Ifs welding al_mmc_b4_c

Friction Stir Welding of Aluminum MMC 6061-XX% Boron Carbide

Han Zhang* and Tracy W. Nelson** T. Haynes*

*Reynolds Metals Company **Brigham young University

Provo, UT Aluminum metal matrix composites (MMC) offer increased stiffness, strength and wear resistance over standard aluminum alloys. However, the weldability of these materials is greatly compromised with the addition of non-metallic reinforcement. Powder metallurgy products of METAMIC materials with B4C reinforcement developed at Reynolds Metals Company is being considered for neutron shielding applications because of the ability of B10 isotope to absorb the neutrons. Fusion welding studies have demonstrated the ability to join in this type of material with minimal welding-related defects using carefully controlled parameters. However, the joint strength is determined by the choice of filler material. Solid state welding method could be used to join this type of material offering better joint mechanical properties. Friction stir welding (FSW) was investigated as a possible means of joining this material. Preliminary results demonstrate that FSW is a viable process for joining producing superior mechanical properties compared to traditional fusion welding. Test results for both fusion welding and FSW will be compared, as well as tool wear and the feasibility of joining 6061-21% B4C METAMIC neutron shielding materials.

1. Background

Friction stir welding (FSW) is a relatively new joining process that has received considerable attention since being patented in 1991 by The Welding institute in Cambridge England (Ref. 1-3). The advantages of friction stir welding (FSW) over conventional fusion welding have been recognized by many industries, especially for joining aluminum alloys (Ref. 11-17). A few of the advantages that pose significant cost reductions in the manufacturing of aluminum aerospace structures include the elimination of cracking in both the fusion zone and heat-affected zone, porosity, filler metals, shielding gases and costly weld preparation (Ref. 1 and 15). Despite these potential benefits and the tremendous development efforts by numerous industries to implement this process into production, there is still a vast amount of research and development needed in order to better understand the microstructure and long term service integrity associated with friction stir welds (FSWs). An understanding of microstructural evolution during FSW and how the associated microstructures affect such mechanical and physical properties as strength, fatigue, creep and corrosion is critical. Such an understanding will bring broader acceptance, and thus, new applications for this new innovative joining technology. Although FSW has gained broad acceptance in joining many aluminum and cooper alloys, there are a number of other materials which could benefit tremendously from this process. Aluminum metal matrix composites (MMC) are among those materials that could take advantage of the benefits that FSW offers over traditional fusion welding techniques. MMC materials suffer from a variety of problems when subject to traditional fusion welding processes. Welds in alloys like 6061-Al2O3 exhibit poor mechanical properties due to loss in composite material and porosity formation. During welding, the aluminum oxide particulate dissociates resulting in free excess oxygen which may for porosity in the weld and a lower composite particulate count in the matrix. Both of the above lead to reduced mechanical properties.

Likewise, aluminum alloys reinforced with SiC or B4C particulate suffer from similar problems. SiC and B4C dissociate during fusion welding resulting in excess Si and B in the matrix. The carbon form these phases combines with aluminum to form an adverse aluminum carbide (Al4C3) phase which is soluble in water. Again, the loss of reinforcing particulate and the formation of adverse aluminum carbide and porosity results in reduce mechanical properties of these alloys when fusion welded.

Despite their problems during welding, these materials offer increased stiffness and specific strength. Alloys strengthened with B4C also exhibit reduced density with increased wear resistance and neutron absorption. Alloy 6061-XX%B4C can exhibit an XX% in modulus with up to XX% B4C. This alloy also exhibits excellent neutron absorption characteristics making this alloy particularly beneficial in the nuclear industry. ????? 2. Experimental Approach Both gas tungsten arc welds (GTAW) and friction stir welds (FSW) were made in ??? in. thick 6061-XX%B4C. GTAW were produced using xXX amps alternating current with pure Argon shileindg gas. ????? FSW were produced at 670 RPM and 4.5 and 5.5 IPM using a tool design developed at BYU.

