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The fracture characteristics of welds of various aluminum alloys were evaluated by means of tear and notch-tensile tests. The tear resistance and notch toughness of welds are generally greater than those of cold-worked or heat-treated base metal, and approach those of annealed base metal.
Subsequent thermal treatment of heat treatable filler metals may appreciably change the fracture characteristics. Except for the 7000 series of alloys, there is no significant decrease in the toughness of the welds between room temperature and -320 or -423°F (-160 or -217°C). Except for 4043 with 6061, the toughness of the weld is equal to or greater than that of the base metal.
Welding is one of the most practical methods of joining aluminum alloys, particularly for applications where components must be leak tight. The welding of almost all commercial aluminum alloys is feasible with one or more of the processes developed since 1950, the two most popular being the gas metal-arc and gas tungsten-arc.
Some alloys are, of course, more difficult to weld than others, in that more cracking occurs. Those generally considered "aircraft" alloys, principally the high-strength 2000 and 7000 series, are weldable with special techniques whereas 2219 and the "pressure-vessel" alloys, the 1000, 3000, 5000 and 6000 series, are readily welded. The choice of filler alloy is also an important factor in determining the relative ease of welding.
Welds of almost all of the aluminum alloys are so tough that accurately describing their fracture characteristics is often a problem, particularly in the case of the pressure-vessel alloys. Although data developed in accordance with fracture-mechanics concepts would be of most value to designers, it is practically impossible to develop unstable crack growth under elastic stress conditions in laboratory tests in sound aluminum-alloy welds.
A wide variety of aluminum alloy base sheet and plate and filler alloys are represented by the test data. The sheet and plate range from 1100-H112 (commercially pure aluminum in the as-rolled condition) with an ultimate tensile strength of 15,000 psi (≈110 MPa) to 7178-T6 (Al-Zn-Mg alloy, solution heat treated and artificially aged) with an ultimate tensile strength of 90,000 psi (≈630 MPa).
There are no consistent differences among the unit propagation energies for the individual alloys dependent upon specimen orientation. In the few instances where there appears to be a significant difference, the specimens in which the crack extended along the center of the weld gave the lowest values, so the use of this orientation as the standard is reasonable.
Welds of the high strength 5000 series alloys have the highest tear resistance. There is a trend toward decreasing tear resistant with increasing magnesium content. Filler alloys 2319 and 4043 have less tear resistance than the Al-Mg or, Al-Mn alloys. Aging or heat-treating and aging increases the tear resistance of 2319 welds in 2219, but heat-treating 4043 welds in 6061 markedly reduces the tear resistance.
The tear resistances of welds of the nonheat-treatable alloys are relatively high, in fact in some cases almost as high as those of the base alloys in the annealed condition (0 temper). This is to be expected since welding partially anneals the weld zone (heat-affected zone). In all cases, the unit propagation energies are about equal to or greater than those of the unwelded base metal in the cold-worked tempers.
In the as-welded or aged condition, 2319 has practically the same tear resistance as 2219-T81, T851 sheet and plate. When heat treated and aged after welding, the tear resistance of the weld metal is greater than that of 2219 T62 sheet and plate.
Welds of 4043, without subsequent treatment, have about the same tear resistance as those of 2319, and about 2/3 that of 6061-T6 plate, with which 4043 is commonly used. Heat treatment and aging after welding reduces the tear resistance of 4043 to a much lower level, to only about 15 % of that of 6061-T6 plate.
These variations in tear resistance, indicated by unit propagation energy, show that tear resistance is no simple function of either strength or ductility (elongation), but rather reflects the particular degree to which and ductility combine the material to resist crack propagation. Although it is true that general trend is toward increasing tear resistance with increasing elongation and decreasing strength, the great ductility of 1100 is more than offset by its lower strength; hence, the toughness is at a level lower than that of 5052 or 5154. These latter alloys have both high strength and great ductility.
The practical difficulty of determining empirically the plane-strain fracture toughness (K1c) of aluminum alloy welds may be seen by spotting the unit propagation energies. Unit propagation energies for all but 4043 and 2319 are above 700in.-lb/in.2. This fact, of course, is a strong indication that problems in unstable crack propagation at elastic stresses in sound welds of these "pressure-vessel" alloys would be quite rare.
