IIW White Paper

4 Needs and challenges in welding and joining technologies

corrosion fatigue. Likewise, stress corrosion cracking may be a problem with some magnesium weldments, with hydrogen (formed in local galvanic cells) and residual stress driving the crack growth. This also opens a research field within the welding of magnesium alloys.

Dissimilar joints Due to growing fuel costs, there is a strong desire to replace steel parts with light metals (Al, Ti, Mg, and composites) in all segments of the transportation industry. With each component evaluated separately, an alloy replacement is typically made based upon an optimisation of cost, availability, mechanical properties, and ease of fabrication. This has resulted in, particularly for automobiles, the need to join together many different alloy types in a multi-component structure. This presents a formidable technical challenge, because fused alloy mixtures (e.g. Al-Mg, Al-Fe, Mg-Fe) result in intermetallic phases that severely embrittle the joint. In some cases this need has been met using braze-welding techniques whereby a filler material, deposited using a conventional welding technique (e.g. gas-metal arc), is made to wet the joint face with only minimal fusion. FSW appears to work well for some dissimilar metal combinations. Likewise, mechanical fastener- adhesive combinations have been used. A further challenge is to avoid or minimise galvanic corrosion which further complicates this problem, necessitating the use of insulators or coatings. Metal-matrix-composites (MMC) Light-weight structures often require high strength and stiffness at minimum weight where MMC materials may provide such property combination. An example can be given from aluminium matrix composites consisting of the matrix AA6061 reinforced with 10 to 20 vol. % aluminium oxide particles. Successful fusion welding of such materials (containing particles, short or long fibres and whiskers of Al 2 O 3 or SiC as reinforcement) is essential for development of various novel components. Responses of the MMC materials to the weld thermal cycle can significantly be different than the unreinforced alloys due to changed viscosity of the weld pool. It remains a challenge to weldMMCmaterials with or without filler wires to obtain optimum weld microstructure (avoidance of porosity and particle clusters etc.) and joint mechanical properties. It is important to improve current understanding of the materials response to various joining processes. 4.1.2 Consumables Welding consumable developments have not kept pace with developments in steels and other alloy systems. Steels of 900 MPa yield strength and higher are commercially available, but they are without matching filler metals with sufficient ductility and fracture toughness. At the moment, most welding of such steels is with undermatching strength filler metal, requiring that this be taken into account in weldment design and flaw assessment. Likewise, high toughness high strength pipeline steels lack consumables of corresponding toughness and strength suitable for field welding. These high strength steels often must be welded in high humidity environments, where hydrogen introduction from the atmosphere, as well as from the consumable, into the weld is difficult to avoid, with consequent concerns about hydrogen induced cracking. This issue poses new opportunities and challenges for the consumable manufacturers. Introduction of flux-cored wire type of consumables opens new technological opportunities in various industrial applications (such as shipbuilding) due to their higher deposition rate and deeper penetration capacities. The power generation needs of the industrialised countries and especially of the developing countries with growing populations, combined with the need to limit greenhouse gas emissions, require higher thermal efficiency in fossil fuel power plants, that in turn require higher operating temperatures, up to 750°C. Available filler metals do not match the creep performance of the advanced steels and nickel base alloys at such temperatures.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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