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