P16 New Development in Aluminum Welding Wire - Alloy 4943

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1 P16 New Development in Aluminum Welding Wire - Alloy 4943 By: Bruce Anderson, Tony Anderson, Galen White and Patrick Berube Maxal International, a subsidiary of Hobart Brothers/ITW Welding North America Abstract Aluminum alloy 4943 filler metal is the first aluminum filler alloy to be developed for wrought commercial applications since the 1960 s and has recently received AWS A5.10 approval. It is designed to provide a high strength alternative to 4043 while maintaining the ease of welding and other advantages of Alloy 4043 filler metal is a popular aluminum/silicon filler alloy for general purpose welding applications but can show significant variability in strength based on welding conditions and the level of base metal dilution. Alloy 4943 filler metal is formulated to be welded with the same weld procedure specifications as 4043, address 4043 shortcomings while maintaining the same excellent corrosion characteristics, low melting temperature, low shrinkage rate, higher fluidity, and low hot cracking sensitivity in most applications. Alloy 4943 welds exhibit low welding smut and low discoloration similar to In addition to the higher as welded strength, the new 4943 filler alloy is heat treatable and has demonstrated its improved strength characteristics in the post weld solution heat treated and artificially aged condition when compared to the currently used heat treatable filler alloy 4643, which has been generally employed for welding the 6xxx series base materials that are post weld heat treated. This article explains the test results of this alloy development project, shows the alloy properties compared to the traditional filler metal alloys and offers potential applications and benefits. The fundamentals of aluminum-silicon alloys are also presented. Also included are 4043 baseline test results highlighting 4043 property variability susceptibility from the weld procedure. Introduction Starting in the 1930 s, the ancient arts of soldering and brazing were adapted to aluminum on a commercial scale. The brazing filler metals have liquidus temperatures above f but below the melting temperature of the base metal. With brazing and soldering, little if any base metal is melted and diffused into the brazing filler molten material pool is encountered. Alcoa developed two basic silicon based brazing alloys: AWS / ASTM Aluminum Association % Si % Mg Melting Range Class designation range Max. ºF BAlSi BAlSi Fusion welding of aluminum was started, on commercial basis, with the advent of high volume aluminum products fabrication in the early 1940 s. TIG welding was first, followed by MIG welding. These processes melt the metal under the arc, both filler and base metal, followed by the solidification of the weld pool. For this reason, arc weld beads have a rapidly solidified cast structure, and the solidified weld is metallurgically bonded to the base metal. The weld bead chemical composition is a mixture of the base metal and the filler metals. As the arc welding process was developed for aluminum, filler metals Page 1

2 were developed for welding various classifications of aluminum base metals. Unlike steel welding, filler metals with significantly different chemical composition than the base metals can be used. Dilution of base metal can range from very little as in many fillet welds and thin or thick (above ½ inch) material butt welds on to almost 100% filler metal weld beads. Because there can be filler metal to base metal dilutions varying from little to high percentages, the filler metal alloys for arc welding have been specifically developed to accommodate this variability. Most arc filler metal alloys contain all the alloying elements necessary to meet the design physical and property requirements of a base metal and filler metal combination without the need for base metal chemistry dilution. AWS A5.10 lists all filler metal classifications that are approved for both Aluminum MIG and TIG joining methods. All of the listed filler metal alloys have been specifically developed for arc welding except the casting repair and joining alloys and wrought alloys AA 4043, 4047 and These three wrought product filler alloys were developed as brazing alloys with 4047 and 4145 having specific end product commercial usage. Alloy 4043, because it was adopted from a brazing application rather than developed specifically as an arc welding alloy, relies on base metal dilution during arc welding to achieve the properties required for some applications. Further, it has been found that total welding heat input, the specific chemical composition of the base alloy and the cooling rate of the weld bead can all affect the 4043 weld properties. Alcoa developed filler alloy 4643 in the early 1960 s to address the specific challenge of getting sufficient dilution from base alloy 6061 to meet property requirements when using filler alloy 4043, especially in the post weld heated treated and aged condition. Alloy AA-4643 was designed by Alcoa to be a blend of 80% 4043 and 20% 6061 chemistry in the filler metal. Alloy 4643 still requires some dilution from the base alloy (approximately 20%) for post weld heat treat and age applications. Alloy 4643, as compared to 4043, has reduced silicon content that decreases the fluidity of the molten metal, reduces the sweating action and the ability for bead contour control. The lower silicon reduces the tensile properties attributable to the different silicon phases that add to the tensile/yield strength discussed later in this paper. As a result, 4643 properties can typically only achieve 90% of the base metal 6061-T6 properties. Further, this alloy was designed to be a specialty alloy which makes it used in very limited quantities because the selling price is many multiples of alloy 4043 s selling price. Development of alloy 4943 As previously stated, alloy 4043 was developed as a filler alloy for brazing. Alloy 4943 has been developed specifically for the arc welding process to optimize product welding characteristics and properties in the as-welded, post-welded-artificially aged and post-welded-heat treated and artificially aged conditions. Alloy 4943 was engineered to maintain the welding physical characteristics of alloy 4043, while solving the negative aspects of using brazing alloy 4043 for an arc welding filler metal application. Alloy 4043 has a proven performance history for over 70 years of welding performance and accumulated specification data. Alloy 4043 is used in approximately 30% of all welds made worldwide. Therefore, it is believed that alloy 4043 needed to be improved but not totally replaced by a different alloy. A significant case can be made that all current 4043 could and should be changed to 4943 for arc welding applications. Alloy 4943 was developed with the objectives to offer a higher tensile and yield strength alternative to 4043 and 4643 while maintaining the same proven welding characteristic of fluidity, shrinkage, Page 2

