August_AMP_Digital

dditive manufacturing (AM) continues to make

significant technology advancements and attract

widespread interest in industry, academia, and the

general media. However, the technology is likely still a

decade away from attaining meaningful implementation in

industry and has yet to find adoption in large-scale or part-

critical manufacturing operations, such as airframe struc-

tural component production. As a bridge technology that

takes advantage of existing manufacturing methods while

AM is being fully developed,

near-net manufacturing

is a

promising alternative. Near-net fills the gap between clas-

sic subtractive manufacturing and AM, and it may be accu-

rately described in terms of grain size.

In any AM technology, parts are built by using an en-

ergy source (laser, electron beam) to join or fuse a large

number of uniformly shaped items to each other. The

process is repeated layer-by-layer until a part is con-

structed. For the purposes of this article, “grain size” refers

to the size of the items used to build the part. As an exam-

ple, consider

laser sintering

. In this 3D printing process, a

computer is loaded with a 3D CAD file of the part to be

made. The computer directs the laser to move around on

the surface of the powdered material, while powder grains

fuse together and harden once the laser moves on. Each

time the laser traces the powder, the hardened material is

lowered incrementally. Over time and many layers, the

hardened material creates a complete part. In this case, the

grain size of the particles being fused is orders of magni-

tude smaller than the part being created, even for small

parts. Parts made this way are considered to have been

built with 100% additive manufacturing.

At the other end of the spectrum are parts made with

100% subtractive manufacturing. The term

subtractive man-

ufacturing

is used to describe a process where the material

used tomake the part is about the same size as the part being

made, a traditional method used for many decades. In gen-

eral, subtractive manufacturing is most suitable when the

part’s overall size, tensile strength, fatigue performance, or

some other physical performance criteria demands that

parts be made from billet or forgings. Aerospace structural

components are a classic example.

In many aerospace applications, due to weight and size

restrictions,

parts must

perform near

the

physical

limits of the ma-

terials used to

make them. Fin-

ished parts must

be uniform, reli-

able, and meet

well defined per-

formance specifications. Parts also must be as compact and

light as possible. Many aerospace parts are manufactured

from blocks of titanium, stainless steel, or other alloys

and then machined away to achieve the final part shape.

Figure 1 is an example of how an aerospace part is manu-

factured today, with much of the material removed to cre-

ate the final part.

This method is known as “hogging out a part” and is

the reason the aerospace industry talks about

buy-to-fly ra-

tios—

how many pounds of a particular material one must

buy to end up with one pound that makes it into the air.

Specific buy-to-fly ratios are extremely proprietary and ac-

tual ratios for existing parts are closely guarded secrets, but

aerospace companies routinely talk about ratios of between

10 and 20 being common. While buy-to-fly ratios allude to

the high material cost associated with subtractive manu-

facturing, the ratio ignores the other cost—manpower. It

takes a lot of time by a skilled machinist to whittle a part

out of a solid block of titanium and wastes a significant

amount of material. Yet many reasons remain with regard

to the continued use of subtractive methods. Figure 2 il-

lustrates how grain size is related to both additive and sub-

tractive manufacturing processes.

The two main reasons that AM is not yet widely imple-

mented both relate to grain size. First, a huge number of

grains must be fused together to make even a small part,

which takes a relatively long time compared to traditional

machining methods. Second, it is difficult to control the

mechanical properties of fused particles—much less the

entire part—with existing technologies. Companies, uni-

versities, and research organizations are making great

strides to overcome these issues and ultimately will suc-

ceed. Today, however, these and a few other smaller issues

are keeping AM from being used to any great extent in the

manufacturing industry. Compared to AM, subtractive

manufacturing is incredibly wasteful and costly but it pro-

vides a well-established, reliable, and accepted way to pro-

duce high quality, high strength parts. As a bridge

technology between these two methods, near-net manu-

facturing must be considered.

Near-net manufacturing

Near-net fills the middle ground of grain size between

100% additive and 100% subtractive manufacturing (Fig. 2).

With near-net manufacturing, companies can gain some of

the benefits of AMwithout having to give up the speed and

mechanical properties available from subtractive methods.

As its name implies, near-net manufacturing means creating

a form that approximates the shape of the final part. This is

accomplished by joining relatively large components (large

grains) of a desired alloy, like titanium, in such a way that

the material’s physical properties are retained. Figure 3 illus-

trates the concept of near-net manufacturing.

ADVANCED MATERIALS & PROCESSES •

AUGUST 2014

TECHNICAL SPOTLIGHT

Linear Friction Welding Enables

Near-Net Manufacturing

Fig. 1 —

Traditional subtractive manufacturing used to

make an aerospace part. Courtesy of TWI Ltd.

25 mm