Fusion Welding of Titanium-Tungsten and Titanium-Graphite Composites

cored boron filaments, which was con trary to the effects of higher energy input fusion welds where fiber crack ing, break-up and misorientation were common. The effect of poor materials compatibility and high energy input during welding on another type of Fusion Welding of Titanium-Tungsten reinforcing fiber has also been dra matically demonstrated in gas tung sten-arc fusion welds in a sapphire- and Titanium-Graphite Composites titanium (single crystal A1203) fila ment composite.9 The sapphire fila ment was completely dissolved in the molten titanium during welding, result Reaction products at the reinforcing ing in a coarse, embrittled micro- structure that severely cracked while cooling. Other work where fusion of filament-metal matrix interface the matrix resulted in little or no appreciable damage to the reinforcing are directly related to welding energy input fibers included resistance welding of steel wire-aluminum10 and fusion welding of tungsten wire-aluminum.9 The only other well known related BY JAM ES R. KENNEDY studies on composite fusion welding have been on dispersion-strengthened materials such as TD-Nickel. The effects of fusion in this case are quite detrimental, causing destruction and agglomeration of the fine thoria dis persion and a drastic lowering of weld ABSTRACT. Fusion welding experiments strength, light weight and economical joint efficiencies; these problems have were conducted on two titanium matrix structural joints. However, composite composite systems: titanium-tungsten led to emphasis on solid state welding welding remains largely problemati techniques.11 wire and titanium-graphite filament. Ob cal, primarily because the effects of jectives: Determine the weldability of An investigation was undertaken to weld thermal energies on fibers and model composites and observe the influ study the effects of fusion welding on matrix-fiber compatibility for many ence of weld thermal energy on fiber- two metal-matrix composite systems: composites are not yet fully under matrix reactions. titanium-tungsten and titanium- stood. Results of mechanized and manual gas graphite. The titanium-tungsten sys tungsten-arc (GTA) welding tests indi Welding techniques requiring fusion tem is characterized by the absence of cated that well diffusion-bonded com of the matrix present the most appar compound formation and the forma posites generally presented no unusual ent critical condition because the rein tion of a two-phase region. Converse problems during fusion. The extent of forcing fibers will be exposed to a ly, the titanium-graphite system is fiber-matrix reactions in both systems superheated molten matrix and some was directly proportional to the welding characterized by matrix-fiber reac times an intense heat source (e.g. an tions that can lead to interfacial com energy input. As energy input increased, electron beam or electric arc). This tungsten wire dissolution became greater pounds. and titanium carbide formation around can give rise to accelerated reaction rates between matrix and fiber, pos The objectives of this work were to the graphite filaments grew thicker. obtain weldability information on sim Welding energy input thus becomes a sibly leading to extensive interdif fusion, fiber dissolution or complete ple model composite systems that significant factor in controlling the would also be generally useful in the nature of fiber-matrix reaction products. fiber destruction. Obviously the extent Tensile properties of titanium-tungsten of matrix-fiber interaction depends development of other composites and composites, both as-diffusion bonded and upon their basic compatibility and the to increase understanding of fiber- as-welded, are compared. weld thermal history to which they matrix reactions by observing com are subjected. posites exposed to the dynamic ther mal conditions of fusion welding. This Introduction Previous work on composite joining paper describes the fabrication and has been generally quite limited, espe welding of composite specimens, met The great potential of composite cially in the area of fusion welding. In materials in providing improved 6 allographic observations and the .ten a study to determine joining tech sile properties of composite-Velds. mechanical properties has led to the niques suitable for boron fiber- realization that the extent of their aluminum composites it was con utilization as structural members de cluded that gas tungsten-arc, electron Experimental Procedures pends on how easily they may be beam and plasma fusion welding re Materials fabricated and joined into complex 15 sulted in severe weld embrittlement Tungsten wire used in this investi shapes. The possibility of welding and fiber degradation. Conversely, gation