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Mechanical performance of laser metal deposited alloy 718

Alloy 718 is a well-established material with applications in many areas of modern and established aeroengine designs. Alloy 718 components can be very large, presenting opportunities for considerable savings in both repair and additive manufacture through the use of laser metal deposition (LMD).

As part of its Core Research Programme, TWI undertook a programme of work to address the industrial needs for performance data of LMD alloy 718, and to optimise process parameters on the basis of performance as opposed to metallurgical quality.

Sample production

Rectangular blocks of alloy 718 were deposited on 316L stainless steel plates using a Huffman HC-205 laser cladding machine (Fig. 1). Samples were subjected to a direct two-stage heat treatment.

Following process optimisation, two parameters were developed for investigation: high energy input (HEI) and low energy input (LEI).

Wrought alloy 718 was also used to provide a benchmark for tensile and fatigue testing. The material was solution heat treated at 980°C before being subjected to the same two-stage ageing process used for the LMD samples.

Fig. 1 Test samples produced using HEI parameters
Fig. 1 Test samples produced using HEI parameters

Performance evaluation

A series of tests was performed including tensile testing in air under ambient and elevated temperature (630°C), fatigue testing in air under ambient conditions and creep rupture testing at 630°C.

Tensile testing found that the LEI samples gave the highest proof strength under both ambient (~1040MPa) and elevated temperature (~827MPa) conditions.

Compared to wrought at ambient temperature (~1221MPa), a reduction in proof stress by a factor of 1.17 for LEI and 1.21 for HEI was observed.

Fatigue endurance data, plotted in the form of an S-N diagram, showed that LEI samples gave a better fatigue performance than HEI with average lives 2x105 to 4.5x105 cycles greater for a given stress range.

A mean line through each test series was compared at 106 cycles. Wrought material gave the highest fatigue strength at 720MPa followed by LEI (525MPa) and HEI (435MPa) (Fig. 2).

The reduction in tensile and fatigue performance as compared to the fully aged wrought material may be due to over-ageing in the deposits due to multiple thermal cycles during deposition, prior to the double ageing post-weld heat treatment.

Creep rupture testing found that the performance of LEI samples was comparable with published data for alloy 718 aged at 649°C. HEI samples gave a slightly higher rupture life by a factor of 1.62 on endurance (hrs) at a stress of 690MPa.

The slightly better creep rupture performance of HEI samples was probably due to a coarser grain structure.

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Fig. 2 Fatigue test results
Fig. 2 Fatigue test results
Avatar Matthew Doré Section Manager – Fatigue Integrity Management

Matthew joined TWI in October 2000 working in the Fatigue Section and was promoted to Team Manager in 2016. He completed a PhD through the Open University under the supervision of Dr Stephen Maddox investigating fatigue crack growth acceleration and how it can be allowed for in fatigue design.

Matthew has led a variety of consultancy projects for a broad range of industries and specialises in fatigue design and fatigue life improvement. Principal project areas have included the assessment of offshore structures, the structural integrity of bridges, and the fatigue life improvement of welded high-strength steels and welded aluminium alloys. He has authored conference and journal publications on fatigue under variable amplitude loading for steel and aluminium alloy, and fatigue life extension focusing primarily on high-frequency mechanical impact treatment. He is also a committee member serving on panels for BS 7608, BS 7910 and the IIW Commission XIII.