3 solidification range and resultant low weld cracking sensitivity of All this without relying on base metal dilution. It exceeds the strength of 6061-T6 upon post weld heat treat and aging. Specifically, it was developed to be a filler alloy for mass production welding at the preset equipment settings, programmed into welding equipment for 4043, sold off the shelf. Value Proposition and Applications Alloy 4943 is suitable for all applications currently using alloys 4043 or These applications typically use 1XXX, 3XXX, 4XXX and 6XXX base alloys. Applications such as automotive and motorcycle frames, wheels, ship decks, pleasure boats, bicycles, scooters, 356 casting repair and high end ladders are ideal for the use of alloy Alloy 4943 is not recommended for 5XXX series alloys with above approximately 2.5% magnesium as coarse Mg 2 Si may form and be detrimental to joint strength and ductility. Alloy 4943 is especially well suited for V-groove welds with plate thicker than 0.5, thin walled sheet or tube applications and fillet welds. It has demonstrated higher weld strength than alloys 4043 and 4643 in the as-welded, post weld aged or post weld solution heat treated and artificially aged conditions. Alloy Design The alloy was designed around two principal ideas: 1) The addition of a strengthening element in this case magnesium, which combines with the available silicon to form Mg 2 Si, an effective strengthening phase as demonstrated in 6XXX series alloys. The range of the magnesium addition was set from 0.1% to 0.5% in order to achieve a specific amount of Mg 2 Si precipitation while staying away from the crack sensitivity peak of 1% Mg 2 Si (see figure 1). Relative crack sensitivity %Mg 2 Si Figure 1. Mg 2 Si crack sensitivity curve. (source: maxal.com\guide for Aluminum Welding ) 2) The adjustment of the silicon range to % to maintain the level of free silicon at the same level as % - knowing that up to 0.5% Silicon will precipitate as Mg 2 Si phase. Maintaining the amount of free silicon to the level of 4043 is essential to maintain the fluidity characteristics and resistance to hot cracking during solidification. It has a beneficial effect on strength and also allows designing for fracture toughness and fatigue performance similar to alloy 4043, which are well known. Page 3

4 Metallurgy of Aluminum-Silicon Alloys Most of this section borrows information and figures from ASM s publication Aluminum-Silicon Casting Alloys, Atlas of Microfractographs, chapter 1. Very little knowledge and test data specific to aluminum welding filler alloys is available. This said, in many cases, aluminum casting metallurgy can be of significant help because MIG and TIG welding create many conditions that are common with casting processes. Silicon is one of the most common alloying elements in commercial aluminum. Its effect of increasing fluidity, reducing solidification and solid state shrinkage, reducing welding distortion, strength, wear resistance, etc. are taken advantage of in many markets and applications, from casting, where up to 25% Si levels are common and in a variety of wrought products, including welding rod and electrodes. Currently the main Al-Si filler wire in terms of volume usage is by far 4043, with a range of % Silicon. This is under the 12.6% eutectic composition, as shown in figure 1. This alloy offers a good compromise in terms of resistance to hot cracking, and this is due specifically to its high fluidity, lower melting temperature and reduced shrinkage rate obtained from the silicon content. It is easy to weld with and produces smooth, great looking welds. It offers a good compromise in strength and manufacturability. Figure 2. Al-Si phase diagram and typical hypoeutectic Al-Si alloy microstructure. Alloy 4047 is the 2 nd most commonly used, near the eutectic composition with 12% silicon. The increased fluidity makes it suitable for applications requiring superior leak proofing such as heat exchangers and pressure vessels. The improved fluidity comes at a cost, as fracture toughness and fatigue life are negatively impacted by the higher silicon content. Figure 3 shows a microstructure typical of eutectic Al-Si alloys such as Hypereutectic alloys are common in the foundry industry but not in the wrought or weld wire product forms as the very high silicon content makes these alloys extremely difficult to hot work or cold work. Page 4

5 Figure 3. Al-Si phase diagram with typical eutectic alloy composition microstructure. Other registered alloys do exist but their use is rather marginal. Examples are 4010, 4643, etc. The silicon addition increases strength of the alloy through 1. Up to 0.1%. Solid solution strengthening via atomic substitution the size of the silicon atom being key, since atoms of smaller radii (e.g. Si, Mn, Cu) are more effective at strengthening the α-phase than atoms of larger radii (e.g. Mg) %. Disperse precipitation strengthening. Small silicon particles precipitate since the matrix is supersaturated following fast cooling. See figure 4, zone II and figure Above 1.65%. The strength increases proportionally to the volume fraction of silicon, up to a point where the morphology and distribution of silicon precipitates offsets the effect of more additions. See figure 4, zone III. Page 5

