Abstract
The welding of cemented carbide to tool steel is a challenging task, of scientific and industrial relevance, as it combines the high level of hardness of cemented carbide with the high level of fracture toughness of steel, while reducing the shaping cost and extending the application versatility, as its tribological, toughness, thermal and chemical properties can be optimally harmonised. The already existing joining technologies often impart either insufficient toughness or poor high-temperature strength to a joint to withstand the ever-increasing severe service condition demands. In this paper, a novel capacitor discharge welding (CDW) process is investigated for the case of a butt-joint between a tungsten carbide-cobalt (WC-Co) composite rod and an AISI M35 high-speed steel (HSS) rod. The latter was shaped with a conical-ended projection to promote a high current concentration and heat at the welding zone. CDW functions by combining a direct current (DC) electric current pulse and external uniaxial pressure after a preloading step, in which only uniaxial pressure is applied. The relatively high heating and cooling rates promote a thin layer of a characteristic ultrafine microstructure that combines high strength and toughness. Morphological analysis showed that the CDW process: (a) forms a sound and net shaped joint, (b) preserves the sub-micrometric grain structure of the original WC-Co composite base materials, via a transitional layer, (c) refines the microstructure of the original martensite of the HSS base material, and (d) results in an improved corrosion resistance across a 1-mm thick layer near the weld interface on the steel side. A nano-indentation test survey determined: (e) no hardness deterioration on the HSS side of the weld zone, although (f) a slight decrease in hardness was observed across the transitional layer on the composite side. Furthermore, (g) an indication of toughness of the joint was perceived as the size of the crack induced by processing the residual stress after sample preparation was unaltered.
Highlights
Tungsten carbide (WC) bonded with 6 to 20 wt% of cobalt (WC-Co composite, cemented carbide or hard metal) is part of a family of materials which are composed of micrometricWC particles embedded in a cobalt matrix
The materials that had to be joined were supplied by Silmax S.p.A. (Lanzo, Italy) and consisted of a full density WC-Co composite rod and a drawn and tempered AISI M35 high-speed steel (HSS)
Boundary has been found to be fine and carbide free. Both features indicate an innate toughness of the capacitor discharge welding (CDW) joint, high thermal and residual stresses build-up on cooling as a result of the large differential thermal expansion of the joint as well as the phase transformations in the steel and Co matrices
Summary
Tungsten carbide (WC) bonded with 6 to 20 wt% of cobalt (WC-Co composite, cemented carbide or hard metal) is part of a family of materials which are composed of micrometric (or sub-micrometric)WC particles embedded in a cobalt matrix. Tungsten carbide (WC) bonded with 6 to 20 wt% of cobalt (WC-Co composite, cemented carbide or hard metal) is part of a family of materials which are composed of micrometric (or sub-micrometric). Materials 2020, 13, 2657 properties to the composite [1]. By varying the cobalt content, a wide range of wear resistance, hardness, strength, fracture toughness, and temperature and chemical stability can be achieved. Most applications require excellent combinations of hardness and toughness, which cannot be achieved by a WC-Co composite alone. The joining of tool steels with WC-Co composites can provide an alternative manufacturing route which combines the toughness of tool steel with the hardness of WC-Co composites. High-speed steel (HSS) usually contains W and V elements, which impart high tribological properties at high temperatures and high fracture toughness
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