3. Results and Discussion Although possible, it is difficult to produce quality welds in 6061-XX%B4C using GTAW. There exist a miriad of problems associated with GTAW in this materials such as porosity, adverse second phase formation and excess eutectic formation in the fusion zone. Optical and SEM photomicrographs of transverse section of GTAW and FSW in 6061-XX%B4C are shown in Figures 1-5.

Figure 1. Transverse photomicrograph showing presence of porosity in an autogenous GTAW in 6061-XX%B4C.

Figure ?. Transverse photomicrograph showing aluminum carbide (black needles) in an autogenous GTAW in 6061-XX%B4C

2 mm

10 µm

Figure ?. Chemical analysis of needle phases (black) in Figure ?.

Figure ?. Photomicrograph of GTAW with 4043 filler metal in 6061-21%B4C.

Al4C3

2 mm

a) b)

Figure ?. GTAW with 4043 filler in 6061-21%B4C, a) showing shrinkage porosity in weld, and b) showing eutectic formation.

a) b)

Figure ?. Transverse photomicrographs of the weld regions in a) GTAW and, b) FSW. All the test results indicate that FSW is a superior joining process comparative to GTAW when joining 6061-XX%B4C. Tensile test results are shown below in Figure ?. From

20 µm

Shrinkage Porosity

1 µm

Si

Figiure ?. Comparison of base metal and weld metal Tensile properties in 6061-XX%B4C.

Figure ?. SEM backscatter of surface of FSW indicating deposits of Fe from the tool at the

advancing side of the welding. 4. Conclusions 1. METAMIC neutron shielding materials containing B4C can be welded using TIG method.

BSFSW-4.5

FSW-5.25TIG

Elongation

YSUTS

36.0

29.4 30.4

22.9

18.0 19.5 19.817.4

12.0

5.04.0

4.00.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0St

reng

th (k

si) +

Elo

ngat

ion

(%)

Conditions

100 µm1 mm

Steels

2. Standard welding procedure used to join I/M monolithic 6061 can not be used to join this type of material due to serve hydrogen-induced porosity and formation of Al4C3 at the fusion zone.

3. The density of hydrogen-induced porosity can be minimized by controlling heat input and dilution.

4. No apparent heat affected zone softening was detected. 5. Fusion zone mechanical properties are determined by the choice of filler material. 6. FSW can be used to join this material with similar tensile properties as parent material.

However, the long weld can not be made due to serve tool wear. 7. References 1. Welding Institute; TWI; Thomas, WM; Nicholas, ED; Needham, JC; Murch, MG; Temple-

Smith, P; Dawes, CJ, “Improvements Relating to Friction Welding [Friction Stir Welding and Friction Plunge Welding],” PCT World Patent Application WO 93/10935. Filed: 27 Nov.1992 (UK 9125978.8, 6 Dec.1991). Publ: 10 June 1993.

2. TWI; Thomas, WM; Nicholas, ED; Needham, JC; Temple-Smith, P; Kallee S, WKW; Dawes, CJ, “Improvements Relating to Friction Welding [Friction Stir Welding and Friction Plunge Welding],” UK Patent Application 2 306 366 A, Filed: 17 Oct.1996 (UK 9521570, 20 Oct.1995; 9523827, 22 Nov.1995; 9605864, 20 Mar.1996). Publ: 7 May 1997.

3. TWI; Thomas, WM; Murch, MG; Nicholas, ED; Temple-Smith, P; Needham, JC; Dawes, CJ, “Improvements Relating to Friction Welding [Friction Stir Welding and Friction Plunge Welding],” European Patent Application 653 265 A2. Filed: 27 Nov.1992 (UK 9125978, 6 Dec.1991). Publ: 17 May 1995.

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13. W.M. Thomas and E.D. Nicholas, “Friction stir welding for the transportation industries”, Materials and Design, Vol. 18, Nos. 4/6, 1997, pp. 269-273.

14. M.J. Johnsen, “Friction Stir Welding Takes Off at Boeing”, Welding Journal, Feb. 1999, pp. 35-39.

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16. O.T. Midling and H.G. Johansen, “Production of Wide Aluminum Profiles by Solid-State Friction Stir Welding”, International Aluminum Extrusion Seminar & Exposition, Chicago, IL, May 1996.

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