was 0.008 in. commercially metal-matrix composite systems resistance welding of these composites pure lamp filament. Typical tensile should be an important consideration has been relatively successful in pro properties in the as-drawn and cleaned because of the need to fabricate high ducing good quality spot welds with condition were 446 ksi tensile strength 6-8 reasonably high strengths. and 4.9% elongation. After heat Interestingly, the molten aluminum treating at 1600F for 1 hr in vacuum, JAMES R. KENNEDY is with the Ma terials Research Group, Grumman Aero matrix of the spot welds had no ap the tensile strength was 328 ksi with space Corp., Bethpage, N. Y. parent adverse effects on the tungsten- an elongation of 1.9%. 250-s I MAY 19 72 The graphite filaments were Graphite was used in the as-received longitudinal to the fiber direction. uncoated "Courtaulds HM," a high condition. Butt welded joints consisted of either modulus and high strength grade pro Before hot pressing, the composite machined edges, in which the edges of duced from a polyacrilic-nitryal assembly was enveloped in a layer of both the matrix and the fiber ends (PAN) precursor. The filaments were 0.005 in. thick stainless steel foil to were coplanar, or of joints in which about 8 turn (0.0003 in.) in diameter protect against contamination from the matrix edges were removed a few and were spooled as a continuous the hot press graphite rams. The foil mils by chem milling to provide fiber length in an unwoven yarn. Typical was tackwelded around its edges and relief for joint intermeshing. Both top tensile properties were 300 ksi tensile then its outside surfaces were coated and bottom surfaces of the titanium strength and 50 to 60 million psi with a thin slurry of 0.05 y.m alumina composites were manually scraped modulus. powder that acted as a parting agent with a draw file in the vicinity of the The matrix for both groups of com between the foil and the graphite fusion zone before welding to mini posite specimens was an annealed rams. The composite specimens were mize possible weld contamination. hot pressed at 1000 psi for 1 hr at sheet of commercially pure titanium, 4 Ti75A, in sheet thicknesses up to 1600F under IO" mm Hg vacuum. The fusion welding operation was 0.017 in. After hot pressing, the foil layers were performed by manual and mechanical removed and the composite specimens techniques, while argon inert gas Composite Fabrication were chemically milled to remove a 5 shielding procedures generally recom mended for titanium were used. Typi Composite specimens containing the mil surface layer from each side. cal manual welding parameters for tungsten wire or graphite filaments in The composite specimens in this the titanium matrix were made by investigation were prepared and tested 0.055 in. thick composites were 60 diffusion welding in a vacuum hot with their reinforcing fibers aligned amp, 10 volts and 6 in. per min (ipm) press. The titanium-tungsten com along the axis of loading (i.e., zero speed. The parameters for mechanized posites were made by wrapping a degree orientation). Fiber volume welding on 0.036 in. thick composites continuous length of tungsten wire percentages of each composite were were 25 amp, 12 volts and 10 ipm around a sheet of titanium, which was approximated during composite speed. then sandwiched with additional ti preparation and later were more ac tanium sheet. The assembly was resist curately determined by metallurgical Testing ance tack welded at the corners to cross sectioning and counting. Room temperature tensile tests de facilitate further handling. The graph termined the strength of all matrix ite-titanium composites were prepared Welding and composite specimens. Subsize ten by placing a portion of the graphite All fusion welded specimens were sile coupons were used in accordance yarn on a titanium sheet, covering it prepared by gas tungsten-arc welding. with ASTM Standards.12 The tensile with another titanium sheet and then Two types of weld specimens were specimens were generally machined in tackwelding the sheets together. tested: square groove butt and simple relation to the fibers, as indicated in Prior to assembly, the titanium was bead-on-sheet. Welds in these experi Fig. 1. Composite and matrix tests cleaned with an alkaline degrease and ments were predominantly without were conducted on an Instron testing water rinse and the tungsten wire filler wire additions. Bead-on-sheet machine at a crosshead speed of 0.02 was cloth-wiped in isopropyl alcohol. welds were made both transverse and ipm. Weld Direction 7^ v <Z7, ' / •> " v ^a Fig. 1—Schematic of composite lay-up and specimen orientation WELDING RESEARCH SUPPLEMENT I 251-s Composite Examination vealed no detrimental effects

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