to cracking when using base alloy filler. Alloys with 3.5% magnesium and more exhibit low sensitivity to weld cracking. The use of a 5% magnesium content filler, such as "5356, to weld 5052" can provide a higher magnesium percentage in the diluted weld metal and reduce sensitivity to weld cracking. The more magnesium an aluminum magnesium alloy contains, the less likely it is to crack; thus, the high magnesium content fillers "5356, 5183, and 5556" are commonly used to weld both wrought and cast aluminum magnesium base alloys."
Here's more on the subject that might just "muddy" the debate a bit:
"Magnesium in aluminum from 0.5 up to 3.0%, produces a weld metal composition that is crack sensitive and should be avoided. Another issue relating to the aluminum magnesium base alloys, which is not directly related to the crack sensitivity chart, but is a very important factor, must be addressed. As a rule, the Al - Mg base alloys with less than 2.8% Mg content can be welded with either the Al - Si (4xxx series) or the Al - Mg (5xxx series) filler alloys dependent on weld performance requirements. The Al - Mg base alloys with more than about 2.8% Mg typically cannot be successfully welded with the Al - Si (4xxx series) filler alloys. This is due to a eutectic problem associated with excessive amounts of magnesium silicide (Mg2Si) developing in the weld structure, decreasing ductility and increasing crack sensitivity."
Important factors to consider:
"Strength of the Weld - This rating is based on the ability of the filler alloy to meet or exceed the strength of the as-welded joint. Most often with aluminum the heat affected zone (HAZ) of a groove weld dictates the strength of the joint, and often many filler alloys can satisfy this strength requirement. Unlike groove welds, the joint strength of fillet welds is based on shear strength that can be affected considerably by filler alloy selection. Fillet weld strength is largely dependent on the composition of the filler alloy used to weld the joint. Typically, the 4xxx series filler alloys have lower ductility and provide less shear strength in fillet joints. The 5xxx series fillers typically have more ductility and can provide close to twice the shear strength of a 4xxx series filler alloy in some circumstances."
"Corrosion Resistance - Most unprotected aluminum base alloy filler alloy combinations are quite satisfactory for general exposure to the atmosphere. In cases where a dissimilar aluminum alloy combination of base and filler is used, and electrolyte is present, it is possible to set up a galvanic action between the dissimilar compositions. Corrosion resistance can be a complex subject when looking at service in specialized highly corrosive environments and may necessitate consultation with engineers within this specialized field."
Here's Alcotec's Technical Knowledge web page that offers a wealth of info regarding Aluminum:
http://www.alcotec.com/us/en/education/knowledge/techknowledge/index.cfmHere's another one that is also helpful.
http://www.aluminum.org/Content/NavigationMenu/The_Industry/Technology_Resources/Technology_Articles/applications.pdfI hope these are useful to anyone concerned.
Respectfully,
Henry
Henry, 
Forum