Even 2319, in various conditions, and 4043, as-welded, have considerable toughness. Some attempts to measure K1c for many aluminum alloys with indicated toughness in this range, such as 2219-T851 and 2219-T87, have been fruitless. Values in the range of 30,000 psi√in. might be anticipated. With heat treatment and aging after welding, the toughness of 4043 is low, with a value of K1c of about 20,000 psi√in indicated.
At -320°F (-160°C), the tear resistances of welds of most aluminum alloys are about as high as or higher than at room temperature. In some cases, notably 2319, as welded and with subsequent aging, the unit propagation energies were appreciably higher at -320°F, while for most of the alloys tested they were in the same general range -- all exceptionally high. There are no suggestions of any sudden transitions in fracture behavior.
Tensile tests of severely notched specimens provide information on the ability of materials to deform plastically in the presence of stress raisers, and thereby resist crack initiation. The very sharp notch was selected because it approximates the most severe stress concentration, a crack, in the structure; and studies of various designs of notched specimens showed that the greater range of data from tests of the very sharply notched specimens provided greater discrimination between materials.
Notch-tensile strengths are calculated by dividing the fracture loads by the net area at the root of the notch. These strengths, in themselves, are not very useful as indicators of notch toughness, but their relation to the tensile yield strength provides an indication of whether or not the fracture took place with appreciable yielding.
The greater the ratio of the notch strength to the yield strength (termed the notch-yield ratio), the greater is the ability of the material to deform (yield) in the presence of the stress concentration. The ratio of the notch-tensile strength to the tensile strength (often called the notch-strength ratio or notch-tensile ratio) is not as reliable for evaluating the relative notch-toughness of materials; although, it may be useful as an indication of tensile efficiency for certain specific structural components.
By way of caution, the common practice of setting some arbitrary value of notch-yield ratio or notch-strength ratio as a limiting value may be misleading, since the value of notch-tensile strength and hence, of the ratios, are dependent upon specimen geometry. No specific value of a ratio can have the same significance with different notch designs. The data are useful primarily for rating the alloys.
Tensile tests of notched specimens have been used to evaluate the notch toughness of welds at subzero temperatures as well as at room temperature. Tensile tests of notched specimens were made at -320 and -423°F (-160 and -217°C) in the same manner as at room temperature, except that the specimens and grips were immersed in liquid nitrogen and liquid hydrogen, respectively.
Welds of 5556 filler alloy in 5456 base alloy are relatively tough, but welds of 5556 in high strength 7000-series alloys 7079-T6 and 7178-T6 are less tough. Alloy 4043 shows a similar variability; welds in alloy 6061 are tough in comparison with those in alloys 2014-T6 and 7075-T6.
For both sets of data however, it is clear that the notch toughness of welds in aluminum alloys, as measured by notch-yield ratio, is generally greater than that of cold-worked or heat-treated base metal. Since welding partially anneals the weld zone, the toughness of the joint approaches that of annealed plate. Aging after welding or heal treating and aging after welding can appreciably reduce the toughness of some combinations of alloys.
Data from tests at -320 and -423°F show that the notch-tensile strengths of the welds, as well as the tensile and joint yield strengths, are higher than at room temperature. Notch-yield ratios are generally about the same at -320°F as at room temperature; some show a decrease between -320 and -423°F, but in these cases, the notch-strength ratio remains the same at the two temperatures.
These variations of the notch-yield and notch-tensile ratios suggest that the ultimate strength of the weld influences the weld notch-strength to a greater degree than the yield strength. In all cases, the notch toughnesses of the weld at -320 and -423°F (-160 and -217°C) are superior to that of the base metal.
Welds in the non heat-treatable alloys (1000, 3000 and 5000 series) are exceptionally tough. Comparison of their tear resistances and notch toughnesses with correlations established between these properties and plane-strain stress-intensity factor, K1c, on the basis of tests of very high strength aluminum alloys, indicates that in many cases K1c, is greater than 40,000 psi√in., so that unstable crack growth is not likely to ever be a problem in welded structures of these alloys.
The influence of subzero temperatures on the fracture characteristics has been evaluated and, except for welds in the 7000 series of alloys, there is no significant decrease in the toughness at -320 or -423°F (-160 or -217°C). Except 4043 with 6061, the toughness of the weld is greater than that of the comparable base metal.