6 Figure 5. Disperse precipitation of silicon following supersaturation from fast cooling. Figure 4. Tensile strength vs. Silicon content in aluminum cast alloys. Additional means can be used to further strengthen the alloy through 1) decreasing the brittle fracture risk associated with the morphology of the silicon particles, 2) increasing the strength of the soft matrix, and 3) increasing the degree of dispersion of the dendritic structure. In the case of alloy 4943, small strontium additions are made during the casting process to spheroidize the silicon particles, making the particles less susceptible to brittle fracture. The silicon particle modification from strontium is effective twice, first when the raw redraw rod is cast, providing ease of manufacturing and a second time when the filler alloy is weld, providing additional strength of the weld joint. See the modified line in figure 4. Alloys 4643 (developed in the 1960 s by Alcoa) and 4943 (recently developed by Maxal-Hobart/ITW) take advantage of magnesium additions to increase the strength of the soft matrix, in addition to all above strengthening means from the silicon. In this case, the magnesium combines with the free silicon to precipitate as Mg 2 Si. The Mg 2 Si precipitates are very effective at strengthening the matrix. Alloy 4043 Page 6

7 takes advantage of this to a lesser extent, when it is welded to a base metal which contains magnesium, as some magnesium from the base metal is diluted into the fusion zone. In this case, the welding practice and the type of weld have a significant impact on the amount of dilution fillet welds, thin, and thick welds are especially susceptible to variations in the degree dilution, thus variations in welds strength (shown in figure 8). The smaller variation for 4943 can be explained by the fact that the magnesium is already in the alloy and not dependent solely on base metal dilution diffusion. The degree of dispersion of the dendritic structure is influenced by the alloy composition and the welding practice. Titanium boron additions used as a grain refiner during the rod casting process also influence the grain structure of the solidified weld, hence its strength. Heat input during welding, cooling rate influenced by ambient conditions or thermal conductivity of the base plate as well as interpass temperature can also impact the weld strength of these alloys. Figures 6a and 6b illustrate the difference between 4043 Tig welded (very high heat input) and 4043 MIG welded with moderate heat input. The result being a slightly coarser microstructure for the TIG weld, and potentially more annealing effect (precipitation of Mg 2 Si phase) between weld passes, resulting in lower tensile strength. Figure 6a TIG weld microstructure (coarser than 4043 MIG weld, 25.2 ksi average ultimate tensile strength) Figure 6b MIG weld microstructure (finer than 4043 TIG weld, 28.0 ksi average ultimate tensile strength) Page 7

8 As-Welded, Post Weld Aged and Post weld heat treated and aged properties Alloy 4943 was evaluated side by side with alloys 4043 and The evaluation was done using different weld joint designs welded to the D1.2 code for alloys in the F23 group. Tensile test specimens were taken in the longitudinal direction, in the all weld metal region and were tested per ASTM B557. A number of tests were done using fairly wide joints with alloy 1100 base plates to prevent any favorable base metal dilution. This is important to recreate the little to no dilution conditions obtained on many thick welds, on very thin joints where heat input has to be limited, and, more frequently, on fillet welds in the field. Figure 7 summarizes the results. Figure 7. All weld metal longitudinal tensile strength of 4043, 4643 and 4943 in the as-welded, post weld artificially aged, and solution heat treated and artificially aged to T6 conditions, with no base metal dilution (Alloy 1100 base plate). Resistance to Hot cracking Shrinkage strains are proportional to the temperature range between the highest temperature of coherence between solidifying dendrites and the solidus - where there is very low cohesive strength between dendrites. The range is at its minimum at approximately 5.1% Silicon. This is key to understand why a magnesium addition in 4943 creates the necessity to the increase of the silicon content target to 5.5%, to compensate the loss of free Silicon through precipitation into Mg 2 Si. This adjustment is done with 4943, maintaining the level of free Silicon to at least 5%, but is not done for alloy 4643, for which the free silicon content is significantly lower, as low as 3.6%. This results in more shrinkage strains during the solidification of 4643, potentially resulting in more incipient cracking during solidification than with alloys 4043 and The 12% silicon level is near the eutectic composition for an alloy such as 4047, where the amount of liquid present during solidification is maximized, allowing constant back-filling of the cracks resulting from shrinkage strains. For this reason, alloy 4047 is typically used in applications where high sweating and pressure tight integrity properties are important such as heat exchangers. Page 8

9 Conclusion Aluminum-silicon alloys of the 4XXX series are widely used in MIG and TIG welding due to their very good welding characteristics, fluidity, reduced shrinkage distortion, and resistance to hot cracking. The moderate and variable strength of 4043 can be improved via magnesium additions to the filler alloy itself and this was done to some extent with alloy The magnesium addition was optimized with a silicon addition in alloy 4943, for optimal strength, fluidity, shrinkage and hot cracking resistance. A significant case can be made that all current 4043 and 4643 could and should be changed to 4943 for arc welding applications. Page 9