Investigating the Linear Thermal Expansion of Additively Manufactured Multi-Material Joining between Invar and Steel

Abstract

This work investigated the linear thermal expansion properties of a multi-material specimen fabricated with Invar M93 and A36 steel. A sequence of tests was performed to investigate the viability of additively manufactured Invar M93 for lowering the coefficient of thermal expansion (CTE) in multi-material part tooling. Invar beads were additively manufactured on a steel base plate using a fiber laser system, and samples were taken from the steel, Invar, and the interface between the two materials. The CTE of the samples was measured between 40 °C and 150 °C using a thermomechanical analyzer, and the elemental composition was studied with energy dispersive X-ray spectroscopy. The CTE of samples taken from the steel and the interface remained comparable to that of A36 steel; however, deviations between the thermal expansion values were prevalent due to element diffusion in and around the heat-affected zone. The CTEs measured from the Invar bead were lower than those from the other sections with the largest and smallest thermal expansion values being 10.40 μm/m-K and 2.09 μm/m-K. In each of the sections, the largest CTE was measured from samples taken from the end of the weld beads. An additional test was performed to measure the aggregate expansion of multi-material tools. Invar beads were welded on an A36 steel plate. The invar was machined, and the sample was heated in an oven from 40 °C and 160 °C. Strain gauges were placed on the surface of the part and were used to analyze how the combined thermal expansions of the invar and steel would affect the thermal expansion on the surface of a tool. There were small deviations between the expansion values measured by gauges placed in different orientations, and the elongation of the sample was greatest along the dimension containing a larger percentage of steel. On average, the expansion of the machined Invar surface was 42% less than the expansion of the steel surface. The results of this work demonstrate that additively manufactured Invar can be utilized to decrease the CTE for multi-material part tooling.

Keywords: Invar; additive manufacturing; coefficient of thermal expansion; multi-material joining; steel; tooling.

Figure 1

Laser end-effector used for the laser-hot wire additive manufacturing process.

Figure 2

(a) Invar bead layout and Invar sample locations on A36 steel baseplate and measured dimensions (mm) of base plate and samples in each direction. (b) Welded invar beads on steel baseplate before sample removal.

Figure 3

A36 steel baseplate and welded Invar bead sample locations for thermomechanical analysis and corresponding cross-sectional views of each Invar and steel section with the respective sample depths in relation to the material interface and top of the weld beads.

Figure 4

(a) Strain gauges bonded to the machined Invar beads, steel, and Invar reference plate. (b) Invar plates in the oven with strain gauges and thermocouples.

Figure 5

Location and orientation of strain gauges on the machined invar beads, steel plate, and the invar reference plate. Dimensions of each plate and the interface between the steel and Invar.

Figure 6

NI 9237 strain gauge module and NI 9211 thermocouple module wiring diagram.

Figure 7

CTE comparison between the samples taken from the top surface of the steel and samples taken from 1.95 mm under the top of the invar bead.

Figure 8

(a) Secondary scanning electron image of interface region and (b) energy-dispersive X-ray spectroscopy (EDS) line scan signals along the interface for the WA36X1 sample.

Figure 9

CTE comparison between the samples taken from the top surface of the steel and samples taken from the interface between the steel and the invar bead (0.45 mm below the top of the Invar bead).

Figure 10

(a) Secondary scanning electron image of interface region and (b) EDS line scan signals along the interface for the M93A36Z2 sample.

Figure 11

CTE comparison between the samples taken from the top surface of the steel and samples taken from 0.13 mm below the top of the Invar bead.

Figure 12

Secondary scanning electron image of interface region and EDS line scan signals of (a) M93Z2 and (b) M93X1 samples.

Figure 13

Back-scattered electron image and electron probe micro analyzer (EPMA) line scan signals of (a) M93A36Z2 and (b) M93Z2 samples.